The Violent Atmosphere
Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Spring 1998
|humidity||saturation||adiabatic warming and cooling|
|mixing ratio||saturation mixing ratio||condensation nuclei|
|dew-point temperature||relative humidity||cloud|
(1) Humidity is a measure of the amount of water vapor in the air. One way to represent humidity is the mixing ratio, defined as the mass of water vapor "mixed with" each unit mass of air. We usually express mixing ratio as the number of grams of water vapor in each kilogram of air. In the atmosphere, the mixing ratio can vary from nearly zero (in deserts and polar regions and at high altitudes) to as much as 30 grams per kilogram (in warm, moist tropical regions).
Other ways of representing humidity include relative humidity and dew-point temperature (described below).
(It is also possible to express humidity in terms of the number of molecules of water vapor in a sample of air as a percentage of the total number of molecules of all gases in the sample. This percentage can range from nearly zero in deserts and polar areas and at high altitudes, to as much as 4% in warm, moist tropical areas.)
(2) There is an upper limitto the amount of water vapor that can be present in a given mass of air. That is, there is a limit to the capacity of air to "hold" water vapor. When that limit is reached, we say that the air is saturated with water vapor.
One way to represent the maximum amount of water vapor that can be "mixed with" a given amount of air--that is, the maximum capacity of air to "hold" water vapor--is the saturation mixing ratio. We usually express saturation mixing ratio as the number of grams of water vapor that would have to be "mixed with" each kilogram of air to saturate the air.
The maximum capacity of air to hold water vapor depends on the temperature of the air. Warmer air is capable of holding more water vapor than cooler air is.
[Note that this does not mean that warmer air actually has more water vapor in it than cooler air. It just means that warmer air has a greater maximum capacity to hold water vapor than does cooler air.]
(3) Relative humidity (RH) is a measure of how much water vapor is actually in the air relative to (that is, as a percentage of) the air's maximum capacity to hold water vapor. The relative humidity of a sample of air can be expressed as:
| Relative humdidity
|=|| (amount of H2O vapor
actually in an air parcel)
(the parcel's maximum capacity
to hold H2O vapor)
| Relative humdidity
|=|| mixing ratio
saturation mixing ratio
This definition of relative humidity tells us that RH could change in two ways:
Since the maximum capacity of air to hold water vapor changes when the temperature changes, it follows that relative humidity depends both on the amount of water vapor in the air and on the temperature of the air.
(4) If unsaturated air were to cool, its maximum capacity to hold water vapor would decrease but the actual amount of water vapor in the air wouldn't change. However, if the cooling continued, the capacity of the air to hold water vapor would continue decreasing until it equaled the amount of vapor actually in the air, and the air would then (by definition) be saturated. The temperature at which this would happen is called the dew-point temperature.
(Note that at a particular pressure, the dew-point temperature--the temperature at which a parcel of air would become saturated if it were cooled--depends only on how much water vapor is actually in the air. The more water vapor there is in the air, the higher the dew-point temperature is.)
If saturated air cools further--below its dew point--then the air cannot hold as much vapor as it actually contains, and the excess vapor must condense to become liquid water.
However, water vapor requires a solid surface onto which to start condensing. In the atmosphere, microscopic, solid particles of dust, salt, ash, pollen, etc. suspended in the air provide solid surfaces onto which water vapor can start condensing. We call these tiny, solid particles condensation nuclei. Each becomes the center of a tiny droplet of liquid water. The aggregation of many, many of these tiny liquid water droplets suspended in the air constitutes a cloud.
(Note that if these liquid cloud droplets freeze, they become ice crystals, so a cloud may consist of an aggregate of enormous numbers of tiny ice crystals as well as liquid water droplets).
(5) There are two important mechanisms (and several less important ones) by which air can cool below its dew point and thereby cause water vapor to condense in it to form a cloud:
Alternatively, air that moves from one place to another may move across a surface colder than itself, and begin losing heat by conduction into that colder underlying surface. For example, air moving eastward from the Pacific Ocean moves across a strip of water off of the California coast perhaps 50 to 100 miles wide that is considerably colder than the water of the open ocean (due to upwelling along the coast). The air in contact with the colder coastal water promptly loses heat by conduction and may cool below its dew point, causing water vapor to condense in it and producing another surface-hugging cloud that we call advection fog. This is the kind of fog that blankets San Francisco much of the summer, for example.
Rising, unsaturated air that cools adiabatically like this will cool by about 5.5 degrees F for every 1000 ft. that it rises, or about 10 degrees C for every kilometer it rises. Adiabatic cooling in rising air is the primary mechanism by which most clouds form.
The converse of adiabatic cooling is adiabatic warming. When air sinks, the pressure on it increases and compresses it. The energy used by surrounding air to compress a sinking parcel is converted to heat in the parcel, and the parcel's temperature rises (by 5.5 degrees F for every 1000 ft. it sinks, or 10 degrees C for each kilometer it sinks). Of course, air that warms becomes capable of holding more water vapor, so clouds cannot form in sinking air, and any liquid water present in the air (as cloud droplets) would begin to evaporate. Hence, clouds are suppressed in sinking air.
The phenomenon of adiabatic warming and cooling is the reason why meteorologists are so interested in where to expect air to rise or sink. Clouds (and possible precipitation) tend to form in rising air; clouds (and hence precipitation) are suppressed in sinking air.