|| Tucson's Climate || Tucson's 5 Seasons || Annual Change / Dramatic || Statistics || Concepts || Why it Rains ||

In this section, we will examine Why Tucson is a Desert and point out that we actually have five seasons, not just one (hot). Visit the annual change section to see how the desert changes through a year and Tucson climate statistics for information on average temperature and precipitation by month. I have included a page of meteorological (climate) concepts and another on why it rains that I recommend as helpful background information.

Why is Tucson a Desert? Definitional Answer.

Because it is arid here. By arid, we mean that the air is almost always "thirsty" for water (robbing the soil and organisms of any moisture they have) and doesn't provide enough water in the form of precipitation (rain, etc.) to ever quench its thirst. Thus, the plants and animals living in deserts must adapt to these conditions of severe water limitation.

A more technical definition for desert is an area where potential evapotranspiration (the potential combined evaporation from plants and soil) is much greater than precipitation (rain, etc.). Potential evapotranspiration (PET) can be estimated by measuring how much water evaporates from a wide pan and multiplying by 60%. A pan in Tucson will evaporate 100" of water, yet Tucson only receives 12" of rain on average (see Tucson climate statistics)! Thus Tucson's PET would be 60". Scientists use an aridity index (PET/Precipitation) to compare the aridity of different places. Tucson's aridity index is 5 (60"/12"), Yuma's is 30, and other deserts may reach 600! Aridity indices above about 3 are sufficient to produce deserts.

Why is Tucson a Desert? Explanation of why we are arid Here.

There are 5 main reasons:

1) We are relatively low in elevation (2548' at the airport). Around Tucson, desertscrub is replaced by desert grassland at about 3500' and oak woodland replaces desert grassland at about 4000' elevation.

2) We are relatively far from sources of moisture (e.g., the Pacific Ocean to the west and the Gulf of Mexico to the southeast). Indeed, it seems much of our summer moisture comes from the Gulf of California which is closer to the south of us.

3) The Pacific Ocean off of California (where much of our air flow emanates) is cold. This means less water will evaporate from it and, because the cold ocean cools the air above it, the air will hold less water (cold air holds less water than warm air). When the air moves over the warmer land, the air heats. As air heats, its capacity to hold water increases, thus the air is less likely to let go of any of its water and it grabs as much water as it can (out of the soil, out of plants, out of skin, etc.)(see Meteorological Concepts).

4) We are in a double rainshadow. Moisture from the Pacific Ocean is blocked from getting here by the north-south mountain ranges of California (e.g., Sierras and San Bernandino Mts). Moisture from the Gulf of Mexico is blocked by the Rockies and Sierra Madre mountain ranges. This is called the Rainshadow Effect. As the moisture-laden air is blown up the mountain range, the air expands and cools. As air cools, its capacity to hold water decreases, thus the water comes out as precipitation. Not only is much of the water wrung out of the air this way, but when the air descends back down the downwind side of the mountain range, the air heats and its capacity to hold water increases, effectively sucking any moisture available from the soil, plants, and animals.

5) Air is descending upon us from the upper atmosphere due to Hadley Cell circulation. Hadley Cells are mass movements of air -- upward at the equator (due to the more direct light striking the equator, creating heat, which then rises), poleward along the top of the atmosphere as the air deflects off the top of the atmosphere (troposphere), then downward at 30 degrees north and south of the equator (Tucson is at 32°N latitude) as the heat in the air escapes to space and the air cools and descends (cool air sinks). When air descends, it heats up. When air heats up, its capacity to hold water increases, thus is less likely to let go of it in the form of precipitation and is more "hungry" for water (increasing potential evapotranspiration).