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Lab 06: Atmospheric
- Netoff, Weather and Climate Lab
Manual, Chapter 6
- Class text as applicable
- 01. In this lab we will concern ourselves with the presence of
moisture in the atmosphere, the various means by which moisture is
removed from the atmosphere and the forms the moisture takes.
Meteorologists are very interested in the amount of moisture in
the air, not only because the presence of water vapor is an
indicator of the potential for precipitation, but the energy
released as water vapor changes states provides the energy for
- Moisture, or humidity, is a general term we will use to
describe the amount of water vapor in the air. There are a number
of ways to express the amount of humidity in the air -- the most
common being absolute humidity, relative humidity and mixing
ratio. We will take up each of these shortly.
02. Let's begin our look at moisture by considering the ideas of
saturation and vapor pressure. In the graphic below, suppose we
have a sealed container that is half full of water. The
temperature of both the water and the totally dry air (the air
contains no water vapor) above the water is 70 degrees F. Assume
that if we were to insert a barometer into the air overlying the
water we would get a pressure reading of 29.92 inches of
03. In the graphic below the process of evaporation begins with
the movement of one molecule of water vapor from the liquid water
into the dry air above. As this first molecule makes its way into
the air, we should expect to see an increase in the air pressure
being registered on the barometer -- after all we have now added
one additional gas (water vapor) molecule to the air above the
liquid water. Now granted, in order to note the pressure increase
of one molecule, we would have to be dealing with a very sensitive
barometer -- but the point is, one additional molecule into the
air would result in some increase in the air pressure within the
sealed container. This portion of the total air pressure that is
accounted for by the presence of water vapor (in this case just
one molecule) is called vapor pressure.
- As the process of condensation continues and the presence of
water molecules grows greater, we will see an increase in vapor
pressure within the sealed container.
04. In our final graphic in this sequence, we have reached the
point where the air can not hold any additional water molecules.
At this point, the air is said to be saturated -- it is at
capacity. Any increase in vapor pressure (the result of any
additional water molecules being moved into the air over the
water) will cause a corresponding number of water vapor molecules
to move from the air to (because the air temperature is above 32
degrees F) water.
05. Air is said to be saturated when a balance is achieved between
those water vapor molecules leaving the water and those being
returned to the water. The air can hold no more water vapor -- it
is filled to capacity.
- You will note that the capacity (ability) of the air to hold
water vapor increases sharply with increases in temperature. In
the graphic below, note that air at 30 degrees F is saturated when
3.3 grams of water vapor are present in a kilogram of otherwise
dry air. Were you to heat the air to 60 degrees F, note that the
capacity of the air increases to 10.7 g/kg (an increase of 7.4
grams). But when we increase the air temperature another 30
degrees F (to 90 degrees F), note that the capacity of the air
increases 19.3 grams.
- As you increase air temperature, the ability of the air to
hold water vapor increases at an increasing rate.
- And finally keep in mind that most of the moisture present in
the atmosphere is found within the first few thousand feet. This
is where the moisture is; this is where our weather is.
06. Take a little time to study the graphic below taken from your
Lab Manual (Table 1). I, of course, do not expect you to memorize
the information presented here, but note what happens to the
ability of the air to hold moisture as temperature is increased.
Look at the capacity of the air at 90 degrees F (a not uncommon
afternoon summer temperature in southeast Texas) -- 30.052 g/kg.
Compare this to the capacity of air with a temperature of 50
degrees F (a fairly typical winter day temperature) -- 7.389 g/kg.
The air in summer holds over 4 times the water vapor of a typical
winter day. Can you begin to see why our summer storms tend to be
more vigorous (more condensation taking place, thus more energy
being released) and result in larger quantities of precipitation
(not only more water vapor in the air, but the summer storm clouds
may tower 30,000 to 40,000 feet compared to winter storm clouds of
maybe a few thousand feet).
- Too, how many of you have ever heard the saying: "It's too
cold to snow." Well it's probably rarely too cold to snow, but a
glance at the Capacity of Saturated Air table below will give you
a good idea of where this saying may have come from. At
temperatures below 0 degrees F, note how little water vapor the
air is able to hold if saturated. And after all, if it's not in
the air, it's not going to be available to fall out.
Let's take a look at the various ways the moisture content of the
air may be expressed.
Absolute Humidity. Keep
in mind that meteorologists are interested in following the water
vapor -- for the precipitation potential, and as the energy source
for our storms. Absolute humidity does not have a lot of
usefulness in the realm of the meteorologist. As you can see in
the graphic below, as air moves about in the atmosphere, its
volume changes dramatically. What is one cubic foot at the
surface, may expand to become tens of cubic feet aloft.
- In the example, a cubic foot at the surface contains 6 grams
of water vapor. Moved aloft, the air expands to 3 cubic feet with
each cubic foot containing 2 grams of water vapor. If you are
trying to keep up with the water vapor (say in a neat one cubic
foot package), you are going to have a major headache as the
moving air expands and compresses with changes in height.
08. Mixing Ratio.
Meteorologists make extensive use of the mixing ratio. Here
atmospheric moisture is measured by comparing the weight of water
vapor in the air to the weight of a unit of dry air. The mixing
ratio is usually measured in grams of water vapor per kilogram of
dry air. As depicted in the graphic to the right, the advantage
for the meteorologist is that the measurement is not concerned
with a volume of air (that changes dramatically with changes in
altitude), but rather with a weight that remains the same
regardless of the volume encompassed. If on a warm summer day a
parcel of air is heated and rises and spreads out, the basic
"container" remains constant -- a kilogram of air is a kilogram of
air regardless of whether it takes up one cubic foot at the
surface or 1000 cubic feet at 15,000 feet. Thus, it is relatively
easy to follow the water vapor that is the focus of the
09. The graphic below depicts a sling psychrometer. This is an
instrument used to measure the amount of water vapor in the air.
You will note that the instrument is comprised of two
thermometers. On the right is the "dry bulb" thermometer. As the
instrument is slung around in a circle, the dry bulb thermometer
measures the temperature of the air. The "wet bulb" thermometer on
the left is covered with a cotton "sock." Before use, the sock is
dipped in water. As the instrument is being slung, water is
evaporated off the sock. The amount of evaporation (a cooling
process) is dependent upon the humidity of the air. If the
humidity is low, a great deal of water can be evaporated into the
dry air and the result will be a relatively low wet bulb reading.
If the humidity is relatively high, then it is more difficult to
evaporate much water off the sock and a relatively higher wet bulb
reading will be obtained. The temperatures of the two thermometers
are read and, with the use of a chart (see Table 3 in your Lab
Manual), the mixing ratio can be calculated.
10. Relative Humidity.
Probably the best known measure of water vapor in the air is
relative humidity. This measurement is commonly used on evening
weather reports, and the term is frequently used by the general
public. And while the concept of relative humidity has its place
in our everyday life, and most of us can surely relate to the
term, it is not the best measurement of the actual amount of water
vapor in the air. Again, it is the amount of water vapor in the
air that is important to the meteorologist.
- Relative humidity can be defined as the amount of water vapor
in air at a given temperature compared to what that air could hold
at that temperature. For instance, in the example, air at 60
degrees F can hold (has a capacity/will be saturated when it
holds) 10.7 g/kg. On a 60 degree F day, if the air holds only 5.35
g/kg of water vapor, the relative humidity of the air will be 50
percent. In other words the air is holding half of what it could
11. We can change the amount of water vapor in a parcel of air by
one of two means. In the graphic on the left we have two parcels
of similar size. Both have a temperature of 60 degrees F and, if
we were to consult the Capacity of Saturated Air table (see
earlier graphic, or Table 1 in your Lab Manual), we would see that
both have a capacity of 10.699 g/kg. Neither is saturated. The
parcel on the left, lying over a land surface, has a mixing ratio
of 5.35 g/kg. Since the mixing ratio of this parcel is half of
what it could hold, it has a relative humidity of 50 percent. The
parcel on the right, lying over water, has, due to increased
evaporation, a mixing ratio of 7.133 g/kg. The relative humidity
of this air parcel is 66 percent. In this instance we have
increased the relative humidity of the air mass by increasing the
amount of water vapor in the air.
- But adding (or removing) water vapor is not the only way we
can alter the relative humidity of an air mass. Note the graphic
on the right. Here we have two air parcels of similar size. The
parcel on the left has a temperature of 60 degrees F. By
consulting the Capacity of Saturated Air table we find that this
air has a capacity of 10.699 g/kg. Because the mixing ratio of
this parcel is 5.35 g/kg, the relative of this air is 50 percent.
Notice what happens when we reduce the temperature of such an air
parcel just 10 degrees F. The capacity is decreased from 10.699
g/kg to 7.389 g/kg (see the Capacity of Saturated Air table).
While there has been no change in the mixing ratio (changing the
temperature does not change the amount of water vapor in the air),
the reduced capacity means that the mixing ratio of 5.35 g/kg is
now a larger part of the capacity (now only 7.389 g/kg) and as a
result the relative humidity rises to 72 percent.
- We can thus change the relative humidity of a air parcel by
either increasing or
decreasing the amount of water vapor in the parcel (this changes
the mixing ratio of the air mass)
or by raising or lowering the
temperature of the air mass (this changes the capacity of the air
mass). Because we are dealing with a ratio, any change in the
relationship of the amount of water vapor (the mixing ratio) in
the air to the capacity (the ability of the air to hold water
vapor) of the air will change the relative humidity of the air
12. One must be careful when interpreting relative humidity. As
you can see in the graphic below, if we had two parcels of air,
both saturated, one with a temperature of 30 degrees F and the
other of 60 degrees F, the one whose temperature was 60 degrees F
would have over three times the water vapor in it. Again, this is
because warm air holds more water vapor than colder air, and
because relative humidity is a ratio between capacity (variable
with temperature) and mixing ratio.
- As an example of the kinds of problems relative humidity can
create for the casual observer, let's consider the climates of
western Europe and northern Africa. Most would associate the
climate of western Europe with high humidities, clouds, drizzle,
green landscapes and the like. Northern Africa (the Sahara Desert)
is generally characterized by extreme heat and dryness.
13. Let's assume we have an air mass over western Europe with the
characteristics indicated on the graphic on the left. As you will
note, at 60 degrees F the capacity of the air is 10.699 g/kg and
the mixing ratio of the air mass is 10.699 g/kg. The air is
saturated and the relative humidity is 100 percent. We are right
at the point of condensation which, should it occur, would result
in rain, drizzle and/or fog since the temperature is above 32
degrees F. Note especially that the air mass contains 10.699
- Now, look at the graphic on the right. Here we have an air
temperature of 115 degrees F -- almost twice the temperature of
the air mass covering western Europe. Because of the higher
temperature, the capacity of this air parcel is almost six times
greater than that its European counterpart. However, upon
measurement we find that the mixing ratio of this air parcel is
only 13.370 g/kg -- resulting in a relative humidity of 20
percent. Now let's see -- almost twice the temperature, six times
the capacity, but only 20 percent relative humidity compared to
100 percent relative humidity for the parcel overlying western
Europe. But note that the actual amount of water vapor in the air
over north Africa is greater by almost a third when compared to
that of western Europe. How is possible that a place with a 100
percent relative humidity reading can have less water vapor in the
air than a place with a relative humidity?
- It all comes back to the fact that with relative humidity you
are dealing with relationships between capacity that varies with
changes in temperature or mixing ratios.
14. Dew Point. As we have
noted a number of times, if you are going to have condensation
within an air mass, you are going to have to have moisture present
and the temperature of the air mass will have to be falling. The
temperature to which the air mass will have to be cooled is called
the dew point. The capacity of air (see the Capacity of Saturated
Air table) at 30 degrees F is precisely 3.368 g/kg; at 60 degrees
F the capacity rises to 10.699 g/kg and at 90 degrees F the
capacity is 30.052 g/kg. What these figures are saying is that if
you have a parcel of air whose temperature is 60 degrees F, that
air can hold (the most the mixing ratio can be is) a maximum
10.699 g/kg of water vapor.
- If any more water vapor is added, or if the temperature is
reduced below 60 degrees F, a change of state will occur. If you
add any more water vapor, you will exceed the capacity of the air
at that temperature to hold water vapor, thus for every additional
water vapor molecule added, a molecule of liquid water will be
- On the other hand, if you drop the temperature below 60
degrees F, the saturated air will be unable to retain all of the
water vapor at the new lower temperature (its capacity will be
exceeded) and a portion of the water vapor will be condensed.
- This temperature where the change of state occurs is called
the dew point. For air containing 3.368 g/kg of water vapor, the
dew point is 30 degrees F; for air containing 10.699 g/kg of water
vapor, the dew point is 60 degrees F; and for air containing
30.052 g/kg of water vapor, the dew point is 90 degrees F.
- If you think about this for a moment, you can see that dew
point is an excellent indicator of the actual amount of water
vapor (the mixing ratio) in the air. Air with a high dew point
will have a great deal of water vapor present; air with a low dew
point will not have a lot of water vapor present. Too, you can see
that the dew point temperature is directly tied to the amount of
water vapor present. If you reduce the amount of water vapor
present in the air, you reduce the dew point temperature (the
point at which the air is saturated); if you add water vapor to
the air, you increase the dew point temperature.
- In the second graphic, we have four parcels of air of varying
temperatures. Each air parcel is saturated (each has a 100 percent
relative humidity reading). As the temperature of a parcel is
reduced, its capacity is reduced. Too, you will note that since
the capacity is less, the mixing ratio is also represented by a
smaller number. Where is the water vapor going? While not
indicated on the graphic (we will look at this idea in the
following graphics), note that, should what is being depicted
actually be occurring over a specific geographic area, water vapor
would be forced from the air as the capacity of the air is reduced
and rain would be occurring).
- Since the dew point is the temperature to which the air must
be cooled in order for condensation to occur (saturation), as you
remove water vapor from the air the mixing ratio changes as does
the dew point temperature.
15. The following series of graphics is provided to demonstrate
the relationships that exist between air temperature, capacity,
mixing ratio, relative humidity and dew point. For purposes of
example, let us assume that we are in a sealed room (no air in, no
air out and no air conditioner or heater in operation -- and no
- We pull out a thermometer and take a reading of the room's air
temperature and find it to be 70 degrees F. Now, where to find the
capacity of air with a temperature of 70 degrees F? Right -- the
Capacity of Saturated Air table. We consult it and find that at 70
degrees F air has a capacity of (it will be saturated when it
contains) 15.260 g/kg. We take out our trusty sling psychrometer,
wet the sock down, give it a few slings, take the readings and
upon consulting the sling table find that the air in the room is
not at capacity -- it only contains (has a mixing ratio of) 10.699
g/kg. OK, it can hold 15.260 g/kg, but it actually holds 10.699
g/kg. It appears just from eyeballing the figures that the
relative humidity -- what's there (10.699 g/kg) compared to what
could be there at 70 degrees F (15.260 g/kg) -- is about two
thirds. When we actually calculate it (10.699 divided by 15.260),
we find the relative humidity to be 70 percent.
- Now as to the dew point. We know we have 10.699 g/kg of water
vapor present. To what temperature will we have to drop the air in
order for what is actually present (the mixing ratio -- the 10.699
g/kg) to represent saturation? Well, we're once again going to
need the Capacity of Saturated Air table. When we run down the
table, we find that air with a water vapor content of 10.699 g/kg
will be saturated when the temperature reaches 60 degrees F. Then
the dew point of the air in this room is 60 degrees F.
16. Now, what happens if we turn the room thermostat up to 80
degrees F? Looking at your Capacity of Saturated Air table, you
can see that as air temperatures rise, there is an increase in the
ability of the air to hold moisture. At 80 degrees F the capacity
of air increases to where it can hold 21.537 g/kg of water vapor.
We have raised the air temperature, but in doing so we have not
changed the actual amount of water vapor in the air, thus the
mixing ratio must be the same as previously -- 10.699 g/kg.
- However, the relationship between the air's capacity and the
mixing ratio has been changed. This will have to cause changes in
the relative humidity in the room. As you can see, the mixing
ratio is now approximately half of the capacity. Increasing the
air temperature has reduced the relative humidity. When we
actually calculate the relative humidity, we find it to be 50
percent. Just as you should expect. If you have to drop the air
temperature in order to have saturation/condensation, then an
increase in temperature should be moving you away from saturation
and any form of condensation.
- And what about the dew point? Well, if we are going to
saturate this air parcel (and we don't alter the mixing ratio),
aren't we still going to have to drop the temperature of this air
mass to 60 degrees F? Then the dew point must still be 60 degrees
F. The dew point is tied to the amount of water vapor in the air.
If you don't change the water vapor content of the air, you don't
change the dew point.
17. We now drop the thermostat to 60 degrees F. Consulting the
Capacity of Saturated Air table we find that the capacity of air
at 60 degrees F is only 10.699 g/kg -- the same as the room's
mixing ratio. If what's there (the mixing ratio) is the same as
what could be there (the capacity), then the air must be
saturated, the relative humidity must be 100 percent and we must
be at dew point (60 degrees F). Now there is no thunderstorm
taking place in the back of the room, and there is no fog nor
condensation on the walls. The air is just saturated. That means
the air can't hold one more molecule of water vapor. If any water
vapor were to be added at this point (thereby placing more
moisture into the air than it was able to hold), or if we were to
drop the temperature any lower (thereby reducing the capacity of
the air to hold the 10.699 g/kg), we would have a change of state
(gas to liquid at this 60 degree F temperature). But right now the
air is clear -- no clouds, fog or the like -- just saturated.
18. We now drop the thermostat to 50 degrees F and all kind of
things begin to happen. Checking the Capacity of Saturated Air
table, we find that air at 50 degrees F has a capacity of only
- Well, if that's all the water vapor the air can hold -- then
that's all it does hold. The mixing ratio drops to 7.389 g/kg.
What happened to the excess moisture (the 3.310 g/kg)? It "fell
out" of the air. Since the air temperature is above freezing, we
have condensation, and most likely condensation has formed on room
surfaces. We want to come back to this point in a moment, but for
now let's complete the table.
- Since we can assume a steady drop of temperature in the room
as a result of the thermostat change, can you see that as the
temperature hit 59 degrees F we condensed a little bit of
moisture, a little more at 58 degrees F, and so on until we
reached 50 degrees F at which point we had lost a total of 3.310
g/kg? As the capacity was steadily reduced, the mixing ratio was
constantly changing downward to meet the downward trending
capacity. The relative humidity remained constant at 100 percent
since between 60 degrees F and 50 degrees F the capacity and the
mixing ratio remained the same. And the dew point? Well, as we
continued to lose water vapor (the mixing ratio) the dew point
also followed a steadily downward trend so that we now find the
dew point and the room temperature to be the same.
- How many of you have ever asked yourself why rain sometimes
comes down in buckets and other times we only get a slow, soaking
drizzle? This line on the table ought to answer the question. We
can see that dropping the temperature from 60 degrees F to 50
degrees F caused 3.310 g/kg of water to condense. If this
temperature drop took five minutes to occur (like we might well
have with a warm/high capacity summer day thunderstorm), then the
3.310 g/kg will condense out in five minutes. On the other hand if
it took three days for the same temperature drop to occur (as you
might get with a slow moving warm front), then it will take three
days for the 3.310 g/kg to be condensed. The rate of precipitation
is largely related to the capacity of the air, the rapidity with
which the temperatures are dropped and the dew points hit.
19. Too cold for us, so let's run the thermostat back up to 80
degrees F. Consulting the Capacity of Saturated Air table we find
that the capacity of the air in the room has risen to 21.537 g/kg.
Again, since changing the temperature does not impact the actual
amount of water vapor in the room (and assuming here no
evaporation off the room's walls, floors, etc.), the mixing ratio
in the room must be where we left it on the last graphic -- 7.389
g/kg). Eyeballing the capacity and mixing ratio we can see that
the relative humidity has dropped to about 33 percent (actually to
34 percent) -- just as you might expect if you radically increase
the temperature since you are moving away from
condensation/precipitation. And the dew point? Well, look at the
Capacity of Saturated Air table and find at what temperature would
air with a mixing ratio of 7.389 g/kg become saturated. It looks
like 50 degrees F.
20. Now let's throw a little wrinkle into the mix. Say someone
opens the door and kicks in a tub of water. Evaporation begins to
the point of adding (for purposes of example) 4.958 g/kg of water
vapor to the air (sort of like an air mass moving over a large
body of water, right?). What kind of impact will this have on our
21. We may have added water vapor to the air, but since there has
been no temperature change, the capacity of the air remains the
same at 21.537 g/kg. However, we must add the water vapor
evaporated from the tub of water to the mixing ratio in the room
prior to the time the tub of water was added. We did have 7.389
g/kg present. By adding 4.958 g/kg from the tub, we have increased
the mixing ratio to 12.347 g/kg. This should increase the relative
humidity (what's there compared to what could be there) -- and it
does. Our relative humidity jumps to 57 percent.
- Since we have changed the mixing ratio, we have changed the
dew point. Consulting the Capacity of Saturated Air table, we find
that in order to saturate the air presently in the room we would
have to drop the temperature in the room to 64 degrees F -- not to
50 degrees F as was the situation before we added the moisture.
And doesn't this make perfect sense -- if you add moisture,
shouldn't it be easier to get condensation (by having to only drop
the temperature to 64 degrees F instead of the lower temperature
of 50 degrees F)?
22. Ok, a fast review. We have a kilogram of air whose temperature
is 60 degrees F. How much water vapor can air at this temperature
hold at saturation -- what is the air's capacity? We get out the
Capacity of Saturated Air table, run the table to 60 degrees F and
find that such air, when saturated, will hold 10.699 g/kg. The
capacity of the air at 60 degrees F is 10.699 g/kg.
23. The air may hold 10.699 g/kg, but it feels fairly comfortable
-- it just doesn't seem like it is actually holding 10.699 g/kg at
the moment. Getting out our sling psychrometer and related tables,
we find that the air is not saturated. It only holds 5.35 g/kg.
The mixing ratio of the air is only 5.35 g/kg.
24. Well, if the air could hold (the capacity) 10.699 g/kg, but in
fact it only contains (the mixing ratio) 5.35 g/kg, then the air
is not saturated and the relative humidity must be something less
than 100 percent. It looks like the relative humidity (and upon
calculation we find that yes it is ...) 50 percent. The air is
only holding half of what it could hold at 60 degrees F, thus the
relative humidity of the air is 50 percent.
25. And the dew point? Well, by consulting the Capacity of
Saturated Air table, we find that air that holds 5.35 g/kg of
water vapor will be saturated when the temperature is reduced to a
temperature between 41 and 42 degrees F. So if we want to condense
moisture out of this air mass, we will have to drop the
temperature to the 41/42 degree F area. That failing, we could
increase the water vapor content (the mixing ratio) which would
have the effect of raising the dew point to a temperature closer
to 60 degrees F.
26. When condensation occurs and water vapor is converted into a
liquid, the results include dew, fog and the precursor to rain,
snow, sleet and hail -- clouds. For condensation to occur, two
conditions must first be met.
27. In the first condition, the air containing the water vapor
must be saturated. As we have indicated earlier this can be
brought about in either of two ways: (1) the air can be cooled
below the dew point. In this instance, the ability of the air to
hold moisture is exceeded and a change of state occurs. By far,
this is the most typical means by which condensation is brought
about. But there is a second way condensation occurs. Condensation
may also result when sufficient moisture is added to an air parcel
to exceed the air's ability to hold the water vapor. Again, in
such instances, a change of state occurs.
- But there is a second condition that must also be met in order
for condensation to occur, and that is there must be a surface
present upon which the water vapor can condense. Such surfaces are
called condensation nuclei and may include dust particles, spores
or even such objects on the ground as leaves, grass and the like.
As a rule, such particles are in abundance in the lower levels of
- A special kind of nuclei, one that actually attracts water
vapor unto itself, is salt. Such water-seeking nuclei are called
hygroscopic nuclei. When present in abundance, condensation may
occur when relative humidities are in the 80 to 90 percent
28. Dew. One of the most
common types of condensation is dew. In many respects dew is like
the "sweat" that forms on that cold drink can or glass of iced tea
you enjoy so much on a hot summer day. We all know that the
"sweat" is not oozing out of the can or glass. Where does it come
from? On the typical warm summer day the air contains a great deal
of water vapor. As this water vapor comes in contact with the cold
beverage container, the cold container, via conduction, chills the
air immediately adjacent to itself thus dropping the air
temperature below the dew point. The excess moisture is condensed
out of the air onto the container.
- Dew is formed in much the same way. Dew is most likely to
occur on clear, calm nights. Such conditions encourage rapid
cooling of the surface and the ground becomes much cooler than the
overlying air. Conduction then cools the air to slightly below its
dew point (which in order for dew to form must be above 32 degrees
F) and the water vapor condenses on the nearest available surface
which may be grass, leaves, the hood of your car or whatever. Dew
does not "fall" (as does rain, snow and the like), but rather it
- Dew can sometimes be very heavy -- say after a very warm,
humid day (high mixing ratio and dew point), then followed by a
relatively cool night (temperatures falling below the dew point on
such a day may well produce large amounts of liquid since the very
warm air is almost saturated to begin with).
- At other times, the quantity of dew is small and soon
evaporates as the morning temperatures begin to rise. These
conditions would be most likely when the mixing ratio is
relatively low (thus a low dew point). Night-time cooling may
cause the temperatures to fall below the dew point, but the air,
because it holds much less moisture at low temperatures, has less
to give up as dew once the dew point is reached.
- Let's see if we can take the following East Texas summer-time
- When the dew is on the grass
- Rain will never come to pass.
- In the summer, one typically awakes (assuming you are up and
moving before 10:00AM or so) to clear skies -- the result of high
pressure (Continentality) over the land. Such conditions have
encouraged night-time cooling and the formation of dew. Typically
as the morning wears on, heating of the land results in the
formation of cumulus clouds. These conditions intensify into the
late afternoon hours. And while afternoon thunderstorms are often
prevalent in the area in the late afternoon, your chance of
receiving rain on any given summer afternoon is about 20 percent
(how many times have you heard the weatherman say that during the
summer months?). So clear skies in the morning often means not
enough time before dark arrives for widespread thunderstorms to
develop to the point of generating rain for you beyond about a 20
- Compare this to the second stanza of the ditty.
- When grass is dry at morning light
- Look for rain before the night.
- A couple of things come immediately to mind that could make
the grass dry at morning light (air temperature didn't get to the
dew point, not enough moisture in the air -- a relationship
here?). Consider the following. You get up in the morning and the
sky is cloudy. The clouds have prevented significant cooling of
the land during the night -- no dew. Too, can you see in this
instance you have about a two to three hour head start (compared
to most summer days) on cloud formation? Could it be that by the
late afternoon we will have, not isolated thunderstorms, but
instead widespread thunderstorms (thus a better chance for you to
get rained on)?
29. Fog. Like dew, fog
is also a form of condensation. We can think of fog as a low cloud
-- in that the two are almost identical in terms of their
appearance. Clouds are usually defined as being above 50 feet, fog
less than 50 feet above the surface. While they may look alike, in
fact the two are formed very differently. Clouds, as we will see
in the next section, are typically formed as a result of adiabatic
cooling brought about by cooling of the air -- normally through
uplift. Fog are typically formed through either radiational
cooling of the surface which in turn then cools the air above, or
by the movement of relatively warm air over a cooler surface. In
both instances, the air is dropped to its dew point.
30. Advection Fog. Close to
home here in southeast Texas, the widespread fogs of fall along
the coast are generally advection fogs. Here, wind brings
relatively warm and humid air inland from the Gulf. As the air
passes over the cooler land surface, the air temperature is
reduced to the dew point and fog forms.
- You will often hear someone say that a fog "burns off." This
statement implies that the Sun heats the fog and evaporates it.
How many of you have ever heard someone say that a fog is
"lifting?" This statement implies that a fog is evaporated from
the bottom upward. Which is correct -- how does fog dissipate?
Well, from our discussion to this point, we know that if cooling
the air causes fog, then heating the air must cause it to
- Let's consider the "burn off" scenario first. The Sun striking
the fog, heating the air with the resulting evaporation and
dissipation of the fog. Sounds good, but think about that for a
moment. What's going to heat the air? Won't the albedo of the air
(the fog) be high? Not likely that a great deal of heat will be
absorbed in this manner. Fogs do "burn off," but the tendency is
to "burn off" from the ground -- up. In other words, we are going
to have to heat the land, which in turn will heat the air above
it, which will raise the capacity of the air to hold water -- the
result being evaporation and dissipation of fog. Fogs do "burn
off" or "lift," but they tend to do it from the ground up and
around the edges first. Of the two, to say a fog "lifts" would
probably be closer to describing the process.
31. Radiation Fog. A second
type of fog, and one that is widespread in southeast Texas, is
radiation (or ground) fog (Photo 1 below). We have all seen
examples of radiation fog. Most typical on crisp fall days,
radiation fog often looks like a low smoke covering area pastures.
Radiation fogs are commonly associated with inversions where the
cold surface cools the overlying air to the dew point. Because air
is a relatively poor conductor of heat, most radiation fogs tend
to be relatively shallow events. While some such fogs may extend
10 or even 20 feet (maybe to 50 or 100 feet with a light, stirring
wind) off the ground, a more typical radiation fog is maybe
chest-high. Even seen a pasture where only the heads of cows were
visible? Such fogs, especially when thick, make excellent
playgrounds for little kids. Stand up -- now I see you. Crouch
down -- where did they go?
- Because the cold air associated with ground fog is
heavy/dense, some of the more striking radiation fog events are to
be seen when these fogs move downslope to lower elevations. Photos
4 and 5 below depict a ground fog in the process of moving from
its place of formation in the uplands into a lower valley. Such
events, if you can catch them as they begin, almost have the
appearance of a "waterfall" of fog.
32. Valley Fog. Valley fogs
represent a ground fog variant. As the name implies, these fogs
are associated with valleys and are frequently caused by either
cold air or ground fog draining into a lower elevation.
33. Steam or Evaporation Fog.
Some "valley" fogs are caused by a completely different process.
In valleys we often find creeks, rivers, lakes, ponds and the
like. In the fall, the temperature of these water bodies tends to
be warmer than the surrounding land. As the colder surrounding air
drains down into these low-lying areas, the temperature of the
moist air above the water is dropped to its dew point and fog
frequently forms. Such fogs are more properly termed "steam" or
"evaporation" fogs. This is the type of fog you may have seen
overlying a swimming pool in the fall, or maybe a small pond in a
- A fall Sunday morning a number of years ago when my son was
maybe about four or five years old, I was relaxing reading the
paper when he screams out, "Daddy, the man's on fire!" I jumped up
out of my chair and rushed to the front door and looked out across
the street to one of the local schoolyards where a bunch of boys
were playing basketball. And sure enough not one, but several, of
the boys were "on fire." Steam/smoke was rising off them in such
quantities as to make a youngster think they were "on fire." In
fact, it was an evaporation fog -- caused by the cold morning air
condensing the evaporating moisture from the player's skin.
34. Upslope Fog. And finally,
there is the upslope fog. As the name implies, this fog is formed
as a body of air moves upslope (and cools). Imagine a parcel of
air moving in off the Gulf. As it moves from Galveston to Dallas
to Oklahoma City toward Denver, the land is gradually increasing
in elevation. At some point the expanding air will cool to its dew
point and as so often-times happens, a fog (a cloud if you will)
just appears out of nowhere.
35. As we have noted, fogs are formed as a result of the
surface cooling the air immediately above to the dew point. The
resultant moisture is condensed as microscopic water droplets --
so tiny and light that they are literally suspended in mid-air,
unable to fall to the ground. While some fogs may be very thick
and extend upward for many tens of feet, keep in mind that because
air is such a poor conductor of heat that it is not possible for
this cooling process (via conduction) to form clouds which are
generally defined as being more than 50 feet above the
- Because of the poor conducting qualities of air, something
else is at work in the formation of clouds -- that something else
is adiabatic temperature change.
36. We earlier discussed the normal/average or environmental lapse
rate (ELR). The normal lapse rate, which averages 3.5 degrees F
per 1000 feet, is experienced when one moves through static
(unmoving air). If you were to ascend from the surface in a
balloon, you would find that the temperature would drop about 3.5
degrees F for every 1000 feet of ascent. By the same token, as you
move toward the surface through static air, the temperature will
increase at the same 3.5 degree F per 1000 foot rate. Again, you
are moving through static/still air.
- When air moves about it's a little different. Unlike you, a
volume of air is expanded (as it ascends) and compresses (as it
descends) as it moves about in the atmosphere. As air expands or
is compressed, it is either warmed (as it descends and is
compressed) or cools (as it rises and expands). In this adiabatic
process, heat is neither added nor removed -- the temperature
change is solely the result of the air molecules being compressed
closer together (molecular motion is increased) or, in the event
of expansion, the molecules are further apart (molecular motion is
decreased) resulting in a cooling of the air.
- On average, so long as there is no condensation (which adds
latent heat to the atmosphere) occurs, this temperature change is
5.5 degrees F per 1000 feet of ascent or descent. This temperature
change is known as the dry adiabatic lapse rate (or the DAR). This
graphic on the right depicts this temperature change -- rising air
cools at the DAR, descending air warms at the DAR.
37. Of course we know that if air rises far enough, it will cool
sufficiently to reach its dew point. Once this occurs, a change of
state is to be expected. In the graphic below, we have a parcel of
air being forced to rise against the side of a mountain. You will
note that as the air moves up the slope to 3000 feet, the
temperature of the air drops at the DAR (5.5 degrees F per 1000
feet). At the 3000 foot level the dew point is reached. As the
temperature continues to drop (assuming the air continues to
rise), condensation/rain occurs and latent heat is released into
the atmosphere. The air continues to rise and cool but, because of
the latent heat being added to the air (the result of heat
released in the condensation process), at a reduced rate -- the
wet adiabatic lapse rate (the WAR). The WAR which only occurs when
air is ascending/cooling is, on average, 3.2 degrees F per 1000
38. Clouds, which can be defined as: "a dense concentration of
suspended water droplets or tiny ice crystals," are formed via
adiabatic cooling -- not conduction. These very prominent, and
often spectacular, atmospheric features are excellent indicators
of what's coming weather-wise. While there are many, many
different cloud types, most clouds can be categorized within one
of four broad groupings. These groupings, summarized by the
graphic on the right, are generally based on a cloud's height and
appearance. Following are a number of graphics meant to convey a
broad overview of each cloud category. If need be, return to Lab
04 for a more detailed look at cloud types. Keep in mind that,
except in the case of the vertical cloud grouping, the lower (and
therefore the warmer/greater the moisture content) the cloud, the
greater the chance for precipitation. Vertical clouds are noted
for producing large quantities of precipitation simply because
their great vertical extent permits them to hold so much
- To view a variety of cloud photographs categorized by type, go
to our cloud
39. Cirrus Clouds. Cirrus
clouds are high clouds (above 18,000 feet). The air at these
altitudes is very cold, hence these clouds tend to be composed of
ice crystals. Too, because the air is so cold, these clouds
typically hold little in the way of moisture. They are typically
always white in color -- the result of sunlight easily passing
through these thin clouds -- not as is so often believed because
of the ice contained within the clouds. Cirrus clouds are often
associated with approaching fronts.
40. Cumulus Clouds. Cumulus
clouds are typically composed of individual cloud masses and are
characteristically white to light gray in color. They are
typically the result of the surface heating, and are very common
during the warmer times of the day and year in southeast Texas.
They often indicate good weather and are frequently referred to as
fair weather cumulus.
41. Cumulonimbus Clouds.
Cumulus clouds can evolve into the larger, more violent,
cumulonimbus clouds. These clouds, which typically generate
thunderstorms, are a towering (vertical), denser and more massive
version of their cumulus cousins. From the ground, these clouds
often take on a very dark gray/almost black color (the result of
the fact that most sunlight is not visible through these clouds to
the eye). Seen from a distance they are not as thick (you are
looking across the cloud and not up through its vertical extent),
thus they often appear whitish is color. The familiar flat top
(anvil head) is the result of these clouds reaching the top of the
troposphere and their tops being swept off by upper level winds.
These clouds, because of the great amount of moisture (energy)
contained within them, often generate heavy rains, lightening,
hail, high winds and some times even tornadoes.
42. Stratus Clouds. Stratus
clouds tend to occur in sheets or layers. They typically have no
distinct cloud units. While usually varying shades of gray in
color, stratus clouds look menacing, but typically, if
precipitation falls at all, it will tend to be light in nature.
These clouds are most characteristic in winter along the Texas
Rain. Following are a
series of graphics designed to illustrate the conditions that
create the most common precipitation types. The graphic below
depicts the formation of rain. As often happens in the middle
latitudes, rain at the surface often begins as snow aloft. In our
example snow exits a cloud whose temperature is below 32 degrees
F. On the way to the surface, the freezing line is encountered,
the snow melts and a cold rain continues to the surface. On the
right we have a depiction of rain as it frequently occurs in the
lower latitudes. Liquid rain falls uninterrupted to the surface
from a cloud whose temperature is above freezing.
44. Snow. The following
graphic depicts the formation of snow. Snow falls from a cloud
whose temperature is below 32 degrees F. For this snow to reach
the surface, two conditions must be met. One, it must not
encounter air temperatures above 32 degrees F, and (2) the surface
temperature must be below approximately 40 degrees F. If the
surface temperature is much above 40 degrees F, the air
immediately overlying the surface is likely to be above 32 degrees
F and the snow will either melt and form rain, or it will
evaporate before it reaches the surface.
45. Sleet. Sleet can form in
one of two ways. On the left we see snow exiting a cloud whose
temperature is below 32 degrees F. On the way to the surface, the
freezing line is encountered and the snow melts forming rain. As
the rain continues toward the surface, it falls through a layer of
sub-freezing temperatures (maybe 3000 to 4000 feet thick) and the
raindrops are frozen to form sleet. On the right we have rain
falling from a cloud whose temperature is above 32 degrees F. As
it falls toward the surface it encounters the same sub-freezing
layer (again, likely to be 3000 to 4000 feet thick) and the
raindrops are frozen.
46. Freezing Rain. Freezing
rain, or glaze, forms in conditions essentially identical to those
that created sleet. The primary difference is that the
sub-freezing layer near the surface is not as thick as that
required to form sleet (maybe a couple of thousand feet or less).
In such instances, the temperature of the raindrops falls below 32
degrees F (it becomes super-cooled water), but not so far as to
freeze into a solid mass. Such precipitation falling to the ground
will freeze upon contact with the surface (the droplets are
agitated). One of the characteristics of super-cooled water (the
name given to liquid water whose temperature is below 32 degrees
F) is that it will freeze (turn to a solid) if it hits something
or something hits it (it is agitated).
The resulting ice storms can be both beautiful and devastating --
note the graphics that follow.
47. Hail. And finally we have
hail. Ranging in size from small pellets the size of a pencil
eraser upward to stones the size of large grapefruit, hail,
falling to the ground at speeds reaching 100 miles per hour or
greater, can severely damage or destroy crops, buildings and
livestock (Photo 1 below). Photo 2 is a cross-section of the
record hailstone that fell in Coffeyville, Kansas in 1970. Photos
3 and 4 below will give you some idea of the power to do damage
that a hailstone possesses as it comes to the surface.
48. Hail is generated from clouds composed of a combination of ice
crystals and super-cooled water. As the ice crystals move about in
the clouds by winds, they encounter super-cooled water droplets
that, upon contact, are agitated and freeze -- providing an
additional coating of ice on the original ice crystal. In the
graphic on the left, in addition to the presence of both
super-cooled water and ice, here must be a considerable amount of
turbulence/wind in the cloud.
49. This process is continued back and forth across the
super-cooled water line with each trip seeing an enlargement of
the hailstone. The ultimate size of the stone being largely
determined by the amount of super-cooled water present and the
number of trips across the super-cooled water line.
50. The longer the hail is kept aloft by winds within the cloud,
the larger the stones will grow until they are finally thrown from
the cloud. Depending upon the winds, hail may either be dropped
beneath the cloud, or it may be thrown out the side of the cloud.
-- and the distances thrown can be substantial. There are reports
by pilots of planes being struck by hail at distances of over a
mile from the generating cloud.
The Various States of Water
- 51. And finally, before we leave the topic of moisture, let's
take a look at the various states of water and the processes that
take place as state changes occur. Like all gases in the
atmosphere, water vapor can change states (gas/liquid/solid). But
unlike many gases found in the atmosphere, water vapor changes
state at temperatures found at the Earth's surface. As the change
of state occurs, heat is either released or absorbed depending
upon the change taking place.
- Consider the example of ice cubes in a glass of tea. If we
fill a glass with ice cube, then add tea and wait 10 to 15
minutes, we might find upon measurement the temperature of the tea
to be 37 degrees F. Now, if we take the glass and place it over a
flame, we will see the ice cubes begin to melt. Somewhat
surprisingly the temperature of the liquid remains at 37 degrees F
until all of the ice has been melted. The temperature of the
liquid was 37 degrees F before we placed the glass over the flame;
it remained 37 degrees F after all of the ice had been melted.
Where did all of that heat from the flames go?
- Let's take a look at the various states water moves through
and whether heat is being absorbed or released. Again, this heat
is the source of the energy that drives our storms.
In the melting process, the solid ice is changed to liquid
water. In this process, heat is absorbed from the atmosphere by
the water molecule. For purposes of example, it takes
approximately 80 calories of heat to produce one gram of water.
This heat is often referred to as the latent heat of melting. The
heat is retained in the water molecule to be released when the
water is returned to the solid state.
53. Freezing. Freezing is the
reverse of melting. Here liquid water is converted into ice. In
this process heat is released into the atmosphere. How much heat?
The same 80 calories absorbed during the melting process. This
heat is referred to as the latent heat of fusion.
54. Evaporation. In the
process of evaporation, approximately 600 calories are required to
convert one gram of water to water vapor. This heat energy is
absorbed by the water vapor molecule and will be available to be
released as heat when the water vapor is returned to the liquid or
solid state. The 600 calories required is called the latent heat
55. There are a number of factors that can aid in the process of
evaporation. Some of the more important are touched on below.
- Air Temperature. You
will remember that warm air has a greater capacity to hold
water vapor than does cold air.
- Degree of Saturation.
Evaporation is also encouraged by the degree to which the air
is saturated. Drier air can hold more water vapor than can more
- Temperature of the
Water. Evaporation is greater over warm water than
over cold water. This is largely due to the fact that the
molecules are more active in warm water and are thus more prone
to break the surface tension of the water. When compared to
warm water, the molecules comprising cold water move slower and
have a reduced tendency to move from the liquid to gaseous
- Wind. And finally we
might note that evaporation is greater on windy days than on
calm days. Think about hanging a wet shirt out on a
clothes-line to dry. Very quickly the more active water
molecules move to the surrounding air as a thin layer of water
vapor surrounding the wet shirt. Now if there is little or no
wind, the air around the shirt is quickly moved to the point of
near-saturation -- it becomes more difficult for additional
water molecules to move from the wet shirt to the surrounding
air. Thus drying the shirt becomes an all day affair. On the
other hand, if there is a wind, the wind removes the
near-saturated layer surrounding the shirt and makes way for
additional water molecules to make their move from the liquid
to the gaseous state.
56. Condensation. In the
process of condensation, water vapor is converted to liquid water.
This process, which releases 600 calories of heat (the latent heat
of condensation), is an important source of energy for our storms.
Though less important than the movement of warm ocean water into
higher latitudes, this process does assist in the transport of
excess heat from the tropical regions to higher latitudes.
While melting, freezing, evaporation and condensation are well
known to most, there are two other change of state processes
whereby water vapor or its derivative makes a state change.
Sublimation. In one of
these, sublimation, ice is converted directly to a gas without
going through the liquid state. Probably the best known example of
this process is dry ice. Another example would be where snow is
evaporated (solid to a gas) without passing through the liquid
state. In the process of sublimation, 680 calories of heat energy
are absorbed by the gas molecule (80 calories coming from the
latent heat of fusion and 600 from latent heat of
58. Deposition. The opposite
of sublimation is deposition. In this process water is moved
directly from gas to the solid state (ice), again without passing
through the liquid state. Snow and frost are probably the best
known examples of deposition. In the process of deposition, 680
calories of heat energy are released into the atmosphere -- 600
calories from the latent heat of condensation, 80 calories from
the latent heat of fusion.
59. The graphic below summarizes these various processes. Note
that melting, evaporation and sublimation result in heat energy
being taken from the environment. Condensation, freezing and
deposition result in heat energy being released into the
- You have now completed the related reading for
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