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- The textbook for this course is entitled: eWeather and Climate by Netoff, Gillespie, Fujimoto-Strait and Tiller. Upon purchase of the Lab Manual for the Course GEO 1401/Lab, you will be provided a link to a downloadable pdf which will serve as the textbook for this course GEOG 1401/Lecture. The Lab Manual/cd text is only available for purchase online at: bearkatsonline.com.
- Note: This
unit contains a great many graphics and photographs. The
load time may
vary depending upon your
computer and web connection.
- Click the
button located on the left
page margin opposite selected graphics for additional information.
the message box when you are done.
01. For the first exam we are taking a look at the two
primary climatic controls responsible for heat: Latitude and
Continentality. In the previous section on Latitude, we discussed
Sun angle and time as factors in the receipt of solar energy by
the Earth. In addition, we took a look at the concepts of
radiation, conduction and convection.
- In this unit we will take up the second climatic control
related to heating of the Earth's surface -- Continentality.
02. As you will remember from the previous unit, the Earth's
surface significantly influences the air temperature. Thus, if the
Earth's land surfaces do not heat up and cool off at the same rate
as water surfaces, one could expect such a difference to have a
very substantial impact on air temperature. And in fact land and
water do not heat up or cool off at the same rate. This notion of
differential heating and cooling is called Continentality and this
concept will be the topic for discussion in this section. A couple
of examples to illustrate the point follow.
- Suppose it is August 15 and you and I are driving across a
large shopping center parking lot. I suddenly open the door and
throw you out in your bare feet. The high Sun (radiation), long
days and minimal albedo have made the parking lot surface an
inferno. You begin jumping and hopping around (conduction works
fast!). Spying a puddle of water, you head for it to soak your
scortching feet. I don't get it! The Sun is beating down on both
the parking lot and water puddle with equal intensity, yet you
find the water to be cooler (I do understand the fact that the
water is soaking up less heat than the parking lot and that there
is some cooling related to evaporation, but still
at work here?
- Or how about a short trip to Lake Livingston east of
Huntsville. The date is June 1. The temperature as we leave
Huntsville is 90 degrees F (why is it warm?). As we cross the
bridge over the lake, you decide to go for a swim. You dive off
the bridge (illegal, of course), hit the water and are immediately
yelling for rescue and a blanket. The land may be warm, but not
the water. Again, what's at work here?
- Now, while the parking lot situation might not have much
effect on the weather, such could not be said of something as
large as Lake Livingston. If in June the lake water is
substantially cooler than 90 degrees F, then the air over the lake
will be cooler as well. And since many people do not like heat, we
find that lakes are especially popular places for summer homes.
The homeowner may be interested in water-related activities, but
more likely than not they just want to get away from the heat.
What's at work here? Why do water and land, both receiving the
same amount of heat energy, not respond in the same way with
respect to heat?
- Continentality is what is going on here. Let's take a look at
the five factors the cause this differential heating and cooling
of land and water.
03. Specific Heat. One reason
land and water do not heat up and cool off at the same rate has to
do with the nature of the substances themselves. They have
different specific heat values. Specific heat has to do with how a
substance responds to the input/output of heat. We could define
specific heat as the heat required to raise the temperature of 1
gram of a substance 1 degree Celsius (C). From the table below we
can see that the specific heat benchmark (pure water) has a
specific heat value of 1.0. What this means is that 1 calorie of
heat applied to 1 gram of pure water will raise the temperature of
that water 1 degree C.
- Now take a look at the substance concrete. The specific heat
of concrete is .20. That means that if you apply .20 calories of
heat to 1 gram of concrete you will raise the temperature of that
gram of concrete 1 degree C. Or put another way, if you were to
apply 1 calorie of heat to the 1 gram of concrete, you would raise
the temperature of the gram of concrete 5 degrees C! Or put still
another way, concrete will heat up (and cool off) five times
faster than pure water.
- You can see from the chart that various substances have
different specific heat values. For purposes of example, let us
say that dirt (just general dirt) has a specific heat of .33. Now
of course different kinds of dirt in reality have different
specific heat values. But we just want to establish a value we can
make some generalizations with. Assuming general dirt to have a
specific heat value of .33, what we are saying is that dirt heats
up and cools off three times faster than water.
- One reason then that Lake Livingston is cooler in June than
the surrounding land is that the specific heat value of water
(dirty water in this case, not pure) is greater than that of the
adjacent land, thus the land (and as a result the air above the
land) is going to respond more rapidly than the lake water to the
rising Sun angle and longer periods of daylight associated with
late spring and early summer days.
04. In addition to specific heat, we need to consider the opacity
of land and water.
- Land is
Opaque. You can't see (and light
won't shine) through very much dirt. Thus, sunlight shining on
land has a difficult time penetrating very far. What heat energy
is received at the surface is then transferred slowly downward via
the process of conduction. And the process of conduction is not
all that efficient. Think about how far you would have to dig a
hole into the ground on August 15th before the ground would begin
to feel cool to the touch. What -- maybe a foot? What this means
is that all of the heat energy being put to the Earth's surface by
the Sun is being concentrated in the top foot or so of the Earth.
And remember, in order to raise the air temperature to 90 degrees
F, we first have to raise the temperature of the surface of that
one foot of the land surface to 90 degrees F. If all of the Sun's
heat energy is being concentrated in the upper-most foot of the
surface, can you see that it will not take very long to raise the
temperature of the Earth to 90 degrees F. And once this is
accomplished, the surface can then begin to raise the temperature
of the air above it to 90 degrees F.
- Water is
Translucent. Water on the other
hand is translucent. It's not opaque like land, nor is it totally
transparent. And of course some water is more translucent that
others. The clear waters of the Texas Lake-country rivers is a far
cry from the muddy Mississippi (or a small farm pond). Note on the
graphic below, that sunlight can penetrate much deeper (maybe 10
or even 20 or more feet) than was the situation with land. Let's
say for this discussion that light penetrates a water body 10
feet. This alone means that compared to the surrounding land, a
pond will take 10 times longer to heat to a similar temperature.
And remember, in the real world, we also have to include the
specific heat values of the two substances. So can we see that to
raise the temperature of a 10 foot deep water body might well take
30 times the heat energy that it would take to heat land to the
- Water is
Mobile. And one other aside
regarding water. Unless you are in Los Angeles or San Francisco,
the land surface does not move. Such is not the case with water.
Water is highly mobile -- constantly mixing upward and downward.
So while heat may penetrate 10 feet into a water body, in fact one
might well end up having to heat a much greater depth due to the
mobility of the water.
Finally there is evaporation. Evaporation (a cooling process) is
greater over water than over land. Because energy is needed for
the evaporation process, the energy used is not available for
- The graphic below summarizes the five
factors influencing the differential heating of land and
06. Consider the graphics below of the Northern and Southern
Hemispheres. The Northern Hemisphere is comprised of approximately
61 percent water vs 39 percent land, whereas the Southern
Hemisphere is approximately 81 percent water and 19 percent
- Now, which of these hemispheres is most
likely to have the greater annual temperature range? Yes, the
Northern Hemisphere -- because it has more land area. As a result,
summers will be warmer and winters cooler in the Northern
Hemisphere when compared to those of the Southern
07. Below is a climate map of the world from your textbook. Note
that, except for Antarctica, there are no cold Continental or
Polar climates in the Southern Hemisphere. This is no doubt due in
part to the greater presence of water (and its
temperature-moderating influence) in the Southern
- But there is another
Continentality-related factor at work here as well that
contributes to the more moderate temperatures found on Southern
Hemisphere landmasses. You will also observe that as you go
poleward in the Northern Hemisphere the landmasses become larger,
but as you move poleward in the Southern Hemisphere, the
landmasses are for the most part actually narrowing. What we have
then is not only more water in the Southern Hemisphere moderating
the temperatures, but we also find as we move poleward that,
instead of encountering what we would expect to be increasingly
colder climates, we in fact find that the narrowing landmasses and
increase in water is actually moderating the temperatures found at
08. If you were to ask the average person on the street what time
of day is the coldest, most would probably indicate the period
just before sunrise. This might seem strange since we know that
the Sun is actually most distant from us (180 degrees around on
the other side of the Earth) at 12:00AM or midnight. Their answer
is, of course, most likely based on their personal experience. And
they are almost right, but not quite. From the graphic below and
our discussion to this point, I suspect you might be able to
provide them a better answer.
- Let's assume that we have an equinox (12
hours of day, 12 of night with the Sun rising at 6:00AM, rising to
its highest point in the sky at 12 noon and setting at 6:00PM) --
see the line labeled insolation on the graphic below. Note the
second line on the graphic labeled temperature. As you can see, it
continues to fall after the Sun has risen. It doesn't begin to
rise until maybe 6:15 or 6:30AM. Why? Keep in mind that before the
air temperature can rise, we have to heat the surface. The Sun may
come up at 6:00AM, but it is going to take some time for the Sun
to warm the land, and the land in turn to warm the air above it.
Remember air temperature is measured at approximately five feet
off the ground, and air is a poor conductor of heat.
- Note the surface continues to heat the
air several hours past the time the Sun reaches it highest point
(and the time of peak insolation) at 12:00 noon. This difference
between the insolation peak and the time of greatest air
temperature (usually between 2 and 4:00PM in the afternoon) is
called the daily lag (or march) of temperature. We are all
familiar with this fact, and this should serve as a reminder to us
that Continentality involves the heating first of the surface, and
then of the air above.
- Finally, we can see from the 2:00 to
4:00PM temperature peak, air temperature falls steadily, past
sunset, past midnight and past sunrise, only to begin rising again
once the rising Sun has heated the land which in turn heats the
- While the above discussion would
describe the typical day, variations can and do occur. Most such
variations are caused by general cloud cover, the passage of
weather disturbances and/or location along coasts subject to
onshore and offshore breezes.
09. The graphic below depicts the seasonal lag of temperature
concept. This is essentially the same idea as just discussed with
respect to the daily lag of temperature. It would seem like it
ought to be coldest in December when the Sun is most distant from
us here in Texas (you remember it is at 23.5 degrees S on December
21). Instead, we typically experience our coldest temperatures in
January or February. And one might think it would be warmest in
June when the Sun is overhead at 23.5 degrees N, yet our warmest
month in Texas tends to be either July or August.
- What we see at work here is
Continentality on a seasonal basis. As with the daily temperature
lag, we cannot begin to heat the air until the surface below it is
heated. Here the lag is measured in weeks rather than hours, but
the concept is the same. Use the accompanying table and compare
the average temperatures for the months of June and December with
the actual high and low average monthly temperatures.
10. Earlier we considered a map that depicted the distribution of
temperature across the United States using isotherms (lines of
equal temperature) and colors to give you a better idea of the
nation's temperature pattern. Take a look at the graphic below.
Here we have an isotherm -- let's call it the 60 degree F isotherm
for the purposes of discussion (all points along the line/isotherm
have a temperature of 60 degrees F). Out over the ocean at 40
degrees N, the air temperature is 60 degrees F. As we trace the
isotherm in off the water onto the land in summer (July) we can
see that we have to go poleward to maintain a 60 degree F
temperature reading. Because land heats up faster than water in
the summer, if we were to continue straight east along the 40th
parallel, we may well find the temperature over the land to be 75
or 80 degrees F or more. In order to maintain a 60 degree F
temperature, we have to swing north (poleward) to areas where the
Sun's rays are less intense.
- Note the reverse situation in January.
At this time of the year, the water is warmer than the land. Thus,
to find warmer temperatures (and to maintain the 60 degree F
isotherm), we have to swing south (Equatorward) over the land
(which is colder at this time of the year than the water due to a
lower Sun ray and shorter days).
11. Finally, let's take a look at several locations on the North
American continent and compare their annual ranges of
temperatures. You can see that Omaha, Nebraska, with an annual
temperature range of 56 degrees F, is located more or less in the
center of the lower 48 states. Compare this with the 70 degree F
annual temperature range of Winnipeg, Canada, a city located in
roughly the center of the North American landmass. While
Winnipeg's climate is more extreme than that of Omaha, both can be
said to have a continental, or land-dominated climate. As you
might suspect, the temperatures of a continental climate would be
characterized by warm to hot summers and cool to cold winters --
the widely varying seasonal temperatures the result of intense
heating and cooling of the adjacent landmasses.
- Now contrast the temperatures of these
two locations with those of Miami (annual temperature range of 14
degrees F) and San Francisco (annual temperature range of 8
degrees F). Both Miami and San Francisco reflect the influence of
the adjacent water by their rather narrow temperature ranges. Both
of these locations can be said to have a marine, or
water-dominated climate. Because of the proximity to water, such
climates will experience cooler summers and warmer winters than
locations at similar latitudes further inland.
12. Taken together then, Continentality causes water to warm more
slowly than land when exposed to high amounts of solar radiation,
to cool more slowly than land and to store more heat than land.
Specific heat, opacity of the land, the translucent nature of
water, water's mobility and evaporation are all factors to be
considered. Too, always keep in mind that the temperature of the
air is directly related to the surface beneath it.
You have now completed Unit 3:
Continentality. You might wish to check your knowledge of the material presented in this section by working through the Short Answer Review Questions, Multiple Choice/True-False Quiz Questions and the Drop-Down Statements available for Exam 1.To
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