Physical Setting
Climate
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Physical Setting
Climate
Adaptations
Tucson Plants
Tucson Animals
External Resources
Climate | Tucson's Climate | Tucson's 5 Seasons | Annual Change | Statistics | Concepts | Why it Rains |

METEOROLOGICAL (CLIMATE) CONCEPTS

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

Rising Air Rains and Dropping Air Dries

Here is some background to understanding this fundamental concept:

Here are some necessary definitions:

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.


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