The summer phase of the North American Monsoon is in full swing. But what, you may wonder, is a monsoon?

While most people associate a monsoon with rain, that is only half the story. It is actually a wind system. More specifically, according to NOAA, a monsoon is “a thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic “mausim”, meaning seasons or wind shift.

In general, a monsoon is like a large-scale sea breeze.  During the summer months, the sun heats both the land and sea, but the surface temperature of the land rises more quickly. As a result, an area of low pressure develops over the land and an area of relatively higher pressure sits over the ocean. This causes moisture-laden sea air to flow inland. As it rises and cools, it releases precipitation. In winter, the dry season, this situation reverses.

Monsoon wind systems exist in many different parts of the world. In the U.S., we have the North American Monsoon that impacts states across the southwest. Summer temperatures in this region – mostly desert – can be extremely hot. Readings in the triple digits are not uncommon. This intense heat generates a thermal low near the surface and draws in moist air from the nearby Gulf of California. In addition, an area of high pressure aloft, known as the subtropical ridge, typically moves north over the south-central U.S. in summer. Its clockwise circulation shifts the winds from a southwesterly to a southeasterly direction and ushers in moisture from the Gulf of Mexico. This combination of heat and moisture rich air produces thunderstorms and heavy rainfall across the region between July and September. In fact, summer monsoon rains are reported to supply nearly 50% of the area’s annual precipitation.

Replenishing reservoirs and nourishing agriculture, these seasonal rains are a vital source of water in the typically arid southwest. Conversely, they can also cause a number of hazards such as flash flooding, damaging winds and hail, as well as frequent lightning.

The summer monsoon officially begins, according to the National Weather Service, when there have been three consecutive days with a dew point above 54°F.

North American Monsoon: Summer Weather Pattern. The thermal low sets up over the southwest, while the subtropical high moves into the southern plains. Their circulating winds draw moisture (green arrows) from the Gulf of California and the Gulf of Mexico.   Credit: NOAA/NWS