concept for understanding why it rains and why it doesn't rain is this:
Air Rains and Dropping Air Dries
is some background to understanding this fundamental concept:
air rises (e.g., out of your oven into your face),
Cool air drops (sinks)(e.g., out of your freezer
onto your feet).
air expands and cools (adiabatic cooling), Dropping
air compresses and heats (adiabatic heating).
air has more capacity to hold water; Cool air has less capacity to hold water.
are some necessary definitions:
the amount of water in the air (e.g., ml of water per liter of air). Starting
with perfectly dry air, as water evaporates into the air, its specific humidity
increases as more water is added to the air. Depending on the temperature
of the air (warm air can hold more water than cold water), and the evaporation
of water into the air, the specific humidity of an air mass will change.
the amount of water in the air as a percentage of the maximum amount
of water that the air at that temperature could hold. For example (see figure
to right), if the air at 70 degrees could hold a maximum of 100 units of water
vapor, but only 50 units of water vapor were actually in the air, then the
RH would be 50%. Now, if you keep the same 50 units of water in the air (specific
humidity held constant), but increase the temperature of the air (by causing
it to drop or descend) so that the air can now
hold a maximum of 150 units of water vapor, then the RH would have decreased
to 33%. Conversely, if you keep the same 50 units of water in the air (specific
humidity held constant again), but decrease the temperature of the air (by
causing it to rise) so that the air can now hold
a maximum of 50 units of water vapor, then the RH would have increased to
- Dew Point:
The temperature at which the air is completely saturated with water (i.e.,
RH = 100%). At this temperature water precipitates (goes from gas to liquid
or solid) and forms clouds (and dew on grass and ice tea glasses) and potentially
rains/snows. In the example above, what is the dew point?(answer).
The dew point is given to indicate the amount of moisture in the air (specific
humidity). The dew point is also used to determine the monsoon season (see
Tucson's Five Seasons)
As air rises,
it expands and cools, decreasing its capacity to hold water (it's effectively
a smaller sponge), increasing the relative humidity, thus increasing the chance
As air drops,
it compresses and heats, increasing its capacity to hold water, decreasing relative
humidity, thus decreasing precipitation and increasing evaporation (its now
more "thirsty" for water), thus potentially causing a desert to form.
put it all together -- What causes air to rise and drop?
on the downwind side of mountain ranges (rainshadow effect) and in major air
circulation patterns called Hadley Cells (see Why
is Tucson a Desert?).
by three major ways (see Why it Rains).
is it often hot in deserts?
little moisture in the air, less of the sun's radiation is absorbed on
the way to Earth and the land (and anything on it) heats up. The flip
side is that the land's heat escapes quickly back to space at night, causing
large temperature swings from day to night. The converse is also true.
Have you noticed that early mornings are warmer when the sky has been
cloudy during the night? Also, this explains why nighttime temperatures
are warmer during the monsoon summer, when the humidity is high in Tucson.