Wyoming and the Monsoon

As we are most of the way through the summer, and thus in the heights of the thundershower pattern that goes hand in hand with summer afternoons, I thought I'd write about the source of those thunderstorms; the Monsoon.

The very word "monsoon" is a bit of a mystery. It's thought the term was drawn from the Arabic word mawsim, which means "season". Arabic traders used the mawsim, depending on the wind’s seasonal direction, to move goods back and forth between Africa and Asia. Portuguese traders from the 1500s make reference to the Arabic word, but Dutch traders later began using the more similar moniker "monssoen" for the wind. Just to cloud the etymology a bit more, when the British began colonizing India they found that the Hindi word mausam meant "weather". It matters not who came up with the term, only that we use it now to refer to a wind pattern, associated with heavy rain, that comes in the warmer months of the year.

The monsoonal precipitation we see in the western United States each summer is brought about via the same science that creates monsoons all over the world. The best known monsoon is Asia's Southeast Monsoon. For four to six months a year, Pakistan, India, Bangaladesh, and southeast Asian countries like Thailand and Vietnam, receive multiple feet of rain. Cherrapunji, India, where the Himalayas slope down near the coast, has seen over 80 feet of rain fall in one monsoon season. West Africa has a monsoon, as does northern Australia. Like Calcutta and Bangkok in their respective locations, cities like Tucson, Arizona, and Nogales, Mexico, receive most of their yearly precipitation from our North American Monsoon.

At it's essence, the monsoon is a wind pattern, and the best way to understand it is to think of how the wind blows at the beach. At mid afternoon, it's common to feel an "onshore" breeze, that being a wind that is blowing from the ocean to the land. This is because there is a difference in temperature between the moist air over the ocean and the dry air over the land. As we all know, hot air rises, and cold air sinks. This is because hot air is less dense than the air around it (the molecules are spaced out more). We also know that bodies of water, like the ocean, tend to hold temperature for a longer time than land. A walk across beach shows this, with the hot sand burning our feet but the water, exposed to the same amount of solar radiation, cools our feet. So on a sunny day, the air over the ocean tends to be cool, while the air over the land can get very hot. As the air over the land heats up, it rises, creating a bit of a vacuum that draws in the cooler air from over the water. That flow of air, from the cool ocean to the warm rising land, is the moist onshore breeze.

The Monsoon is this on a grand scale, like for an entire continent. The hot air of the Sonoran, Chihuahuan, Mojave, and Great Basin deserts, collectively known as the Great American Desert, rises, and in doing so draws cooler, moist air in from the ocean. This moist air forms into thunderstorms all over the desert southwest that fill (hopefully, with help from rivers that start in Wyoming) the reservoirs of cities like Phoenix, Albuquerque, and Las Vegas.

That explains how coastal Mexico and Arizona get rain in the summer, but it doesn't explain how all that rain gets up here to Wyoming. To understand that we again have to look at the heat of the desert. If you track around a globe, looking at the area between about 20 and 40 degrees of latitude, you will note a lot of famous deserts. This partially has to do with a phenomenon known as Hadley Cells, which is something we won't need to discuss here. Suffice it to say that there are spots that the atmosphere tends to be drier and warmer, and those spots help to make the great deserts of the world. The Sahara, Takla Mekan, Australian, and Great American deserts all fall into this area. But deserts also tend to perpetuate themselves by creating areas of intense high pressure.

Over all of these great deserts of the world you will find a fairly dry, stable mass of high pressure. Air masses of high pressure, due to something called the Coriolis Effect, rotate over the surface of the earth. In the northern hemisphere, high pressure areas rotate in a clockwise motion (fyi, low pressure areas rotate counter clockwise). Every summer, the Great American Desert, running from Idaho to central Mexico, from the Sierras to the Rockies, creates one of these big high pressure areas. Picture it like a clock laid above the surface, with noon in Montana, three in Kansas, six somewhere in Mexico, and nine at about Reno, Nevada. That clockwise motion pulls air from the desert southwest, along the eastern Sierras, over Nevada and Idaho and eventually to Wyoming. The air over us was thus not only over Las Vegas two days ago, it was over the Pacific Ocean four days ago, and that ocean made it moist.

We have to remember that just because the air is humid, does not mean we will have rain. If we can again remember that hot air rises, we can put the final piece of the puzzle in place. All climbers know that thunderstorms are more common in the late afternoon and early evening, and this is because of that warm, rising air. As the sun heats the air through the day, the air rises. Remember, that is now moist air from the Pacific. As the air rises it cools, and as it cools it becomes more dense. As the moist air condenses it forms clouds, commonly cumulonimbus clouds, that we like to call thunderstorms.

So lets put all of it together. Warm air in the American desert draws moist air in from the ocean along the Mexican coast. That moist air meets the upper level winds of a high pressure area. The winds of that high, rotating like a wagon wheel lying on its side in the western United States, draws the damp air north. That damp air heats up during the day, rises, and thus condenses the moisture. It falls as rain, and Wyoming thus feels the effects of the Monsoon.


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