Monsoons are seasonal changes wind direction that affect the Indian sub-continent.
They are caused by extreme heating and cooling of large land masses in relation to the surrounding sea. This affects both pressure and winds!
WARM AIR RISES
and as it does so it causes
LOW PRESSURE on the earths surface
RULE NUMBER 2
COLD AIR SINKS
This causes HIGH PRESSURE
RULE NUMBER 3
Winds always blow from
HIGH PRESSURE TO
March – the sun is overhead at the equator.
June - the sun is overhead at the Tropic of Cancer.
As the sun is at a high angle in the sky, insolation over Northern India increases. The heat causes warm air to rise and therefore low pressure develops.
A high pressure system is centred over Bay of Bengal
SUMMER MONSOON 2
Remember rule 3 - air moves from areas of high pressure to areas of low pressure. So the SOUTH WESTERLY winds pick up moisture as it crosses the sea. These ONSHORE winds therefore bring much RAINFALL to Bangladesh
Sept – Sun overhead at the equator
Dec - Sun overhead at Tropic of Capricorn
Sun is at a lower angle in the sky and temperatures are lower.
Low temperatures over central Asia create an area of high pressure. (cold air sinks)
WINTER MONSOON 2 Rule 3 again – winds blow from high to low pressure. Winds therefore blow out from Northern India and Bangladesh out towards the low pressure area. These OFFSHORE NORTH EASTERLY winds are DRY