Weather Station

A Recipe for Storms

by Lindsey Michaels

Thunderstorms are like pots of boiling water. Combine water with heat in exactly the right proportions. Add something to stir the mixture. The result is a thunderstorm.

In Arizona's Salt River valley, getting moisture used to be a real problem for the people who lived here. Today, we know this area as Phoenix and Maricopa County. On average, the entire Phoenix area gets only about seven inches of rain each year.

Hundreds of years ago, native people had to live near rivers if they wanted to survive. As years went by, they learned to design and build canals that carried water from the rivers to areas where they grew crops and built homes.

Canal water also helped new rain storms to form. Water from irrigated fields evaporated quickly into the hot, dry desert air. Eventually, that moisture returned to the fields as rain.

Today, huge concrete dams block many of Arizona's rivers. People use these dams to harness the power of moving water. The water turns giant turbine blades to make electricity. Lakes and reservoirs form behind the dams. The lakes also store water for future use when there is little or no rainfall.

Of course, there is a trade-off. Water is not evenly spread throughout the Phoenix area as it was before the dams were built. Today, Arizona relies on moisture picked up over the Pacific Ocean and Gulf of Mexico to generate rain storms. That moisture is carried to Arizona by strong wind currents.

Heat is the easiest storm ingredient to find. Arizona gets plenty of heat from a blazing sun that shines nearly every day. During the day, rocks, roads, buildings, and parking lots absorb the sun's heat. In fact, sometimes they absorb so much heat that it actually gets too hot to make a storm. Liquid moisture contained in the air changes to steam before winds can whip up a rainstorm. Steam, or water vapor, is water in the form of a gas, and it is invisible. For clouds to form, water must remain in liquid form. Tiny droplets of water small enough to float in the air make clouds and fog appear white.

When water and heat are present in large enough quantities, wind can stir the pot to form a thunderstorm. Hot air rises. Certain structures can make normally gentle air rise fast enough to "stir" the thunder pot. Mountains are one such structure. Rocks and mountains store heat. As a result, desert mountains and the air around them are usually much hotter than non-rocky areas.

Air glides across these non-rocky areas until it bumps into hot mountains. That bump starts the stirring process. Hot mountain air pushes cooler air upward at rapid rates. This is one reason that many Arizona thunderstorms begin in the mountainous Mogollon Rim area northeast of Phoenix.

Local parking lots and small hills can cause smaller rain storms. During such a storm, for example, it might be raining at your house yet be perfectly sunny at a friend's house located just a half a mile away.