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Latitude
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Reading Assignment
- Gillespie, Netoff and Tiller,
eWeather &
Climate, as applicable
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01. In this unit, we will be
discussing Latitude as a climatic control. By latitude we are not
talking about geographic location. Rather our interest will center
on the effect of latitudinal location on the receipt of solar
energy at the Earth's surface. To a degree the Greeks were on to
the idea when they proposed their Torrid, Temperate and Frigid
temperature zones over 2000 years ago.
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02. Specifically in this section on Latitude, we are going to
cover four topics:
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- Earth-Sun
Relationships. This section
will deal with the actual receipt of solar energy at the
Earth's surface. The receipt of solar energy is directly
related to the relationships which exist between the Earth and
the Sun across the year. All of you would recognize the basic
relationships. Each day we can see that the Sun "rises" in the
east and sets in the west. Most would also recognize that over
a 12 month period the Sun "moves" from north to south in our
sky. In other words, the Sun not only "moves" from east to
west, but also north to south. And how about the fact that days
tend to get longer in the northern hemisphere from December 21
to June 21, and then grow shorter from June 21 to December 21?
Earth-Sun relationships cause changes in the amount of
insolation received day to day and seasonally. And, depending
upon the relationship, the amount of insolation changes
locationally and through time. The resultant temperatures
created due to these changing relationships create pressure
differences which are largely responsible for winds. The winds
in turn drive the ocean currents and our weather.
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- Temperature.
We will also consider the measurement and transfer of heat --
giving special attention to conduction, convection and
radiation as these processes relate to the
atmosphere.
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- Atmospheric
Gases. While we will touch
upon the major gases comprising the atmosphere, we will devote
most of our attention to the most important gas related to
weather -- that being water vapor -- the gaseous state of
water.
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- The Layered
Atmosphere. Finally, we will
take a look at the various atmospheric layers. While there are
in fact four atmospheric layers based on temperature, our
interest will rest primarily with the first layer, the
troposphere. The troposphere, which extends upward from the
Earth's surface an average of maybe 5-10 miles, is where most
of our weather takes place. While we think of weather as being
all-encompassing, it is actually relatively shallow. Think
about the last time you took a trip by air. You most likely
flew at about 30,000 to 35,000 feet. As you looked out of the
window, most of the clouds were far below you. Or maybe you
have stood outside after a front passes and felt a strong north
wind in your face, and then upon looking up in the sky are
surprised to see clouds moving the opposite direction (heading
north). Remember our earlier comments about mountain barriers?
Weather is shallow.
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03. Before we begin, let me take a minute to define the terms low,
middle and high latitudes as I will be using them in this class.
Low latitudes are those locations found between the Equator (0
degrees N/S) and 30 degrees N/S. The middle latitudes are found
between 30 degrees N/S and 60 degrees N/S. And the high latitudes
are found between 60 degrees N/S and the poles (90 degrees N/S).
As we look at these locations in the lesson ahead, we will be
especially interested in the annual and seasonal temperature
differences found in each of these latitudinal belts, and what
causes these differences to occur.
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04. Earth-Sun
Relationships. Our first topic is
Earth-Sun relationships. This is an extremely important idea
because it is these varying relationships that cause differences
in the amount of solar energy that comes to the Earth's surface.
For instance, you get a lot better Sun tan in June than you will
in December -- because the Sun's rays are more intense. It is the
Sun's energy that heats the Earth and drives the ocean currents,
winds and our weather.
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- I have found that one of the best
analogies to use in discussing Earth-Sun relationships is that of
baking a cake. Because that is really what we are talking about --
the Sun baking the Earth. If I take the ingredients for a cake,
put them in a pan, stir them up, and then turn on the oven, to say
20,000 degrees F, and throw them in the oven and then immediately
jerk the pan right out -- what I end up with is a raw cake. Or
what if I stir up those same ingredients, throw them in the same
pan, slide the pan into an oven that is heated to only 80 degrees
F and leave it in there for 6 weeks. What do I have? Well, most
likely a raw, moldy cake! At 80 degrees F, the cake never cooks,
no matter how long I leave it in the oven. In other words it takes
TWO things to bake the cake (or the Earth) -- heat and time
working together -- not just one or the other.
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05. Let's begin by taking a look at insolation
(incoming
solar
radiation).
If we were to take a flashlight and project a beam of light onto a
surface (and let us assume that 1X of insolation is actually
reaching the surface), note that the 1X of insolation actually
covers 1X of surface (for example, 3 square inches). Now, if we
tilt the flashlight a bit and send the light beam to the surface
at an angle (again, the same 1X of insolation is being put to the
surface), you will observe that the surface area covered by the
beam is increased. So, the same amount of insolation, but spread
over a larger area -- the result is less heat energy per square
inch -- thus (if insolation were all that were involved), a cooler
temperature at the surface.
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- Look at the graphic below. Let us assume
that it is March 21 and the Sun (see the high Sun position) is
directly overhead the Equator. Note: 1X of heat energy onto 1X of
land. All of the insolation available (1X) is absorbed by the
Earth at the Equator. Contrast this with what one might see on
that same March 21 date from a position of 45 degrees N latitude.
At 45 degrees N latitude, the same 1X of insolation is being
received at the surface, but, since we have almost 50 percent more
area covered, you should expect the temperatures (again, if this
were all that were involved in Earth temperatues) at this location
to be less than those experienced at the Equator. In other words,
the closer one is to the point at which the Sun is vertically
overhead, the greater amount of insolation received at the
surface.
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06. The graphic below depicts this Earth-Sun relationship at three
different points on the Earth's surface. The Sun is off the
graphic to the right. If the Sun is directly overhead the Equator
(Point C), then 1X of insolation is being put to 1X of surface at
the Equator. It's just like the flashlight being directly over a
spot. In short, the Equator is receiving the maximum insolation
available (1X of solar energy is being put on 1X of area).
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- If we move poleward, to say 45 degrees N
(Point B), here we find the solar energy coming in at a greater
angle than was the case at point C. Just like the flashlight when
we tilted it. The graphic indicates that at 45 degrees N, 1X of
insolation would be spread across 1.4X of surface. It would stand
to reason that on a per square mile basis, Point B would receive
less heat energy than Point C. And at Point A (60 degrees N), 1X
of insolation would cover 2X of surface (approximately half the
heat energy per square mile as is being put onto the surface at
Point A). And, should we carry the example to 90 degrees N (the
pole), the Sun angle would be coming in on the horizon and the
heat energy received would be minimal per square mile. So, that
even though light is available 24 hours a day for several months
on end at or near the poles, the heat energy actually received is
minimal.
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07. Notice in the graphic below, that all things being equal,
solar radiation moving to the Earth's surface from a point
directly overhead will pass through ONE atmosphere at the Equator
on March 21. But, on this day as you move toward a pole, the Sun
angle is less (remember, you are tilting the "flashlight") and
that the angle will in fact be 0 at the pole). Because of
increasing amounts of atmosphere that the solar radiation must
pass through in order to reach the Earth's surface, we can see
from the graphic below that while the number of atmospheres is one
(1.00) over the Equator and increases gradually and only slightly
as you move toward the poles, there is a sharp and very
significant increase in atmospheres as the Sun angle approaches
the horizon at the poles.
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- The point then -- the closer you are to
where the Sun is directly overhead, the more insolation per square
mile. The farther away you are from the Sun's direct ray, the less
the angle and the less heat energy per square mile.
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08. In San Antonio on June 21, the Sun is not directly overhead;
but it is only 6 degrees off the vertical. On this same day in
Winnipeg, the Sun is some 26.5 degrees off the vertical. Note the
difference in the amount of surface covered by each ray and
consider the path through the atmosphere each is taking to the
surface. And contrast June 21 at both locations with the situation
on December 21.
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09. Let's apply this idea to the house you live in. Many of you no
doubt live in a house that has a roof overhang (an eave). In the
southern United States, the eaves of houses tend to overhang the
wall maybe three to four feet. In the South in the summer, when
the Sun is high in the sky and the temperatures are very high,
every effort is made to reduce the heating of one's house. Houses
with substantial eaves are able to intercept the Sun's rays and
thus reduce sunlight entering the windows. In winter, when the Sun
is lower in the sky, heat energy is admitted to the home through
the windows unhindered by the presence of the eave.
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- If you are in the northern part of the
United States, it is relatively cool the year round and here one
is more likely to find much smaller, narrower eaves. Such
construction permits sunlight to come into the house year
round.
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- You can see this idea reflected in the
location of ski resorts. Think about Colorado (or wherever) the
next time you are on one of those slopes. What direction do the
slopes face? North in the Northern Hemisphere (south in the
Southern Hemisphere). Here temperatures are somewhat cooler (less
direct Sun and less evaporation) than those found on say the
south-facing slopes in the Northern Hemisphere. Thus the snow will
tend to melt on the north-facing slopes last.
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10. Heat then is one of the items needed to bake the cake. But it
takes two things to bake the cake. Let us now consider time.
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- From the graphic below you can see that
on March 21 and September 21 (the equinoxes -- equal day, equal
night), the Earth is oriented to the Sun in such a way as to place
the vertical ray directly overhead the Equator. Now, shinning a
light on a ball such as the Earth will cause only half (180
degrees) of the ball to be lighted. From the point where the light
is directly overhead, light will extend in all directions 90
degrees. So that in this example, if the Sun is directly overhead
the Equator, light will be visible 90 degrees to each pole. If you
are at the Equator, the Sun will be directly over your head -- at
the poles the Sun will be on your horizon. But regardless where
you are (pole to Equator) all locations will experience 12 hours
of day and 12 hours of night since all parallels are
bisected.
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11. Things look considerably different on June 21. For openers,
the Earth-Sun orientation is not the same. On this date the
Northern Hemisphere is pitched toward the Sun and, as a result,
you can readily see that the Sun's vertical ray is not striking
the Earth at the Equator but rather is overhead at 23.5 degrees N.
This is the location of the Tropic of Cancer. The tropic marks the
northern-most point at which the Sun will be overhead during the
year.
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- When you see the Tropic of Cancer on
maps and globes, there is usually some distinctive marking (dots,
dashes, or the like) to set this parallel off from others.
Poleward of 23.5 degrees N there is never a day when the Sun is
directly overhead. Poleward of the Tropic of Cancer you will
always have to look south to see the Sun.
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- On June 21, when the Sun is directly
overhead 23.5 degrees N, note that in shining 90 degrees, the Sun
will extend not only to the North Pole (66.5 degrees away from the
Tropic of Cancer), but in fact an additional 23.5 degrees (to make
a total of 90 degrees) beyond the pole to the back side of the
Arctic Circle (66.5 degrees N). Light shines 90 degrees in all
directions from the point where the Sun's ray is vertically
overhead.
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- Light also shines 90 degrees to the
south from the Tropic of Cancer. This means that light will extend
to the Equator (23.5 degrees) plus an additional 66.5 degrees (to
total 90 degrees) to the front side of the Antarctic Circle (66.5
degrees S).
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- If we could spin the Earth one
rotation/one day, we would find that if you are anywhere between
66.5 degrees N and the North Pole, you would always be in light
(24 hours). No darkness would occur because at no time would you
rotate into the dark area. So with 24 hours of light, why is it
not warm to hot in these polar areas? Keep in mind that you are a
long way from the vertical ray of the Sun (it is at 23.5 degrees
N). You have long days, but the low Sun angle means little heat
energy is actually being received at the Earth's surface. Also
keep in mind the earlier graphic depicting the number of
atmospheres sunlight must pass through on its way to the
surface.
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- Now, if you are located between 66.5
degrees S and the South Pole, you will on this date experience 24
hours of darkness because such locations are more than 90 degrees
away from the Sun's vertical ray.
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- Finally, if you are on the Equator you
will have 12 hours of day and 12 of night on June 21. On the
Equator we will see that days and nights are equal 365 days a
year. The Circle of Illumination always bisects the Equator. So
why then is it not hot on the Equator? After all, the Sun is
overhead or near overhead 365 days a year and sunlight has minimal
atmosphere to pass through. Think about it. Just about the time
you begin building up heat energy on the Earth's surface, the Sun
begins to fall and night is upon you. They don't have those 13 to
14 hour days we have here in the mid-latitudes in the summer. They
have the solar radiation (heat energy), but they just don't have
the time in the oven. And it takes both to really bake the
cake.
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12. Finally, let's take a look at December 21. On this date you
will notice that the Southern Hemisphere is pitched toward the Sun
and, as a result, you can readily see that the Sun's vertical ray
is not striking the Earth at either the Tropic of Cancer or the
Equator, but rather it is overhead at 23.5 degrees S. This is the
location of the Tropic of Capricorn. The Tropic of Capricorn marks
the southern-most point at which the Sun will be overhead during
the year.
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- And like the Tropic of Cancer, when you
see the Tropic of Capricorn on maps and globes, there is usually
some distinctive marking (dots, dashes, and the like) to set this
parallel off from others. Beyond 23.5 degrees S there is never a
day when the Sun is directly overhead. Poleward of the Tropic of
Capricorn you will always have to look north to see the
Sun.
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- On December 21, when the Sun is directly
overhead 23.5 degrees S, note that in shining 90 degrees, the Sun
will extend not only to the South Pole (66.5 degrees away from the
Tropic of Capricorn), but in fact an additional 23.5 degrees (to
make a total of 90 degrees) beyond the pole to the back side of
the Antarctic Circle (66.5 degrees S). Light shines 90 degrees in
all directions from the point where the Sun's ray is vertically
overhead.
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- Light also shines 90 degrees to the
north from the Tropic of Capricorn. This means that light will
extend to the Equator (23.5 degrees) plus an additional 66.5
degrees (to total 90 degrees) to the front side of the Arctic
Circle (66.5 degrees N).
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- And again, if we could spin the Earth
one rotation/one day, we would find that if you are anywhere
between 66.5 degrees S and the South Pole, you would always be in
light (24 hours). No darkness would occur because at no time would
you rotate into the dark area.
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- Now, if you are located between 66.5
degrees N and the North pole, you will on this date experience 24
hours of darkness because such locations are more than 90 degrees
away from the Sun's vertical ray.
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- Finally, if you are on the Equator you
will have 12 hours of day and 12 of night on December 21. On the
Equator we will see that days and nights are equal 365 days a
year. Light always bisects the Equator.
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13. Take a look at the graphic below and note that, depending upon
where you are latitudinally on a specific date, the hours of
light/dark change. Always 12 and 12 on March 21 and September 21
all over the Earth. Always 12 and 12 on the Equator -- 365 days a
year, but note the length of day and night at various latitudes on
June 21 and December 21.
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The previous graphics have
demonstrated that Earth-Sun relations change across the year. We
well understand that the Sun is not doing anything. It is just
there -- immobile, relative to the Earth. It is the Earth that is
presenting itself differently to the Sun at various times of the
year. Some times during the year the Northern Hemisphere is
pitched toward the Sun (June 21). Here proximity to the ray,
combined with long days and short nights, creates a period of
considerable heat. At other times of the year the Southern
Hemisphere is pitched toward the Sun and the Southern Hemisphere
experiences summer conditions. During this period (December 21)
the resulting distance from the Sun's vertical ray, short days and
long nights makes for cold winter conditions in the Northern
Hemisphere. And finally, at other times of the year, neither
hemisphere is pitched toward the Sun (March 21 and September 21
periods), and at these times the insolation received from the Sun
tends to be more moderate in both hemispheres resulting in the
more temperate spring and fall seasons. Now there is a lot going
on here -- remember, the Sun is not doing anything other than just
sitting there.
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14. What are these things that are causing the Earth to be
oriented to the Sun differently at different times of the year?
There are four factors at work in Earth-Sun relationships. They
are: Rotation, Revolution, Inclination, and Parallelism. Let's
take a look at each one of these in the graphics that
follow.
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15.
Rotation.
Again, keep in mind the Sun, relative to the Earth, is NOT moving.
The Earth rotates on its axis in a west to east direction. Think
of the axis as an imaginary line extending from pole to pole and
passing through the center of the Earth. That said, how do you
know that the Earth is in fact rotating from west to east? Think
about it for a minute. Doesn't New York City come into the light
(experience dawn) while it is still dark in Huntsville? If it's
8:00AM here in Huntsville, isn't it 9:00AM in New York City? They
are an hour ahead (earlier light) of us. If that is the case, then
won't the Earth have to rotate from Huntsville (west of New York
City) toward New York City in order for Huntsville to experience
the light -- which it generally receives about one hour after New
York? And when Huntsville comes into the light, isn't Los Angeles
still in the dark? So for Los Angeles to receive light (usually
some two hours after Huntsville) won't it be necessary for Los
Angeles (to the west of Huntsville) to rotate from the west TO the
east in order to reach the light? And finally, as dawn breaks over
Los Angeles, hasn't New York City been in the light for several
hours? One rotation takes approximately 24 hours. Again, all this
assumes that you keep in mind that the Sun is immobile relative to
the Earth.
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- The photograph below was taken by the crew on board the
Columbia during its last mission February1, 2003. The photograph,
taken on a cloudless day, catches the sun setting over Europe and
Africa. The Circle of Illumination is striking as is cuts across
western Africa, Spain and France. The more urbanized areas may be
clearly seen in the light pattern -- the lights are on in Paris
and Barcelona, but it is still daylight in London and Gibraltar.
In the middle of the Atlantic Ocean are the Azores and to their
southeast are the Canary Islands. On to the south, off the west
coast of Africa are the Cape Verde Islands. Across the top of the
photo, from right to left is snow-covered Norway, Iceland and
Greenland.
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16. Consider for a moment the speed of the Earth's rotation. You
might think it is the same everywhere, but such is not the case.
Look at the graphic below. We know that the distance around the
Earth at the Equator is approximately 25,000 miles. One turn
(rotation) in 24 hours. Thus the speed of the Earth at the Equator
is a little more than 1000 mph. If you are standing on your big
toe on the North Pole, what is the speed of rotation? Well, since
you will turn once in 24 hours, but in so doing you will actually
go nowhere, the speed at the pole must be 0 miles per hour -- the
Earth is just turning in place for you.
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- Therefore, intermediate locations
between the Equator and poles will have speeds between 1000 and 0
miles per hour with the speed declining toward the pole. And while
we will come back to it later, consider for a moment the speed of
the Earth's atmosphere. On average, do you see why the speed of
the atmosphere above any given point on the Earth's surface is the
same as the Earth's surface? If such were not the case -- for
instance say the Earth were turning 800 miles per hour, but the
atmosphere above it were only turning 700 miles per hour --
individuals on the Earth would be experiencing a 100 mph
wind!
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17. Kind of as an aside, yet related to rotation (which causes day
and night), is the idea of the Circle of Illumination. As the
Earth rotates, some of the time we face the Sun and are in light;
other times we are away from the Sun and in darkness. Keeping in
mind that only half of the globe can be lighted at one time, what
are we to make of the transition area between light and dark? You
know these times as dawn, and dusk or twilight. This area is known
as the Circle of Illumination. In the Circle of Illumination it is
not really dark nor is it light. Any of you who have ever flown in
a plane and crossed the Circle will well remember it. You can look
up toward the front of the plane at 35,000 feet and see the light,
and look to the rear and see darkness.
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18.
Revolution.
The second factor influencing Earth-Sun relations is Revolution.
As the Earth is rotating on its axis, it is also revolving around
the Sun in a counterclockwise direction. 365 days to make one
circuit. In revolving about the Sun, the Earth moves along on what
is called the Plane of the Ecliptic (its orbital plane). As you
can see in the graphic below, at some times in the orbit the Earth
is closer to the Sun than at others. Two dates come to mind:
January 3 and July 4. Living in the Northern Hemisphere you might
think that the Earth and Sun would be closest on July 4 because
this date is warmer than January 3, but to do so would be in
error. Do you think you would have made the same assumption if you
were living in Australia? In fact, the Earth is closest to the Sun
on January 3 and most distant on July 4. January 3 is called
perihelion; July 4 is called aphelion (think Aphelion --
Away).
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- What does all this mean? This basically
means that distance from the Sun is not what is causing the
seasons.
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19.
Inclination.
Inclination refers to the tilt of the Earth on its axis. Note that
when you see models of the Earth, the North Pole is not at a 90
degree angle with reference to the Plane of the Ecliptic. Rather
it is tilted or inclined 23.5 degrees off the perpendicular. Now,
where have you heard this 23.5 degree business? Yes, the Tropics
of Cancer and Capricorn. Anywhere else? How about the two circles
(Arctic and Antarctic)? They are at 66.5 degrees N/S, but where
are these points relative to the adjacent poles? Right, they are
23.5 degrees Equatorward of the poles. This is why we give special
significance to these two special parallels -- the two tropics and
the two circles. They are related to the tilt of the Earth off the
perpendicular.
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- Let's try you out on something here.
What if, instead of the Earth being tilted 23.5 degrees, it were
tilted 10 degrees off the perpendicular? If such were the case,
would summers in the Northern Hemisphere be: warmer than they are
now, cooler than they are now, about the same, or can't
tell?
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- Think about this for a moment. What is
it that makes the Earth's surface warm or cold? Isn't it distance
from the vertical Sun ray and length of day (the cake analogy)?
Well, that being the case, take it to the extreme. Don't tilt the
Earth 10 degrees -- just don't tilt the Earth any degrees. In
other words imagine there is no tilt. The Earth's poles are at a
90 degree angle to the Plane of the Ecliptic. Now, if this were
the case, would the Sun's vertical ray be striking the Earth at
the Equator every day, 365 days a year? Yes. Well, if the Sun is
on the Equator all year, doesn't that mean that the Sun's vertical
ray does NOT travel into either hemisphere, but instead remains on
the Equator all year? Too, that will mean that every day (and
night) is 12 hours long. If that is the case, then wouldn't
summers be cooler than they are now? True, the question didn't ask
for no tilt, but it did ask for LESS tilt (10 degrees) than we now
have (23.5 degrees), so doesn't it follow with the example above
that, if such were the case, that summers would be cooler than
they are now?
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- And do you see that if the tilt were
increased to say 40 degrees, we would all be in big trouble from a
temperature standpoint? In such an instance, wouldn't the Sun's
vertical ray (and related surface heat) extend to 40 degrees N in
the Northern Hemisphere's summer (thus reaching far beyond the
present 23.5 degrees) into the hemisphere, and in addition
providing much, much longer days (and shorter nights) than we now
experience? Summers of 130 degrees F or more might be common. Too,
do you see that winters in such a situation (with a 40 degree
tilt) would be much, much colder than we now experience. The cold
being related to the fact that the Sun's vertical ray (the heat
energy) would not be at its maximum distance as is now the case
just 23.5 degrees south of the Equator, but instead would be 40
degrees south of the Equator. Northern Hemisphere locations would
be much more distant from the vertical ray than is now the case.
Too, Northern Hemisphere days would be much shorter (not as much
time to absorb any insolation headed their way) and nights would
be much longer than is now the case -- thus more time for the
meager heat energy to leave the Earth's surface.
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- If you understand all this, then you've
got it!
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20.
Parallelism.
Parallelism is the idea that as the Earth revolves around the Sun,
the axis is always pointed to the same place in space. Where is
this place that it is pointed? The North Star. The North Pole is
always pointed to the North Star no matter where it is in its
orbit about the Sun. Look closely at the graphic below and you can
get an inkling of the importance of this idea. Note that in all
the views of the Earth, the axis is pointed the same direction (to
your right). Consider for a moment the implications of this across
a year. Can you see that at some points in the orbit, the North
Pole is pitched toward the Sun while at other points the South
Pole is pitched toward the Sun?
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Now, let's put all this together by taking a look at the Earth-Sun
relationships that are to be found on the four critical dates as
the Earth orbits the Sun.
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21. June
21. We will begin with the
position of June 21. Note that you are viewing the Earth graphic
on the left from the position indicated by a red "X" on the
graphic to the right.
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- On June 21 you will note that the
Northern Hemisphere is pitched toward the Sun. This means that the
Sun's vertical ray is striking the Earth at the Tropic of Cancer
(23.5 degrees N). Light extends 90 degrees in all directions from
this point. On this date, you will note that light extends from
23.5 degrees N toward and then beyond the North Pole to the back
side of the Arctic Circle, and that light also extends toward the
South Pole to the front side of the Antarctic Circle (66.5 degrees
S).
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- Note that at the Equator we have equal
day and night, but as we move toward the North Pole from the
Equator days get longer finally reaching 24 hours at the Arctic
Circle (66.5 degrees N). It is a combination of the long days and
the fact that the Sun's vertical ray is at 23.5 degrees N that
makes this the day of maximum solar radiation receipt in the
Northern Hemisphere.
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- And we have the reverse situation in the
Southern Hemisphere. Here, as you go toward the South Pole, days
get shorter until we get to the Antarctic Circle beyond which
there is no light. Too, keep in mind that the Sun's vertical ray
(the heat energy) is centered 23.5 degrees north of the Equator --
a long way from points in the Southern Hemisphere. Thus solar
energy is at its minimum in the Southern Hemisphere on this date
(the first day of the Southern Hemisphere winter).
-
- Let's see now. The date is June 21. It
is the beginning of summer in the Northern Hemisphere and the
beginning of winter in the Southern Hemisphere. I get the June 21
date in the Northern Hemisphere -- but June 21 in the Southern
Hemisphere? Are we sure about this? What are the chances, since it
is winter in the Southern Hemisphere, that it is December 21 in
the Southern Hemisphere? The chances are ZERO. The season may
change as you move from one hemisphere, but the date stays the
same. Just checking!
-
- What's the best day for a sun tan? June
21. You may think the best day is sometime in August when the
temperatures are the highest, but keep in mind you are not baking
yourself to get the tan, you are absorbing the Sun's rays. These
rays are most intense when the Sun is closest to you. So while
June 21 may be the absolute best day for a tan (assuming clear
skies), keep in mind that you can move off that date an equal
number of days in either direction and get a similar
tan.
-
-

-
-
Now let's revolve the Earth counterclockwise 90 degrees (3 months)
from the June 21 date. The date is now September 21/22. Note that
the North Pole is still pointed toward the North Star. Keep in
mind that what we just did (the counterclockwise revolution)
happens gradually and that what we will now discuss has evolved
over three months from conditions we previously discussed for June
21.
-
22. September
21/22. Note where you are -- you
are in front of the Earth in its orbital path. You are looking
back on the Earth from a point in FRONT of it. The Sun is to your
right, thus the right side of the Earth as you look at is lighted.
The Earth is moving in its orbit toward you. The Circle of
Illumination is bisecting all parallels. This is the fall or
autumnal equinox in the Northern Hemisphere, and the spring or
vernal equinox in the Southern Hemisphere. Equinox -- equal night
(and equal days at all locations). For us in the Northern
Hemisphere, the days are getting shorter as the Sun falls/moves
lower in the southern sky. Winter is approaching. If you are in
the Southern Hemisphere, the Sun is rising in the northern sky and
the days are getting longer. Summer is approaching.
-

-
-
23. December
21. We will now revolve the Earth
90 degrees along its orbit in a counterclockwise direction. Note
that you are viewing the Earth graphic on the left from the
position indicated by a red "X" on the graphic to the right.
-
- On December 21 you will observe that the
Southern Hemisphere is pitched toward the Sun. This means that the
Sun's vertical ray is striking the Earth at the Tropic of
Capricorn (23.5 degrees S). Light extends 90 degrees in all
directions from this point. On this date, you will note that light
extends from 23.5 degrees S toward and then beyond the South Pole
to the back side of the Antarctic Circle, and that light also
extends toward the North Pole to the front side of the Arctic
Circle (66.5 degrees N).
-
- At the Equator we have equal day and
night, but as we move toward the South Pole from the Equator days
get longer finally reaching 24 hours at the Antarctic Circle (66.5
degrees S). It is a combination of the long days and the fact that
the Sun's vertical ray is at 23.5 degrees S that makes this the
day of maximum solar radiation receipt in the Southern
Hemisphere.
-
- And we have the reverse situation in the
Northern Hemisphere. Here, as you go toward the North Pole, days
get shorter until we get to the Arctic Circle -- beyond which
there is no light. Too, keep in mind that the intense energy
associated with Sun's vertical ray is centered 23.5 degrees south
of the Equator -- a long way from points in the Northern
Hemisphere. Thus solar energy is at its minimum in the Northern
Hemisphere on this date (the first day of the Northern Hemisphere
winter).
-
-

-
-
24. March
21. We will now revolve the Earth
90 degrees in a counterclockwise direction. Look at where you are
-- you are behind the Earth in its orbital path. You are looking
at the Earth from a point in BACK of it as it orbits the Sun. The
Sun is to your left, thus the left side of the Earth as you look
at is lighted. The Earth is moving in its orbit away from you. The
Circle of Illumination is bisecting all parallels. This is the
spring or vernal equinox in the Northern Hemisphere, and the fall
or autumnal equinox in the Southern Hemisphere. Equinox -- equal
night (and equal days at all locations). For us in the Northern
Hemisphere, the days are getting longer as the Sun rises/moves
higher in the southern sky. Summer is approaching. If you are in
the Southern Hemisphere, the Sun is falling in the northern sky
and the days are getting shorter. Winter is
approaching.
-

-
-
25. Heat
Transfer. As you well realize,
with the Sun's vertical ray appearing to move between the two
tropics, there is a lot of insolation being put into the
tropical/low latitudes. Your textbook indicates that between 36
degrees N and 36 degrees S there is a annual surplus of heat
energy. In other words there is more heat energy being put into
this area by the Sun over a 365 day period than is leaving.
Poleward from 37 degrees N/S there is a heat energy deficit -- in
other words, more heat energy is being lost than is being
received. Keep in mind that if you are at the poles, day may be 24
hours long in the summer, but you are not getting a lot of
insolation because of the low angle. And in the winter months, the
Sun is extremely low or non-existent angle.
-
- Well, a situation like this exist can't
exist for very long. If it did, then soon all of the world's
population would be fighting for the area around 36 degrees N and
S. Toward the Equator from this point it would be too hot to live,
and toward the poles it would be too cold. Since this is not the
case, something must be causing cold air to be transferred out of
the cold deficit areas into the heat energy surplus areas, and
warm air to be transferred out of the heat energy surplus area
into the heat energy deficit areas. For us, the most obvious
factor at work is fronts that bring cold air from the higher
latitudes into the lower/warmer mid-latitudes. That air is coming
in from Canada -- that's cold air from the heat energy deficit
region.
-
- And think about the typical summer
weather forecast for Houston -- such and such a high temperature,
such and such a low temperature AND warm, humid air moving in from
the Gulf of Mexico -- or some variation of that. This warm air
from the Gulf of Mexico is going over us (the typical south wind
of summer) as it heads out of the tropics into the middle and
upper middle latitudes. In other words we can all see this heat
transfer at work right here in southeast Texas the year
round.
-
- Air is important, and the winds do move
a lot of heat/cold about the Earth, but even more important in the
distribution of temperature is water (ocean currents). Water
moving from the high latitudes into the mid and low latitudes
(cold currents), and water moving from the tropical latitudes into
the mid and high latitudes (warm currents) are carrying huge
quantities of energy with them.
-
- Of lesser importance are hurricanes.
They contain huge amounts of heat energy (acquired via evaporation
over tropical oceans) that are brought into the mid-latitudes and
released in the condensation process (rain).
-
-

-
-
26. When we consider Latitude alone as a control, we know that the
low latitudes (say from the Equator to approximately 30 degrees
N/S) are the warmest across the year (on an annual basis). Why?
Well, let's consider the question in terms of our cake analogy. We
have to consider both temperature and time.
-
- We know that in these latitudes the
Sun's ray is vertical or near vertical at all times, thus a
considerable amount of heat energy is available to the region
throughout the year. In addition, the length of day is always 12
hours at the Equator, and as we move poleward toward 30 degrees
N/S the length of day gradually increases to maybe 13/14 hours in
the summer (and running some 10 to 11 hours in the winter). Thus,
with a high Sun ray and days of moderate length all year, you
would expect the annual temperature to be relatively
warm.
-
-

-
-
27. And as you should expect, on an annual basis the high
latitudes (60 to 90 degrees N/S) are the coldest. Consider: during
the cooler late fall, winter and early spring months we have
little or no Sun angle (insolation) and thus little or no daylight
(time) for these roughly six months. During late spring, summer
and early fall months, while we do have extremely long days
(ranging up to six months at the pole) the Sun is either on the
horizon or barely above it, thus little heat energy is
available.
-

-
-
28. So far so good. All is just as you would have suspected. The
Equatorial areas are the warmest and the polar areas are the
coldest across the year. Now let's take a look at things
seasonally. Keep in mind as we talk about seasons that we have two
hemispheres, and that the seasons are reversed in these
hemispheres. When it is summer in the Northern Hemisphere, it is
winter in the Southern Hemisphere. Too, keep in mind that while
summer is always summer (summer is always the "warm" season),
summer is not always July. It depends on what hemisphere you are
in.
-
- On a seasonal basis, the high latitudes
are the coldest -- just as you thought would be the case. After
all, when you have a place that during its cold season has both
little Sun angle and little daylight, you should expect it to be
cold.
-
-

-
-
29. Let us now consider the summer (high Sun) situation at say 25
to 35 degrees N/S. We can actually use Texas as the example since
most of the state falls within these latitudes -- and I suspect
that most of us would agree that our summers seem awfully hot. Why
is this?
-
- In our summer season, the Sun is
approximately 82 to 83 degrees above the southern horizon in
Houston. Not 90 degrees like we get at the Equator at equinox, but
still 80 plus degrees is pretty high. This means that the Sun,
while not vertically overhead, is very close to vertical, and as a
result we do get a great deal of insolation. In addition, our days
run approximately 14 hours in length. It's warmest along 25 to 25
degrees N in the summer season because here you have both the ray
(insolation) and time.
-
- Then why doesn't it get warmer as you go
poleward? Why are 25 to 35 degrees N/S the warmest latitudes and
not 40 to 45 degrees N/S? After all, since these latitudes are
experiencing longer days than we are here in south Texas,
shouldn't they be warmer?
-
- Remember to keep in mind that it takes
TWO things to bake the cake -- time and temperature. As one
continues to go poleward from 23.5 degrees N, it is true that the
days get longer (being 24 hours poleward of 66.5 degrees N on June
21), but, while days are getting longer, the Sun angle is dropping
in the sky and thus there is less heat energy
available.
-
-

-
-
30. There are, of course, other controls that have an impact on
temperature in addition to latitude. While we are discussing the
climatic controls one at a time, if we really wanted to explain
why a place experiences the weather/climate that it does, we would
need to consider all of the controls together. We are not there
yet -- but we will be at the end of the course. However, you do
need to keep in mind that there are other controls at work. These
controls would include:
-
- Air Masses. Fronts
and winds can bring large bodies of air into a region whose
temperatures and humidities are very different from the
resident air mass. Think here about the very cold and dry "blue
northers" that occasionally visit Texas in winter.
-
- Continentality.
This is the idea that land and water do not heat up and cool
off at the same rate. Think about Galveston and
Dallas.
-
- Ocean
Currents. There are both warm
and cold currents. Jacksonville, FL has a warm current off its
coast, Los Angeles has a cold current. While both are coastal
in their location, their climates are considerably
different.
-
- Mountain
Barriers. Mountains can block
warm or cold air and in so doing have a considerable impact on
a location's climate.
-
- Elevation.
Places located between 25 and 35 degrees N/S may well be the
warmest seasonally, but such locations can still have
snow-covered mountain peaks the year round.
-
-

-
-
31. Temperature is a measurement of molecular activity. The faster
the speed of the molecules, the greater the temperature. The
slower the speed of the molecules, the colder the temperature.
Temperature is measured with instruments called thermometers
(thermo/heat,
meter/to
measure). A number of scales have been devised and are currently
in use to measure temperature. The most popular (with the melting
point of ice and the boiling point of water) are: Fahrenheit (32
and 212 degrees F), Celsius (0 and 100 degrees C), and Kelvin (273
and 373 Kelvins). In this class we will use the Fahrenheit scale
because this is the most commonly used scale in this country --
note your evening weather report. If you are taking the lab with
this course, the emphasis will be on the Celsius
scale.
-

-
-
32. In your day-to-day life, you will encounter many different
expressions of temperature. Our interest in this class will be
with air temperature. But there are other types of temperatures
you should at least have some acquaintance with. One of these is
what is called sensible temperature. Sensible temperature is how
it feels on your skin as contrasted with air temperature that is
measured with a thermometer. Sensible temperature is affected by
not only the air temperature, but also the relative humidity, wind
speed and solar radiation.
-
- You have no doubt heard the weatherman
talk about something that is sort of like sensible temperature --
wind chill. Wind chill is a measurement that takes into account
air temperature and wind speed. But you no doubt recognize that
there are other things that go into determining the "temperature"
you feel. Cold, dry air can be very cold, but cold, wet air can
make one miserable. And to combine either with a good stiff wind
adds but another layer of misery.
-
- Think about those times you have seen on
the news people snow skiing in shorts and without a shirt. Are
they cold? They are probably cold only if they fall down in the
snow or if they ski into the shade. The air temperature is
certainly cold, but in the dry mountain air, and with the bright
Sun above and the Sun being reflected off the snow, the skiers are
getting a lot of radiation thus helping to offset the chilling
tendencies of the air temperature.
- If you have ever been to a high
elevation, you can relate to the skier's situation. If you stand
out in the Sun, the temperature can be very pleasant. But if you
step back into the shade, the temperature drops very quickly. The
air temperature in both the Sun and the shaded area is the same,
but they feel very different on your skin. This is why when you
hang a thermometer outside you should hang it in the shade. With
the thermometer you are attempting to measure air temperature. If
you hang the thermometer in the Sun, you will in fact measure the
air temperature, but, in addition, the reading will include any
heat generated as a result of radiation striking the thermometer
resulting in a higher reading of the air temperature.
-
-

-
-
33. We may well make passing reference to sensible temperature
from time to time in this course, however, our primary focus will
be on air temperature.
-
- While temperature and other weather
element readings have in the past typically been taken by
instruments placed in shelters such as those pictured in your
textbook, readings today are much more likely to be taken by
stations such as pictured below. There are literally thousands of
such measuring stations in this country and around the world
taking weather readings on a daily basis and sending them in to
central collection points where they are complied and mapped. This
information forms the basis for the weather forecasts all of us
are familiar with. This same information, over the long term,
provides the basis for climatic statistics.

-
-
34. As we move through the course we will make use of a number of
terms related to temperature. For the moment, you should be sure
that you understand each of the following:
-
- Daily Mean
Temperature. The daily mean
temperature is arrived at by securing a daily temperature
average. This may be calculated in a number of ways. The most
common is to average the maximum and minimum temperatures for
the day. Another method would be to average the hourly
temperature readings. You have to be careful in using this
temperature when trying to provide someone with an idea of what
kind of temperatures they might could expect at a given
location. Galveston, influenced by its proximity to the coast,
may have an average daily mean temperature of 80 degrees F
during the summer (75 degrees F for a low, 85 degrees F for a
high). Las Vegas may have the same average (80 degrees F) but
their low may be 50 degrees F and their high 110 degrees F.
Same daily mean temperature, two very different sets of
temperatures to contend with.
-
- Daily Range of
Temperature. The daily range
of temperature is the difference between the high and low
temperatures for a given day. In the example above, Galveston's
daily range would be 10 degrees F while Las Vegas would have a
daily range of 60 degrees F.
-
- Monthly Mean
Temperature. The monthly mean
temperature is the average temperature for the month. This is
usually arrived at by adding the daily means and dividing the
result by the number of days in the month.
-
- Annual Mean
Temperature. The annual mean
temperature is simply an average of the 12 monthly temperature
means.
-
- Annual Temperature
Range. The annual temperature
range is the difference between the means of the warmest and
coldest months.
-
-

-
-
35. As you can well imagine, trying to make sense out of hundreds
or even thousands of temperature readings across the United States
would be a daunting task. A weather map exhibiting such data would
be no more than a great mass of incomprehensible numbers. How to
make some sense out of all these numbers? Meteorologists make use
of isotherms
(iso/equal,
therms/temperature
-- lines of equal temperature) to accomplish the task. Isotherms,
usually drawn in 5 to 10 degree intervals, are a convenient method
for mapping temperatures. In principle, this idea is very similar
to that of the contour line in geology -- contour lines being
lines connecting points of equal elevation.
-
- The graphic below depicts an uncolored
weather map with several weather stations on it and a series of
isotherms drawn in at 10 degree F intervals. Such a graphic is a
major improvement over a map depicting the simple plotting of
data. But such a map can itself be significantly enhanced by the
addition of color. Here, at a glance, once can get an overview of
the temperature gradations across the country.
-

-
36. We now have some idea of what is involved in getting the heat
energy of the Sun to the Earth's surface. Now the question is how
is the heat energy absorbed by the Earth and then distributed
upward to the atmosphere? Three mechanisms are involved in this
process: conduction, convection and radiation. Let's now take a
closer look at each of these.
-

-
-
37.
Radiation.
Radiation can be defined as the transfer of heat energy by
electromagnetic waves -- no medium of transport is required -- no
solid, no liquid no gas, no land, no air is required. It is in
this manner that energy from the Sun is transferred to the Earth.
Here the term "insolation" is frequently used. Think of the Sun's
warmth on your face on a cold winter afternoon. You step into the
shadows and it gets cold. Radiation comes to the Earth from the
Sun as short wave radiation -- which just means that the distance
between the electromagnetic waves is "short." The radiation is
absorbed by the surfaces it reaches and is then re-radiated back
to the atmosphere as long wave (or infrared) radiation.
-
- Consider: a winter morning with a heavy
frost. As you drive to work or school, you notice that your roof
along with several others in your neighborhood are covered with
frost, while on others frost is only visible around the edges of
the roof. What's going on here? Well, if you are one who has a
frost covered roof, could it be that you have a lot of insulation
in your ceiling? Next door you notice that the only frost on your
neighbor's roof is around the edges (above the eaves). Could it be
that with so little insulation that he has a lot of heat escaping
from the inside of the house thus heating the underside of his
roof and preventing frost from forming?
-
- And just as you turn the corner you see
a small airplane from your local power company flying overhead. As
you later learn, the plane is snapping infrared photographs of the
neighborhood. About a week or so later you get a postcard from
them in the mail indicating that, based on their analysis of their
photos of the neighborhood, you have one of the "tightest" houses
in the area -- very minimal loss of heat. Smiling at your good
luck, you later learn that your next door neighbor has decided to
make use of the energy company's services to run an audit on his
home to find out how he might make his home more energy
efficient.
-

-
-
38.
Conduction.
Once radiation has heated the surface, how is heat energy
transferred to the air above (to then be moved about by the wind)?
By the process of conduction. Conduction might be defined as the
transfer of heat from molecule to adjacent molecule with the heat
flowing from the warmer object to the colder object.
-
- Look at the graphic below. The bar is
placed over the flame and heat is transferred to the bar and then,
with the bar held in place, heat slowly works its way down the
flame to the point that it becomes too warm to be held. While this
bar may conduct heat very efficiently, such is not the case with
all substances. Overall, solids tend to conduct heat better than
liquids, and liquids tend to be better conductors of heat than
gases. Since our concern is with the atmosphere, our interest is
primarily with gases -- the poorest conductors of
heat.
-
- In your everyday life you can see plenty
of evidence of the poor conducting qualities of air. Think about a
winter coat. The purpose of a coat on a cold winter day is NOT to
keep the cold air OFF of you, but rather to keep the warm air IN.
Isn't this the same idea that is at work when one layers their
clothing? Each layer of clothing is surrounded by a thin layer of
air, and it is this thin layer of air (a poor conductor of heat)
that inhibits the escape of warm air from beneath the
clothing.
-
- If you are wearing a coat and the coat
gets wet, you are in big trouble quickly as the warmth of the body
is conducted to the cooler water and you get cold (take on the
temperature of the water). At the extreme, think about the problem
of a person forced into the cold ocean after a shipwreck.
Conduction transfers the warmth of the body (98.6 degrees F) to
the much colder ocean water that is completely surrounding, and in
contact with, the body. The body temperature will drop quickly
with the result often being death.
-
- Or think about the use of insulation in
a house. When insulation is placed in the attic, it contains a lot
of air. That area of air is very difficult to heat or cool. The
air in the insulation helps prevent the escape of warmth (or
coolness) from the living space below. In time, as the insulation
settles, its insulating qualities diminish.
-
- Because air is such a poor conductor of
heat, as you might suspect it is very difficult for conduction to
heat the air to any great height. A few feet above the ground, the
temperature may be much cooler or warmer than the surface. For
instance, it is generally recommended that when placing a
thermometer outside to measure air temperature that the instrument
be placed some five feet off the ground (and in the shade). The
five feet gets it away from the very warm layer right at the
surface caused by conduction (for instance it may be 90 degrees F
at five feet, but 110 degrees F at the ground on a warm summer
day). You should place the thermometer in the shade to get it out
of the Sun's radiation that will provide a false reading by
increasing the temperature of the instrument itself (and, by
conduction, the temperature reading).
-
-

-
-
39.
Convection.
And finally there is the process of convection that we might
define as the transfer of heat within gases or liquids as these
gases/liquids move. We can also think of this as the vertical
movement (up or down) of air in the atmosphere. Some of you may
have heard someone refer to a summer afternoon thunderstorm as a
convectional storm. What is meant here is that the storm is caused
by the intense heating (radiation) of the land in the afternoon by
the high Sun angle and long summer day. The heat of the warmed
surface is then conducted to the air above. This warm air then
rises (convection) and cools to the point of condensation. Clouds
are formed, often followed by heavy rains. Convection is important
for moving air vertically through the atmosphere from the surface
upward (and sometimes from aloft downward to the surface).
Convection is also at work in moving air from the warmer regions
of the Earth (the Tropics) to the higher latitudes and vice
versa.
-
- Convection is the vertical movement or
air. But what about what you and I feel daily -- the horizontal
movement or air? Such air is termed "advection."
-
- Note on the graphic below that warm
rising (lighter) air is being uplifted. As this air leaves the
surface, other cooler, more dense air moves in to replace the
rising warm air. As the air rises aloft, it does not go on out
into space. A number of forces are at work that cause the rising,
cooling air to spread out (diverge) and then begin to fall back
toward the surface (subside). Upon reaching the surface the air
begins to move toward areas that are being heated and uplifted.
This is a classic convection cycle. We will discuss this in
greater detail when we get to the control: Pressure.
-
- We will discuss condensation and
precipitation later as well, but for now keep in mind that there
are at least two conditions that must be present in order to have
condensation and/or precipitation. One: moisture (water vapor)
must be present in the atmosphere; and (2) the temperature of the
air containing the moisture must be falling. You can see by this
graphic why we tend to get a lot of afternoon rain in the summer
months. Sun heats the land; the warm (and humid) air rises; it
expands and cools to the point of condensation and rains. Think of
the rainforest located along the Equator. Is it warm along the
Equator year round? Then do you think almost every afternoon we
would find warm air rising, cooling, condensing and
raining?
-
-

-
-
40. Let's put it all together with a couple of examples. You drive
to school for a class. The outside temperature is 25 degrees F.
You lock your car, go in for the class and upon returning find as
you open the car door that the temperature inside is almost 80
degrees F! What's going on here?
-
- Short wave radiation from the Sun is
penetrating the car's windows, it strikes the dash, steering wheel
and seats and is absorbed. The warmed surfaces inside the car, by
conduction, warm the air above. The warmed air then rises to swirl
about inside the car. You return to the car, open the door and are
hit with a blast of hot air -- not to your feet, but to your face
as the warm (and light) air exits the car and moves upward through
the atmosphere.
-
- Or say you are going to visit your 105
year old grandfather in deep East Texas. This old guy is not into
modern conveniences -- like central heat. It's cold -- 20 degrees
F. No heat in the house as you get ready to go to bed. Now you
have been through this drill before on previous visits so you know
just what to do.
-
- Before getting into bed, you carefully
lay out the clothes you will be putting on in the morning. Now,
ready to get into bed you note that the bed is covered with four
or five hand-made quilts -- not the kind with the cheap,
light-weight batting in it, but rather the real McCoy -- with
plenty of heavy cotton batting. You know that you have to decide
right now, as you get into bed, what position you are going to
want to take for the night. Because once in bed there will be no
turning over -- the quilts will keep you pressed to the lumpy
mattress in the same position all night. Heavy, but effective.
Isn't there a layer of air between each quilt? The air contained
within will keep you warm.
-
- Morning arrives at 4:30AM with a
forceful bang on the door by Granpa. "Time to get up! It's sinful
to sleep past daylight!" Again, you've been here before. You know
that the only heat in the house is the single gas heater in the
kitchen. The trick is to jump out of bed, grab the clothes you
laid out the night before, get them on while making your way to
the kitchen. And Granpa will demand that you be fully dressed
before you show up in the kitchen. You make it without freezing to
death along the way.
-
- As you ease up to the fire, you are
careful not to let the jeans touch the back of your legs. The
raging heater will, within minutes, bring the back of the jeans to
a boil. A slight tilt of the knees in the wrong direction will
cost you all of the hair on the back of your legs. Ain't radiation
wonderful!
-
- Got
it?
-
41. The graphic below is a relatively famous photo taken by the
NASA astronauts from the moon looking back to the Earth. Why is it
that you can see the Earth? It's because a percentage of the total
light being sent from the Sun is being reflected from the Earth
back into your eyes. This reflected light is called the planetary
albedo -- some might call it "Earthshine." We can define albedo as
the reflection of radiant energy expressed as a percentage, and
this percentage varies with the surface encountered. Of all of the
insolation coming to the Earth, approximately 30 percent is
reflected back into space and never gets to the Earth's surface.
Again, this is why you can see the Earth in the photo taken from
the surface of the moon.
-
- In addition to the planetary albedo
lost, there is another approximately 20 percent that is lost via
scattering, absorption and reflection as the radiant energy passes
through the atmosphere on its way to the Earth's surface. Of the
solar radiation that reaches the outer layer of the atmosphere,
only about 50 percent, on average, actually reaches the Earth's
surface. And of the 50 percent that does get through then,
depending upon the surface encountered, maybe as much as 95
percent of that may be reflected back and lost to the
surface.
-
- You will note in the table that
accompanies the NASA photo that the albedo rate varies
considerable. Let me call your attention to two items in
particular.
-
- First, in looking at the percentages,
notice that they vary widely depending upon the surface
encountered. First, in looking at the percentage for fresh snow
(75 percent to 95 percent ) think back on our earlier discussion
of Sun angle at high latitudes. Does this 75 percent to 95 percent
figure help you to better understand why during the winter, the
snow-covered surfaces of Canada (or Russia) have such low
temperatures? Not only is (1) the Sun angle low, thus little heat
energy is received per square mile, and (2) the days short so
there is relatively little time for solar radiation to be
received, and long nights allowing plenty of time for the meager
amount of solar radiation received to be lost, but (3) even of
what little radiation may actually get to the surface, a very high
percentage is reflected back and thus lost to the surface for the
purposes of heating.
-
- Secondly, note the wide variation in
albedo for water (from 5 percent to 80 percent ). Why such a wide
variation? Well, note in the NASA photo that the darkest color to
be seen is that of the ocean. For sure the clouds would be
associated with a high albedo, as would the lighter colored
landmasses. But the ocean water? When the Sun is overhead or
nearly so, can you see that a huge percentage of radiation would
be absorbed by the dark blue oceans relative to the land? However,
as one moves poleward, the Sun angle would decrease, reflection
would increase as would the albedo. As a result, what we generally
find in high latitude locations is very cold land (among other
things, the result of high albedo over the snow-covered surfaces)
and relatively cold water (among other things, the result of high
rates of albedo over the water).

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42. The atmosphere is comprised of a number of gases, and several
of them are directly and immediately useful to life on Earth --
oxygen for animals, carbon dioxide for plants being the two that
come quickly to mind. In addition, certain of these gases screen
out harmful rays (most notably those related to skin cancers), and
of course the atmosphere is very important in that it
insulates/holds in terrestrial radiation -- without which the
Earth would experience extreme swings of temperature.
-
- In terms of their presence in the
atmosphere, the most prominent gases are nitrogen, oxygen and
argon -- none of these are directly important in weather. However,
these three gases make up, on average, some 99.9 percent of our
atmospheric gases. Combined, the rest of the gases comprising the
atmosphere make up just 1/10 of one percent of the gases present
-- these include two gases that are meteorologically important:
carbon dioxide and water vapor.
-

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43. The importance of carbon dioxide lies more in the realm of
climatology than meteorology. This gas has the ability to absorb
out-going long wave radiant energy and thus to heat the Earth.
Coming from a variety of sources, most notably volcanic eruptions,
decaying vegetation and the burning of fossil fuels, carbon
dioxide, as a percentage of the atmosphere, has been rising for
several centuries. This gas is most closely associated with those
who attribute its increase to global warming. This is a
controversial theory and is covered at some length in your text.
For now, let us just remember that all notions such as this have
two sides -- and besides, a little global warming might be good
for a lot of people. Try telling a Russian or a Canadian that
global warming is all bad!
-

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44. Water vapor is the colorless, odorless, gaseous form of water.
This gas, largely confined to the lower-most layers of the
atmosphere, has the ability to absorb BOTH incoming, short wave
solar radiation AND outgoing, long wave terrestrial radiation. In
addition, water vapor, unlike most atmospheric gases, changes
states at temperatures found at the Earth's surface. As water
vapor changes state, heat is either absorbed or released by the
water molecule. This heat is the source of energy that drives our
weather and storms. Consider a hurricane. The storm forms over the
tropical oceans. Here it absorbs huge quantities of heat as water
is evaporated into the storm. The hurricane then moves into the
middle latitudes where both moisture and heat are
released.
-
- The amount of water vapor in air varies
widely depending on a number of factors. Generally, warm, tropical
locations contain considerably more water vapor that do colder
polar locations. Isn't this what a clothes dryer is all about?
Clothes dryers do not make use of cold air (such air can't hold
much water). Instead, liberal use is made of warm air to hot air.
Such air, with its increased ability to hold moisture, encourages
evaporation off the wet clothes.
-
- Many of you have no doubt heard the
saying, "It's too cold to snow." This is just a reminder that warm
air can hold more moisture than cold air. And if we have really
cold temperatures, there may be so little moisture in the air that
only very small amounts of snow are to be expected.
-
- Consider Antarctica. You might think
such a snow and ice covered location would experience considerable
annual snowfalls. Such is not the case, and we should all be
grateful. The temperatures in Antarctica are so cold that the air
holds very little moisture, thus there is little to fall out
(maybe a couple of inches across the year). Then where does all
the snow/ice come from? It's been there a long time. Think about
it. If it did snow a lot in Antarctica, and it continued to be as
cold as it now is, we would have very little melting or
evaporation. Thus, in short order, the snow pack would increase;
the thickness of the ice sheets would increase and we would likely
see a return to an Ice Age.
-
-

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45. One other gas we might take note of is ozone. Most of this gas
is concentrated about 12 to 18 miles up. Ozone, whose "hole" is in
the news from time to time, has the ability to absorb certain
parts of the short wave spectrum (ultraviolet). Acting as a
screen, ozone is important in keeping these harmful rays which
have been shown to be a major contributor to certain types of skin
cancers to a minimum. The graphic below depicts the famous "hole"
in the ozone.
-

-
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46. Gases are not the only items found in the atmosphere. Solid
particles such as dust, pollen, spores and the like are found in
abundance, especially in the lower-most levels. Think about the
last time you gave your house a real good cleaning. Vacuuming,
dusting, Spic and Span -- the whole nine yards. Then you walk by
one of the newly cleaned rooms and see Sun streaming in a window
and all kinds of "dust" floating in the air. The moral of this
story -- atmospheric particles are always present, especially in
the lower-most levels of the atmosphere. It is sunlight, being
reflected and scattered off these particles that give rise to the
beautiful orange and red sunrises and sunsets that are so
prominent in the more arid portions of our landmasses and along
our coasts (salt particles).
-
- Particulate matter in the atmosphere is
also important from a weather standpoint. In the condensation
process, it is necessary for particulate matter to be present in
order for there to be something for the water vapor to condense
around. We will discuss this topic at much greater length in a
later unit.
-
- The graphics which follow depict several
of the major sources of particulate matter.

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47. Volcanoes (Mt. St. Helens, USA).

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48. Dust Storms. Kansas (Photo 1) and over Ethiopia and the Red
Sea (Photo 2).

-
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49. Fires. Forest fire in California (Photo 1) and large forest
fires in Florida (Photo 2, see the red color along the east
coast).


-
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50. Pollution. Air pollution over Mexico City (Photo 1). Oil fires
in Kuwait (Photo 2).


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51. The atmosphere is usually divided into four layers on the
basis of temperature. The lower-most layer, the troposphere
(tropo/to turn over or change), will be the focus of our
attention. The troposphere extends upward some 10 to 12 miles over
the Equator and gradually declines in height to some five to six
miles over the poles. As one ascends into the troposphere, the
temperature drops on average approximately 3.5 degrees F per 1000
feet. This temperature change is fairly consistent and is often
called the environmental lapse rate, or sometimes the normal lapse
rate. And it follows that temperatures will rise by 3.5 degrees F
per 1000 feet as one descends toward the surface.
-
- It is in the troposphere, and especially
in the first 3000 to 5000 feet of the troposphere (the so-called
Boundary Layer), that we find virtually all of our weather. This
is where most of the water vapor is found as well as most of the
particulate matter. Here are the clouds, the fronts and storms.
You will often hear the troposphere referred to as the "Weather
Sphere."
-
- Above the troposphere, from 10 miles to
about 30 miles above the Earth's surface, lies the stratosphere.
Here temperatures begin to rise due to the presence of ozone.
Little weather is to be found in the stratosphere as this layer
contains very little moisture. While volcanic eruptions may throw
particulate matter into this layer, little particulate matter is
typically found at these altitudes. Because of the lack of clouds
and storms, this is the preferred area for air travel. Think about
the last time you flew. Looking out the window, most of the clouds
(except for a few powerful thunderstorms) were below
you.
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- The layers above the stratosphere (the
mesosphere at 30 to 50 miles and the thermosphere at 50 to about
300 miles) are not meteorologically significant.
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-

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You have now completed Unit 2:
Latitude. You might wish to check your knowledge of the
material presented in this section by working through the Multiple
Choice, and True-False Quiz Questions as well as the essay-style
Review Questions available through The
Course dropdown located in the header of this page.
To return to the top of the
page.
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