Overview
The atmosphere is primarily composed of nitrogen and oxygen.
In dry air, these gases comprise about 78% and 21% of the atmosphere,
respectively, by volume, leaving about 1% for all other gases,
including argon, carbon dioxide, and ozone. However, the atmosphere
is not completely dry; it typically contains 0 to 4% water
vapor, concentrated near the earth's surface. It is this small
amount of water vapor that greatly affects the weather.
Water
vapor is simply water (H2O) in gaseous form. It readily exists
in Earth's atmosphere at temperatures cooler than the boiling
point of water, even at temperatures below freezing. Water
vapor is removed from air primarily by condensation and added
to air primarily by evaporation and transpiration.
Atmospheric
water vapor has many important effects on the weather and
climate of Earth. It is the most important "greenhouse
gas" in the atmosphere, trapping enough warmth for life
to exist. Its presence is essential for clouds and precipitation
to form and, hence, it is a vital component of Earth's water
cycle. And its phase changes (from gas to liquid or vice
versa, for example) produce significant changes in temperature,
either adding energy to or removing energy from the atmosphere. |
Evaporation
and Condensation
Evaporation is the process by which a liquid is transformed
into a gas. Conversely, condensation is the physical process
by which a gas becomes a liquid. Energy is exchanged during
both of these processes. The energy that is released or
absorbed by water vapor during these phase changes is called
latent heat.
When liquid water evaporates, energy is required
to separate the molecular bonds which hold the water molecules
close together in liquid form. This energy is removed from
the nearby environment, whether that be the air or an object
onto which the liquid water is attached. For example, when
you step out of the shower into a drier environment, your
skin suddenly feels cooler. This physical sensation is a
result of the evaporation of the water on your skin into
the air. Heat is taken from your body to change the water
from liquid to gas. Hence, evaporation is a cooling process.
When
water vapor condenses into liquid water, energy is released
to the environment. In the atmosphere, this energy causes
an increase in the air temperature in the region where condensation
occurs. Hence, condensation is a warming process. As a result,
the air becomes more buoyant, because warmer air is less
dense than cooler air. The heating of the air has a significant
impact on thunderstorm development and enhancement, adding
to the thunderstorm's updraft.
On a molecular
level, water molecules on the surface of liquid water (e.g.,
the ocean, a puddle, a raindrop in the air or on a leaf)
that have enough energy will escape from the surface of the
liquid and become a gas (that is, they will evaporate). Water
molecules in the air that do not have enough energy to continue
being part of the gaseous air will condense onto the surface
of the liquid water. This exchange of molecules between gas
and liquid occurs regularly at the temperatures and pressures
common to Earth's atmosphere.
If
more water molecules leave the liquid surface than join with
it, the air is said to be unsaturated. If every molecule
that evaporates from the liquid surface is accompanied by
another molecule that condenses on the liquid, the air is
said to be saturated. Hence, when air is saturated, there
is a balance between the number of water molecules condensing
and the number evaporating. |
Water Vapor
Content and Air Temperature
The amount of water vapor that can be maintained in the air
at saturation changes depending on the air temperature. Because
energy is required to allow water molecules to escape liquid
form to become a gas, the cooler the air temperature, the
lesser the energy available for evaporation to occur. Conversely,
the warmer the air, the more the energy available for evaporation
and, hence, the more water molecules able to evaporate into
the air.
This point is vital to understanding atmospheric
water vapor. The warmer the air, the larger the number of
water vapor molecules can be mixed into the air before saturation
occurs. The cooler the air, the smaller the number of water
vapor molecules can be mixed into the air before saturation
occurs. |
Density
of Moist Air versus Dry Air
If asked, few people would conclude that moist air is less
dense than dry air. Their reasoning would be that if you
add water vapor to dry air, there will be more molecules
in the volume and, thus, more mass in the volume.
However,
to make appropriate comparisons, the pressure in the volume
cannot change. So the addition of water vapor means the removal
of some other molecule. Because air is primarily composed
of nitrogen and oxygen, these are the two most likely molecules
to be removed. Molecular nitrogen (N2) has a molecular weight
of 28 and molecular oxygen (O2) has a molecular weight of
32. Water vapor, on the other hand, has a molecular weight
of 18, substantially less than either nitrogen or oxygen.
So
when water vapor takes the place of nitrogen or oxygen in
a given volume of air, the mass of the entire volume decreases.
Because density is defined as the mass per unit volume, the
density of the given volume also decreases. Thus moist air
is less dense than dry air (at the same pressure). |
Dew Point,
or Dewpoint Temperature
Because of water vapor's
importance, no weather forecaster would make a prediction
without taking into account the amount of atmospheric water
vapor. Hence, its measurement is as vital as that of air
temperature and winds. One variable that is used as a measure
of atmospheric water vapor is dew point, which is also called
dewpoint temperature. The dew point is the temperature to
which air must be cooled for saturation to occur.
Dew point is expressed in units of temperature (e.g.,
Celsius or Fahrenheit). In weather forecasting, a dewpoint
temperature near the ground of 55°F and higher is considered
adequate for thunderstorms to form. Dew points of 65°F
and higher are desirable for thunderstorm development, and
those of 75°F and higher are considered high dew points.
When the dewpoint temperature is lower than about 40°F,
the air is considered reasonably dry. However, clouds can form
at any dewpoint temperature as long as the air temperature
decreases to equal the dew point.
To understand dewpoint temperature
better, take a glass of ice water outside on a hot day. If
beads of water condense on the outside of the glass, then the
air next to the glass was cooled to its dew point, allowing
the water vapor in the air to condense onto the outside of
the glass.
Dewpoint temperature is an excellent measure of the
actual amount of water vapor in the air. The higher the dew
point, the more water vapor there is in the air. Conversely,
the lower the dew point, the less water vapor there is in
the air. For this reason, meteorologists use dew point as
the primary variable to describe atmospheric moisture content. |
Relative Humidity
Relative
humidity is the ratio of the amount of water vapor actually
in the air compared to the maximum amount that can be mixed
in air at that particular temperature. Hence, when the temperature
changes, so does the relative humidity, even without changing
the amount of water vapor in the air.
Relative humidity is expressed
as a percentage. If the relative humidity were 0% (unrealistic
near Earth's surface), there would be no water vapor in the
air. When the relative humidity is 100%, the air is saturated
and the air temperature and dewpoint temperature are equal.
In this case, a decrease in the air temperature (and, consequently,
a decrease in dew point) would result in water vapor condensing
into cloud droplets or dew.
Because relative humidity is dependent
on air temperature so strongly, it is not a good measure of
the actual amount of water vapor in the air. However, relative
humidity is a good indicator of the potential for evaporation
to occur. When the relative humidity is high, little evaporation
occurs. When the relative humidity is low, evaporation likely
will occur, especially with moderate to strong winds and warm
temperatures. |
Dew Point
versus Relative Humidity
Many people have difficulty in understanding the relationship
between dew point and relative humidity. An analogy is appropriate
to describe the differences (Master
#2A).
Imagine that you
have two glasses, each of which can contain 1 cup of liquid.
You fill one full of water and the other you fill halfway
with water. How can you describe the amount of water in each
of the glasses? One way is to say that one glass contains
1 cup of water and the other contains 1/2 cup of water. Another
way that is just as valid is to say that one glass is 100%
full of water and the other is 50% full of water.
The first description of
the amount of water in the glass is analogous to water
vapor measurement using dew point. The second description
is analogous to the measurement using relative humidity.
The importance of the difference
is found when you find a second set of glasses. These two
new glasses will contain 2 cups of liquid. Again, fill
one full of water and the other fill halfway with water.
The full glass now contains 2 cups of water and the other
glass contains 1 cup. Or, similarly, one is 100% full and
the other is 50% full.
Now
compare the half full glasses. They are both 50% full,
but they do not contain the same amount of water. One contains
1/2 cup of water and the second contains 1 cup. Analogously,
two regions of air may have the same relative humidity
but vastly different amounts of water vapor. Because warm
air can sustain more water vapor mixed within it than cold
air can, the warm air is analogous to the larger set of
glasses; cooler air is analogous to the smaller set of
glasses. Thus, meteorologists typically examine the dewpoint
temperature rather than the relative humidity when making
their forecasts. |
Sources of
Atmospheric Water Vapor
Weather forecasters try to
predict the changes in the water vapor content of the air
in order to enhance their forecasts of clouds, precipitation,
and high and low temperatures. To predict these changes,
forecasters are aware of the sources of moisture in and around
their forecast area.
Most near-surface
moisture results from evaporation over warm ocean waters.
The moist air is blown over land by near-surface winds. Indeed,
the moisture in the humid South and Southern Great Plains
of the U.S. predominantly is blown by the wind, or is advected,
from the Gulf of Mexico. Smaller sources of evaporation include
lakes, rivers, irrigated fields, and wet soil. Although solar
heating and warm temperatures enhance evaporation, the single
most important aid to evaporation is strong winds. Sometimes
strong winds also stir drier air from above toward the surface,
lowering the moisture content of the air even while evaporation
is adding moisture.
Plants have an important role in Earth's
water cycle. They absorb water from the soil through their
roots in order to remain healthy. As a source for atmospheric
moisture, plants transpirate, exchanging carbon dioxide in
the atmosphere for oxygen. The rate of transpiration increases
as a result of photosynthesis as plants become greener and
healthier and as their water source is replenished with precipitation. |
Diurnal Cycle
of Dewpoint Temperature
During a quiescent warm day (e.g., autumn high pressure system),
it is possible to observe a few characteristic changes in
the dew point over a 24-hour period. After sunset, the air
temperature decreases steadily throughout the night. If the
air temperature cools to the dew point, dew will form on
objects near the ground. The formation of dew will remove
water vapor from the air; hence, the dewpoint temperature
will decrease throughout the night after dew begins to form.
At
sunrise, dew on the ground quickly evaporates, adding moisture
to the air and increasing the dewpoint temperature. As the
day gets warmer, the winds increase, creating a competing
effect of increasing and decreasing moisture. First, warm
temperatures and windy conditions increase evaporation at
the surface. This effect tends to increase surface moisture.
However, strong winds also mix the air from the ground upward
into the atmosphere and move air from above toward the ground.
Because the air aloft typically is drier than that near the
surface, this effect tends to decrease surface moisture.
As the sun begins to set,
the winds calm and the dewpoint temperature may rise as
a result of continued evaporation and transpiration. |
| |
|
 |
|
| Condensation |
| · Water vapor
-> liquid
water |
| · Energy released |
| · A warming process |
| |
| Evaporation |
| · Liquid water
-> water vapor |
| · Energy required |
| · A cooling process |
| |
| Fun Facts |
| Ninety-seven percent of
Earth's water resides in the oceans. Only 0.0012% of Earth's water
is in the atmosphere. |
| A molecule of water will
stay in Earth's atmosphere for an average duration of 10-12 days.
In contrast, a molecule of water will remain in the ocean for an
average of 3600 years. |
| According to the U.S.
Geological Survey, less than 1% of Oklahoma's water budget is consumed
by humans in irrigation, industry, and household use. |
| |
| Funding
for this publication was provided by the Environmental Sciences
Division of teh U.S. Department of Energy (through Battelle
PNL Contract 144800-A-Q1 to the Cooperative Institute for
mesoscale Meteorological Studies) as a part of the Atmospheric
Radiation Measurement Program. |
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