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The fundamental concept for understanding why it rains and why it doesn't rain is this:

Rising Air Rains and Dropping Air Dries

rising air

Here is some background to understanding this fundamental concept:

  • Warm air rises (e.g., out of your oven into your face), Cool air drops (sinks)(e.g., out of your freezer onto your feet).
  • Rising air expands and cools (adiabatic cooling), Dropping air compresses and heats (adiabatic heating).
  • Warm air has more capacity to hold water; Cool air has less capacity to hold water.

Here are some necessary definitions:

  • Specific (Absolute) Humidity: 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.relative humidity
  • Relative Humidity (RH): 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 100%.
  • 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)

In summary:

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 of rain.

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.

So let's put it all together -- What causes air to rise and drop?

Air drops on the downwind side of mountain ranges (rainshadow effect) and in major air circulation patterns called Hadley Cells (see Why is Tucson a Desert?).

Air rises by three major ways (see Why it Rains).

Why is it often hot in deserts?

With 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.