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Monsoons
Cross-Curricular Connection
Many locations on Earth experience prevailing winds that blow in one general
direction for long periods of time, but then change course. Such seasonal shift
in the wind’s direction is called a monsoon. The term monsoon comes from the
Arab word mawsim, which means “season.” This changing of the direction of
the wind can have a dramatic effect on both the weather and a population’s way
of life. Although monsoons are regular occurrences in places such as North
Africa, West Africa, and Southeast Asia, perhaps the most famous monsoon of
all takes place in India.
The Arrival of the Rainy Season: India’s Summer Monsoon
Early each summer, two
ingredients mix to create
India’s famous wet-weather
monsoon: cool, moist air
from the Indian Ocean; and
hot, dry air over the
country’s landmass.
Because this mix of
ingredients is often just
right, the monsoons that
affect India each year are
among the most potent on
Earth. Between June and
September, India will
receive as much as 75% of
its annual rainfall.
Moisture from the Indian Ocean feeds powerful monsoons that
The moisture that feeds
affect India each summer. In this map, the red arrows show the
India’s monsoon rains comes movement of winter monsoons (dry winds) and the blue arrows
show summer monsoons (wet winds).
from the Indian Ocean, a
body of water that is
constantly warm because of its proximity to the equator. As the sun warms the
ocean, water evaporates into the atmosphere, creating clouds. An increase in
direct sunlight caused by the arrival of solar summer in the northern
hemisphere intensifies this process. Simply stated, more warm water means
more evaporation to fuel the formation of clouds. The first important
component of the rainy monsoon season – moist air – is now in place.
The second ingredient in India’s wet monsoon season is hot, dry air over the
mainland. Because land absorbs more heat than water, the same seasonal
warming trend that causes water over the Indian Ocean to evaporate is even
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Monsoons
Cross-Curricular Connection
more intense on India’s land. Each spring, the sun’s direct rays create heat that
is stored in the ground, and warmer ground creates more heat in the
atmosphere. This warming is intensified by the presence of the Himalayan
Mountains in the north. The Himalayas are a massive mountain range that
contain many of the highest mountains in the world, landforms that create a
barrier between India and much of the rest of Asia. This mountainous wall
blocks the flow of cooler air into India from areas such as Tibet and China. As a
result, each spring temperatures in many parts of India regularly exceed 100
degrees Fahrenheit. During this time, the air is still and lifeless. But the
stillness creates a sense of anticipation. Things are about to change.
How do the moist air over the Indian Ocean, and the hot, dry air over India’s
mainland combine to create the rainy monsoon season? It is important to
remember that warm air rises. So each spring, as the hot air over India’s
mainland rises, it slowly pulls moist air from the Indian Ocean towards the
land. At first, the arrival of this moist air brings uncomfortable humidity but
little rain. But as moist air moves higher into the atmosphere, it cools and its
water molecules condense, becoming too heavy to remain suspended in the air.
The moisture falls to the earth as rain.
India’s physical geography also impacts the annual progress of the monsoon.
The Western Ghats, a mountain range in the country’s southwest, blocks winds
and rainclouds arriving from the Indian Ocean. This ensures that the country’s
southwestern coast receives the earliest and most rainfall. This region has a
tropical monsoon climate. The coastal town of Kundapura, for example,
receives an average of more than 120 inches of rainfall during the monsoon
season alone. Areas of India north and east of the Western Ghats receive less
rainfall from the monsoon, and therefore are home to a variety of drier climate
regions. In addition to blocking the arrival of cool air from other regions, the
Himalayan Mountains are the final barrier to wet weather from the monsoons.
They keep virtually all rain from the monsoons in India, and their northern
slopes receive little annual precipitation.
The Winter Monsoon
Each December, the monsoon changes direction as the
northern hemisphere’s solar winter arrives. Since
solar winter is the quarter of the year when a
hemisphere receives its least amount of sunlight,
India’s land, and the air masses above it, begin to cool.
There is no longer any hot air to pull moisture from
Flooded streets are common during
India’s summer monsoon.
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Monsoons
Cross-Curricular Connection
the Indian Oceans. As a result, winds begin to change direction, blowing from
India’s land instead of toward it. The dry monsoon season of January and
February begins. Although there
can still be precipitation, rainfall totals
decline dramatically. In fact, much of
India experiences drought conditions
during the winter monsoon season!
Predicting the Monsoons
During the time periods bridging the
monsoon seasons, scientists collect data
to predict how much rain the country
can expect during the next wet monsoon
season. The India Meteorological
Department releases forecasts in April of The winter monsoons come from
the mountain regions. The Indian
each year for the four geographical
subcontinent is shown here. The
regions of India. These predictions are
Himalayas are to the north. The
important because monsoon rains are
Bay of Bengal is the water to the
right of India.
vital for the agricultural economy of
India. More than 50 percent of the
farmland in India relies on rainwater alone to water the crops. Accurate
forecasts help farmers know what to expect for each growing season.
The Indian monsoons are the lifeline of India. Due to the unique geographical
features of the land, monsoon winds protect the land from becoming a desert
environment. So the next time you hear about a monsoon, don’t just think
about rain and floods. Think about a great wind that helps propel a country.
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