Most people think of a monsoon as a deluge of rain. And, while the monsoon in one of its phases brings increased precipitation, the complete monsoon cycle produces a variety of weather.
What is a Monsoon?
A monsoon is a gigantic seasonal sea breeze circulation. It derives from differential heating with the changing elevation of the sun. Where a large body of water abuts land, the land heats relative to the water when the sun is high; cools more when it's low. The differential heating can occur on a daily basis or a yearly one.
Water is a much better conductor of heat than most land surfaces. When the sun is high in the sky, the land heats relative to the water and heats the air above it. Hot air rises. A circulation that includes a wind from water to land is established. When this circulation arises from a single day's heating of the land it is simply called a seabreeze; when the heating occurs over a season, summer, the result is a monsoon.
The reverse flow develops during the opposite part of the cycle, when the land cools by radiation of its heat content to space, and a flow from land to water ensues, but the result is normally not anything that attracts much attention. A summer monsoon, on the other hand, can bring copious, sometimes catastrophic, rains, as the unrelenting wind from sea to land carries with it large amounts of moisture.
Major Monsoons of the World
The best known monsoon brings periodic disasters to the Indian subcontinent. The warm ocean and high humidity coupled with uplift from the nearby Himalayan Plateau, sets the stage for a very pronounced summer monsoon which is responsible for frequent flooding in parts of India, Pakistan and Bangladesh. But the monsoon is not all bad. Successful crop harvests rely on the rain. The return flow, with wind blowing from land to sea, brings dry and cool weather during winter.
The Gentle Monsoon of the American Southwest
The desert areas of Arizona and surrounding states are not thought of as monsoonal locations, but the precipitation record clearly shows that there is one. Beginning sometime in June or early July, average precipitation rises from less than a quarter inch per month to over 2 inches per month. This doesn't seem like a lot of rain compared to the tens of inches per month in other monsoons, but the change in weather pattern is quite pronounced, with winds generally shifting from west in June to south in July, and afternoon thunderstorms occurring with regularity.
Further Reading:
- The Monsoon Season in India, Sharell Cook
- South West Monsoon in India
Jon Plotkin - The author was a math major at Cornell and has a master's degree in meteorology from MIT.
