winds as they blow from that direction. These winds blow only in the summer.
the rain cloud They are called Monsoon winds. They are also called south ows in Map I show us the direction of
other
We
nds,
The
Ghats
them
wate
Seisagar
Map 1: Monsoon
winds in India
Towards which parts of the country
will the winds take the clouds on
the Bay of Bengal?
Towards which parts of the country
will the winds take the clouds on
the Arabian Sea?
From which direction will the
winds blow to bring monsoon rains
to West Bengal, Lucknow and
Delhi?
From which direction will the
winds blow to bring monsoon rains
to Mumbai, Hyderabad, and
Bengaluru?
ascer
is co
Hov
otho
hap
the
Areas of Heavy Rainfall
Wind direction
Delhi
br
th
Arunas
Sikkim
Bedest
Lucknow
Mahaba
с
Meghala
Bhopal
Tapuesi
Kolkata
Rainfall in Andhra Pradesh
Nagapur
ashion
Mumbai
Hyderabad
• In which months does it rain most
in your place? List three rainiest
months.
Name the three driest months.
• Do you have normal' rainfall every
year or does it vary every year?
• Have you ever experienced
drought?
• Have you ever experienced floods?
ARABIAN
SEA
BAY OF
BENGAL
Bengalore
Chennai
Thiruvananthapuram
T
When the south west Monsoon sets
around the beginning of June, the winds
carrying the clouds also reach Andhra
Pradesh. As you can see from Map 1, these
winds reach Rayalaseema districts o
Chittoor and Kurnool first. However the
bring very little rain as most of th
INDIAN OCEAN
10 Diversity on the Earth
Answers
At the Equator the area near India is unique in that dominant or frequent westerly winds occur at the surface almost constantly throughout the year; the surface easterlies reach only to latitudes near 20° N in February, and even then they have a very strong northerly component. They soon retreat northward, and drastic changes take place in the upper-air circulation (see climate: Jet streams). This is a time of transition between the end of one monsoon and the beginning of the next. Late in March the high-sun season reaches the Equator and moves farther north. With it go atmospheric instability, convectional (that is, rising and turbulent) clouds, and rain. The westerly subtropical jet stream still controls the flow of air across northern India, and the surface winds are northeasterlies.
Monsoon Onset And Early Developments
As the high-sun season (that is, the Northern Hemisphere summer) moves northward during April, India becomes particularly prone to rapid heating because the highlands to the north protect it from any incursions of cold air. There are three distinct areas of relative upper tropospheric warmth—namely, (1) above the southern Bay of Bengal, (2) above the Plateau of Tibet, and (3) across the trunks of the various peninsulas that are relatively dry during this time. These three areas combine to form a vast heat-source region. The relatively warm area above the southern Bay of Bengal occurs mostly at the 500–100-millibar level. (This atmospheric pressure region typically occurs at elevations between 5,500 and 16,100 metres [18,000 and 53,000 feet] but may vary according to changes in heating and cooling.) It does not appear at a lower level and is probably caused by the release of condensation heat (associated with the change from water vapour to liquid water) at the top of towering cumulonimbus clouds along the advancing intertropical convergence. In contrast, a heat sink appears over the southern Indian Ocean as the relatively cloud-free air cools by emitting long-wavelength radiation. Monsoon winds at the surface blow from heat sink to heat source. As a result, by May the southwest monsoon is well-established over Sri Lanka, an island off the southeastern tip of the Indian peninsula.
Map of climatic heat sources and heat sinks for Asian summer and winter monsoons.
Map of climatic heat sources and heat sinks for Asian summer and winter monsoons.
Encyclopædia Britannica, Inc.
Also in May, the dry surface of Tibet (above 4,000 metres [13,100 feet]) absorbs and radiates heat that is readily transmitted to the air immediately above. At about 6,000 metres (19,700 feet) an anticyclonic cell arises, causing a strong easterly flow in the upper troposphere above northern India. The subtropical jet stream suddenly changes its course to the north of the anticyclonic ridge and the highlands, though it may occasionally reappear southward of them for very brief periods. This change of the upper tropospheric circulation above northern India from westerly jet to easterly flow coincides with a reversal of the vertical temperature and pressure gradients between 600 and 300 millibars. On many occasions the easterly wind aloft assumes jet force. It anticipates by a few days the “burst,” or onset, of the surface southwesterly monsoon some 1,500 km (900 miles) farther south, with a definite sequential relationship, although the exact cause is not known. Because of India’s inverted triangular shape, the land is heated progressively as the sun moves northward. This accelerated spread of heating, combined with the general direction of heat being transported by winds, results in a greater initial monsoonal activity over the Arabian Sea (at late springtime), where a real frontal situation often occurs, than over the Bay of Bengal. The relative humidity of coastal districts in the Indian region rises above 70 percent, and some rain occurs. Above the heated land, the air below 1,500 metres (5,000 feet) becomes unstable, but it is held down by the overriding easterly flow. This does not prevent frequent thunderstorms from occurring in late May.
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