What is a Monsoon, and When is the Monsoon Season?
Monsoon has Several Meanings, which have Changed Over Time and from Area to Area
The term monsoon season suggests the mysterious east, hinting at the aroma of exotic spices in the air as the storm clouds build.
And the term is an eastern one, derived from the Arabic word for season or seasonal winds. In the region around the Arabian peninsula, the winds are indeed seasonal - about six months each from the northeast followed by the same from the southwest. These winds werevery important in trade with India and points to the east - traders set out in the southwesterlys and returned in the northeasterlys.
The Indian Monsoon Season
But the word is most often associated with India, where monsoon, or monsoon season is a term used for the wet or rainy season. The rainy season lasts from around June to September, when warm moisture laden winds blow from the southwest. The monsoon season provides relief from the hottest, driest time of year which precedes it.
The seasonal reversal of winds in the Indian area is related to the different ways land and sea react to air temperatures. In summer the land heats up faster than the sea, and the hot air rises with the formation of a low pressure zone - a heat low. The sea heats up more slowly, and the air above it is cooler, as well as being moisture laden.
As the warm inland air rises, it is replaced by the cooler, moister air. A combination of high land surfaces and instability causes the humid air to rise, resulting in large masses of convective cloud, including numerous thunderstorms.
Rainfall can be heavy and persistent, and the world's wettest places have earned their records because of the effect of the monsoon in the foothills of the Himalayas. How about Cherrapunji, with up to 90 inches of rain (2290mm) in the monsoon season.
In winter the reverse happens - the land cools faster than the sea, cool dry air moves seaward, and the monsoon season is over.
The situation is a little more complex than that, but that describes the basic elements. and if you are reminded of sea breezes and land breezes you are correct - it's partly a matter of scale.
Other Examples of Monsoonal Conditions
So from being a name for a seasonally reversing wind, monsoon has come to mean a whole weather system, including both wet and dry periods. And it is not just restricted to the area including India and the Arabian peninsula. Monsoonal conditions occur in much of tropical and subtropical Africa, and south east Asia. South America and north west Australiaalso have monsoon seasons, but being in the southern hemispherethey occur between December and March.
In fact, anywhere where persistent low air pressure zones form over relatively dry areas adjacent to cooler seas has some form of monsoon. And the effect can extend over considerable areas.
Central Asia includes a large arid area which becomes very hot in summer, and it is affected by monsoonal winds derived from the western Pacific.
In fact the effect is felt much further away in places like Indonesia and the Philippines, neither of which have semi desert areas but are affected by winds blowing towards the Asian mainland.
So it's the combination of an extensive land area which heats up in summer situated reasonably close to a large sea or ocean. A bit like the south western USA and parts of northern Mexico, perhaps.
Yep, that's right. South western USA, centered on Arizona, has its own monsoon season.
The Arizona Monsoon Season
More strictly speaking, the monsoon is more typical of northern Mexico with a spillover into the USA. So although it is more helpful to think of a Mexican Monsoon, the term Arizona Monsoon is also widely used.
The Arizona Monsoon comes into play in early July and persists to September. It is marked by relatively high humidity, atmospheric instability, and regular afternoon thunderstorms. And here we come across another use of the term monsoon - for individual thunderstorms that build up during summer.
This is stretching things a bit - that would mean that Arizona would have numerous monsoons, whereas there really is only one each year, marked by the change to the wetter stormy season.
Apart from Arizona and Mexico, Colorado, Nevada, Utah, New Mexico, West Texas and California are also affected to some degree.
And like India and north west Australia, the monsoon season is preceded by some of the hottest weather of the year, with most Arizonan records being set in June.
The official start of the monsoon season is the beginning of a three day period where the dewpoint exceeds 55°F, indicating the presence of moister air. Thunderstorms can develop in several ways during this period, but the key feature is the entry of moist air from the south, possibly derived from the Gulf of California. As this damper cooler air moves in to replace air rising from a desert heat low, it is forced to rise over ranges in its path, creating condensation which in turn is caught up in rising air currents in unstable atmospheric conditions.
From a United States perspective, this all looks pretty impressive, and the rainfall in Arizona and neighbouring states is very useful and much needed. But while Phoenix, Tucson and other centers might average 5 to 10 inches of rain (125-250mm) during the monsoonal period, the real action is south of the border, where the heat low is centered. This results in an average rainfall in Acapulco of 51.8 inches (1316mm) from June to October, and more in the coastal ranges.
As things cool down in early Fall, the heat low in Mexico degenerates and a large high pressure system, which has been pushed to the north, begins to reassert itself and the storm season is over.
Unless some other major feature, such as a Pacific hurricane interacts with the monsoonal flow, scattered thunderstorms are the order of the day rather than long rain periods.
These thunderstorms provide two other examples of severe weather typical of Arizona's climate - microbursts and flash floods. Microbursts are savage descending air currents which occur to some degree in all thunderstorms. When the downbursts hit the ground they spread out as fast moving horizontal winds, often carrying dust, and reaching destructive speeds of over 100mph (160kph).The effect of wind destruction over a small area is reminiscent of tornadoes, but although they do occur, this is not really tornado country.
Like thunderstorms everywhere, desert storms dump a lot of rain and hail very quickly over a fairly small area. The sparse vegetation of the area does little to hold the waterback, and flash floods develop very frequently, both in dry gullies, streets and town watercourses.
I can't present any detailed statistics, but I suspect Arizona could lay claim to being the flash flood center of the USA. That's a good reason for taking more than ordinary care on the roads - water over the road when a storm's around either means the start of a flash flood, where water rises dangerously quickly, or the end of one where it would be wise to check the road for washaways before driving on.
So that's the story behind Arizona's summer weather - winter is a different story. But in the warmer months, the monsoon season has Arizona firmly in its grip.
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