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 landmass and the adjacent ocean that reverses its direction seasonally.” In fact, the word monsoon is derived from the Arabic word “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, this situation reverses and a dry season takes hold.
Monsoon wind systems exist in many different parts of the world, with the most famous one setting up over India and Bangladesh. In the US, we have the North American Monsoon that impacts states across the southwest. Summer temperatures in the region, which is 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 northward over the southern Plains in summer. Spinning clockwise, this shifts the winds in the area 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. Monsoon rains reportedly supply 50-70% 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, dust storms, hail, and frequent lightning.
The wet phase of North American Monsoon typically runs from mid-June to the end of September.
The North American Monsoon pulls most air (green arrows) inland over the typically arid southwest region of the US. Source: NOAA/NWS