About monsoon season: where do all the clouds and water come from?

Question

Are they accumulated during the hot months preceding the monsoon season? If the answer's yes, then why doesn't the water come down normally (i.e. like normal rain), instead of in heavy torrents for days on end?

Answer 1

The answer is not yes. The clouds and water don't accumulate over the area. They are drawn in by the low pressure caused when the heated air rises.

A monsoon is a wind pattern that reverses direction with the seasons. The term was originally applied to seasonal winds in the Indian Ocean and Arabian Sea. The word is also used more specifically for the season in which this wind blows from the southwest in India and adjacent areas that is characterized by very heavy rainfall, and especially, for the rainfall associated with this wind.

In terms of total precipitation, total area covered and the total number of people affected, the monsoon affecting the Indian Subcontinent dwarfs the North American monsoon (also called the "Mexican", "southwest", "desert", or "Arizona" monsoon).

The North American Monsoon (NAM) occurs from late May or early June into September, originating over Mexico and spreading into the southwest United States by mid July. It affects Mexico along the Sierra Madre Occidental as well as Arizona, New Mexico, Nevada, Utah, Colorado, West Texas, and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of southern California but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast during the monsoon). The North American monsoon is known to many as the Summer, Southwest, Mexican or Arizona monsoon. It is also sometimes called the Desert Monsoon as a large part of the affected area is desert.

The North American monsoon is associated with an area of high pressure called the subtropical ridge that moves northward during the summer months and a thermal low (a trough of low pressure which develops from intense surface heating) over the Mexican Plateau and the desert southwest of the United States. The monsoon begins in late May to early June in southern Mexico and quickly spreads along the western slopes of the Sierra Madre Occidental, reaching Arizona and New Mexico in early July. The monsoon extends into the southwest United States as it matures in mid July when an area of high pressure, called the monsoon ridge, develops in the upper atmosphere over the four corners region, creating an easterly to southeasterly wind flow aloft. This wind flow pattern directs moisture originating in the Gulf of Mexico, Gulf of California and the tropical Pacific by way of northern Mexico into the region, setting off brief, but often torrential thunderstorms, especially over mountainous terrain. This activity is occasionally enhanced by the passage of easterly waves or the entrainment of the remnants of tropical storms.

As much as 70% of rainfall in the region occurs during the summer monsoon. Many desert plants are adapted to take advantage of this brief wet season. Because of the monsoons, the Sonoran and Mojave are considered relatively "wet" when ranked among other deserts such as the Sahara.

Monsoons play a vital role in managing wildfire threat by providing moisture at higher elevations and feeding desert streams. Heavy monsoon rain can lead to excess winter plant growth, in turn a summer wildfire risk. A lack of monsoon rain can hamper summer seeding, reducing excess winter plant growth but worsening drought. The Southwest has been in continuous drought status since the mid-1990s.

Flash flooding is a serious danger during the monsoon season. Dry washes can become raging rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away (never camp in a dry wash in the desert). Lightning strikes are also a significant danger. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.

The North American monsoon affects much of the United States and Mexico. Major drought episodes in the midwestern United States are associated with an amplification of the upper tropospheric monsoon ridge, along with a weakening of the western edge of the "Bermuda high" and the low-level jet stream over the great plains.

Answer 2

The average start date for the "Monsoon Season" passed last Saturday (July 7). The official start date is determined when the Dewpoint reaches 55 degrees for the 3 consecutive days. In Arizona, our winds shift to a more southerly direction in the hot part of the summer increasing humidly enabling the development of thunderstorms. July and August are the wettest months of the year in most of Arizona. Storms will kick up late in the afternoon (just when everyone is leaving work), and only last for about 15 minutes. Occasionally they contain very strong winds, enough to down tree limbs. This is also the time of year when it's tough to keep your car clean. It rains, then blows dust on the car, and presto, you have spot of mud on the side of the car.

Answer 3

If this answer on "monsoon" (aka mausam) is for the Indian sub-continent then here is the phenomenon.

The tropical waters of Indian ocean are very warm due to (a) the Warm water currents passing through and (b) the beginning of summer season at the equator. As a result there is a mass of clouds built up on the ocean but the land is relatively cool during Jan thru March as compared to the water (remember it is still winter at Tropic of Cancer and above).

Correspondingly, as heat increases from March through June/July on the sub-continent land-mass, the land heats up more as compared to water(thermal conductivity soil more than water).The air on the land mass also therefore heats up more than the air on the ocean.

As a result, the hot air on land rises at a significant rate leading to the clouds to be pulled in at a rapid rate....don't be fooled since the rain clouds come from Indian ocean but due to the earth's spin, they seem to enter from the Arabian Sea.

The first hit is on the Western Ghats (Kerala, Bombay, Goa,etc) which blocks the path of entry causing heavy (water bearing) clouds to hover over land and loose momentum. This causes the downpour. After which, slightly light clouds cross-over the ghats and rush onto plains only to be obstructed by the Himalayas. This turns them onto Bengal/Bangladesh and onto Bay of Bengal.

It takes a series of such cloud trains to pour down on the land and cool it down over next 3/4 months.....

Hence, you have torrents of rain and not drizzles.....

Hope that answers.