Concentration
How humid is air?
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| Distribution of water
vapor |
The amount of water vapor that air can
contain is strongly dependent on the temperature.
The warmer the air is, the more water
vapor it can contain without it condensing into droplets.
When air contains the maximal amount of water
vapor, we call it saturated. The relative humidity is then 100%.
In and around clouds, for instance, the air is saturated. When there is more than 100% relative humidity in the air, the water
condenses to become cloud droplets. This is what we see when we look at a cloud: numerous
small water droplets that form because the air is saturated
with water vapor.
Because of the temperature dependence of the maximal relative humidity, air in the tropics can take up much more water
vapor than air at the poles. Almost everywhere in the troposphere, temperature decreases with
altitude, and therefore the largest possible concentration of water
vapor is higher near the Earth's surface than higher
up in the troposphere.
Water vapor is dynamic. Rather
than staying in one place, it moves around with wind and clouds, goes over mountains and through valleys, rises and cools, and drops and warms.
When air is uplifted from its original height, it cools, and thus
can contain less water vapor. As
it rises, the air parcel's decreasing maximal relative humidity is getting nearer and nearer to the actual value of relative humidity, and so it can happen that water
vapor condenses and clouds are formed.
When this process is turned around, and the temperature in an air parcel rises as the parcel sinks, clouds can evaporate again.
This effect also becomes clear in our weather system.
In high pressure areas, the air sinks.
This process makes the air less humid, and possible clouds disappear. In low pressure areas, the air rises and clouds tend to
form (frequently causing rain).
 Source: ESPERE
Tropical forests = LOW PRESSURE - Deserts = HIGH PRESSURE
These processes are commonly observed in the tropics and over the deserts.
Over the tropics, the humid air rises,
condenses in the colder higher altitudes (in this picture: blue = cold, red = warm) and forms
large, gray rain clouds. Typically they
partially rain out in the tropics, with
the air then transported in the direction of the deserts
higher in the atmosphere and finally
sinking towards the surface. The clouds evaporate on the way down
as the air warms, thus eliminating the
clouds and creating arid
conditions.
The water vapor in our atmosphere
primarily comes from ocean evaporation into the
atmosphere, with a smaller contribution
from land and soil evapotranspiration.
When water vapor condenses to make
clouds, and the cloud droplets become big enough to rain
out of the clouds, it lands either back in the
oceans or flows from the land via rivers and ground water back into the ocean.
This is the water cycle.
The amount of water vapor in the air can be given in different units. Very commonly used
are specific humidity units, i.e. grams
of water vapor per kilogram of air.
This number doesn't change with temperature, unless of course the water
vapor condenses out of the air, thereby
decreasing the mass of the water
vapor. For example, the maximal specific humidity at the poles and at high altitudes in the tropics is much less than 1 g/kg.
In the tropics at lower altitudes, however, it can amount
to more than 30 g/kg.
Source: ESPERE
Another measure exists for the overall amount of water
vapor over a single point at the Earth´s surface.
It is the water level which remains if all steam in the air would rain out.
This amount can vary from a few millimeter at the poles to over 400 mm in the tropics.
This means that if we would force all
of the water vapor up through the
atmosphere above a bathtub down from the atmosphere into the bath, the water would reach a level of four
centimeters.
In addition, at each point a relative humidity between 0% and 100% can be indicated.
This number, however, is dependent on temperature and
is different for locations within
even a few kilometers of each other,
and drastically different between
the poles and the tropics.
Thus, although it has its
usefulness, other measures are
typically used to describe amounts
of water vapor in the air.
All numbers we mentioned before show how much humidity varies over the troposphere. This variation is mainly determined by dynamic processes,
e.g. the rise and fall of air parcels, and has implications for the weather and
for the longer term climate on Earth.
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Summary:
The water
vapor content of air is highly variable and depends strongly on air temperature. Cold air
is very dry (polar air,
for example) and cannot
hold much moisture,
whereas warm air can
hold a much larger
amount (as in tropical
air). In warm
regions, however, the relative humidity
can vary between 0% and 100% (deserts
vs. rain forest). Because of the strong variation depending on location and altitude, and the influence of water on the climate, it is very hard to forecast the climate in a reliable way.
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Text: Susanne Nawrath - Jülich Research Centre
Pictures: Elmar
Uherek - MPI for Chemistry Mainz
Translation: Heleen de Coninck - MPI for Chemistry Mainz
Review and
Edit: Stephen Gawtry - University of
Virginia
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