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Monsoons
Bring Rain and More Rain
Patrick
J. Kiger
In the summer
of 2019, four states in India were pounded by heavy rains that caused severe
flooding and landslides,
killing at least 244 people and forcing 1.2 million others to flee their homes
and take refuge elsewhere, mostly in government-run relief camps, according
to Agence
France-Presse.
But the
disaster wasn't some freak storm. Instead, it was the result of the
annual monsoon,
a reoccurring phenomenon in various parts of the tropics,
the warm middle section of the globe between the Tropic of Cancer and the
Tropic of Capricorn.
In a monsoon,
the prevailing, or strongest, winds in a region change direction, bringing
moist air with them, which causes heavy precipitation.
As the U.K.'s Met Office explains,
the word "monsoon" comes from the Arabic word mausim,
meaning season, which is fitting because monsoons last for months at a time.
And though
they aren't storms themselves, they can cause very powerful ones, such as the
deluge that India experienced.
"In the tropics, we don't have summer or winter and
spring," explains Jenni Evans,
a native of Australia who is now a professor of meteorology and atmospheric
science at Pennsylvania State University, and also serves as director of the
school's Institute for CyberScience,
in an email.
"We have wet and dry. It's basically broken up into wet
season and the dry seasons. The monsoon means the wet season."
When Do Most Monsoons Occur?
Monsoons
typically are active between May and November in the Northern Hemisphere, which
is when three of the world's major monsoons — the African, East Asian and
Indian ones — occur, as well as the smaller-scale, less intense North American
Monsoon, which affects the southwestern U.S. and northwestern
Mexico.
Monsoons occur
between November and March in the Southern Hemisphere, where the Australian
monsoon is the only big one.
As Evans says,
monsoons are caused by the sun heating up the land and the air over it during
the warmest half of the year, so that they become warmer than the ocean and the
air above the water.
Cold air is
more dense, so it pushes warm air out of the way and changes the wind
direction, blowing onto the land.
The monsoon "is
on the scale of a continent," Evans explains.
"It's huge. If you look at the high and low pressure
systems that come across North America and affect our weather, it's that
big."
That scale
means that the monsoon is affected by Earth's rotation,
so that in the Northern Hemisphere, the low-pressure system that it creates
spins counter-clockwise, Evans says.
(In the
Southern Hemisphere, it spins in the opposite direction.)
Rain, Rain and More Rain
The most
prominent — and at times, potentially catastrophic — characteristic of monsoons
is that they cause rainfall.
A lot of it.
In fact, according to NASA,
India gets between 50 and 75 percent of its annual rainfall from the monsoon.
Monsoons bring rain, according to Evans, "because that
air coming off the ocean is not just cooler but it's moist. It's got a lot of
water vapor in it that evaporated over the ocean. The ocean is cooler than the
land, but it's still warm water. It might be 80 degrees Fahrenheit [27 degrees
Celsius] instead of 90 degrees F [32 degrees C]. Or 75 degrees F [24 degrees C]
instead of 85 degrees F [30 degrees C].
"And so all of that ocean air carries all that moisture on
shore," she continues. That air is spinning and rising, and when it gets
to a higher altitude and cooler temperature, the water vapor condenses and
forms clouds. "You get so much moisture there, that you make big heavy
raindrops and they fall."
Additionally,
some of the water vapor forms snow and ice in the clouds, which can sometimes
result in hailstorms. Mostly, though, what a monsoon produces is thunderstorms
and accompanying rain, she says.
The normal
pattern of the monsoon can be influenced by the Madden-Julian
Oscillation, a shorter-term fluctuation of atmospheric pressure that
can add moisture to the monsoon at some points and take it away at other times,
causing either wet or dry spells during the season.
Monsoons and Cyclones
Monsoons often
produce tropical cyclones, which go by various names. "In the western
Pacific, we call it typhoon," Evans explains.
"In Australia we call it a tropical cyclone or tropical
storm. In Africa we call it a hurricane."
Some of the
most intense storms on the planet, in fact, are the hurricanes formed off west
Africa during monsoons.
(It should be
mentioned that not all hurricanes are spawned by monsoons. Subtropical cyclones
formed around Bermuda, for example, can move into the tropics and become
hurricanes, according to Evans.)
The Indian
monsoon behaves a bit differently from the monsoons that occur elsewhere in the
world because of the subcontinent's topography.
"Early in the season, when the monsoons get stronger
everywhere else, over India, the monsoon is moving from the south and the land
is getting warmer and warmer," Evans says.
"So, the monsoon moves to the north, then it gets stuck at
the Himalayas. And then later in the season as the land is cooling off, it
moves back down south and over the ocean again."
Monsoons
eventually ease toward the end of their cycle, as the seasonal temperature
shifts and the land starts to cool down, creating less of a contrast with the
water.
"What that means is that the ocean is relatively warmer
compared to the land," Evans says.
"So, the air isn't flying over the land as much, and you
don't get that rotation and all that moist air. So, the monsoon kind of dies
off."
Though
monsoons sometimes can lead to punishing storms, they also have an important
beneficial effect.
Farmers in
India, for example, rely upon the monsoon to provide rainfall to nurture crops,
and the nation also relies on the water from them to fill reservoirs that are
harnessed to produce hydroelectric power, as this article from the UCAR Center for
Science Education details.
In years when
the monsoon doesn't produce as much rainfall, crop yields can decrease and hurt
the Indian economy.
NOW THAT'S INTERESTING
Scientists
are concerned that climate change and growing
atmospheric pollution could alter the Indian monsoon, making its storms more
erratic.
Patrick J. Kiger has written for HowStuffWorks since 2008 covering a wide
array of topics, from history and politics to pop culture and technology. He
worked as a newspaper reporter for the Pittsburgh Press, and the Orange County
Register in California, where he covered one of the biggest serial murder cases
in U.S. history, and also as a staff writer at Baltimore Magazine. As a
freelancer, Patrick has written for print publications such as GQ, Mother Jones
and the Los Angeles Times, and on the web for National Geographic Channel,
Discovery News, Science Channel and Fast Company, among others. In recent
years, he's become increasingly interested in how technological advances are
altering urban life and the design of cities, and has written extensively on
that subject for Urban Land magazine. In his spare time, Patrick is a longtime
martial arts student and a fan of crime fiction, punk rock and classic
Hollywood films.
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