Arizona Precipitation throughout the Year

Arizona is characterized by a bimodal precipitation regime. This means that the majority of precipitation falls in two distinct seasons, in this case, winter and summer. Winter and summer precipitation amounts are independent; a wet winter is not necessarily followed by a wet summer, and a wet summer is not necessarily followed by a wet winter. This is because the mechanisms responsible for winter and summer precipitation are largely unrelated.

Right: Climograph for the state of Arizona; data are long-term averages (1971-2000). Note the bimodal precipitation regime.

Winter Precipitation in Arizona

Winter precipitation comes from large-scale low pressure systems. These systems traverse the southwest, drawing in moisture from the Pacific Ocean and produce widespread, soaking rains and snowfall. These storms are generally low to moderate in intensity and can last for a day or longer.

Winter storms typically enter North American over Washington or Oregon (dark blue arrows on graphic). The winter storms that do reach Arizona are those which follow a more southerly track, entering North America over California (light blue arrows on graphic).

Summer Precipitation in Arizona

Summer rainfall is the result of very different mechanisms than winter precipitation. Summer precipitation results from convective thunderstorms that arise through the combination of solar heating and moisture. Convective storms are so named because they are mainly the result of convective circulation, air circulation involving the upward and downward transfer of heat. As the layer of atmosphere closest to the earth’s surface heats on warm, sunny days, parcels of warm air rise, transporting heat to greater heights. The warm air displaces cooler, denser air, which then falls to the earth’s surface. Convection can be visualized in a lava lamp: as the liquid warms, parcels rise, as hot liquids are less dense than cool liquids. As the parcels of liquid move away from the heat source, they cool off and increase in density. The dense parcels of liquid then sink back to the bottom.

This “turning over” in the atmosphere is convective circulation and can lead to thunderstorm development. The photograph to the left shows the variation in temperatures typical of a thunderstorm cloud. Rising air cools considerably, increases in density, and then sinks.

Left: Temperatures within a thunderstorm cloud vary over a great range.

In the summertime in Arizona, sunshine and solar heating are plentiful, but moisture levels adequate for thunderstorm development are not always present. A subtle change in circulation patterns during the summer opens up a flow of moisture from the south that dramatically increases convective thunderstorm activity across the state. This shift marks the beginning of the Arizona monsoon season.

North American Monsoon System

A monsoon is a circulation system driven by the heating of land and shift in dominant winds, resulting in a dramatic increase in rainfall. These most often occur in typically arid areas. The North American Monsoon, or Arizona monsoon, is a circulation that originates over Mexico beginning in May or June and spreads up into Arizona in mid-July. As large land masses heat up in June and July, the major air flow over Arizona shifts from westerly to southern flow, bringing in moist air from the Gulf of Mexico.

The North American Monsoon is characterized by high-intensity, localized, short-duration bursts of rainfall, mainly occurring in late afternoon as a result of convective heating. Rain events can produce as much as 25 mm of precipitation per hour. Flash flooding is common following these events.

More information on the North American monsoon is provided in this University of Arizona Cooperative Extension Bulletin.