Location and Background
The southern tip of India lies in the Equatorial low pressure trough (ITCZ) -> this air is wet and full of excess energy si it expands and cools forming cumulonimbus clouds and heavy rainfall
The north of India, however, lies in the Subttropical high pressure cell -> a zone of hot, dry air. the hot dry air carries more water which is why it is so dry. In the zone fo high pressure there is little cloud cover or precipitation.
Season Controlled by:
- The northward movement of the ITCZ during the Northern Hemisphere's summer
- Pressure differences caused by extreme heating and cooling of large land masses in relation to the smaller heat changes over nearby sea areas. These differences determine the strength adn direction fothe associated winds
- The mountain barrier of the Himalayas, with its numerous peaks over 8000m, is high enough to influence the general atmospheric circulation in the region - it forces air to rise
Dry Season (Winter)
The centre of Asia experiences intense cooling as a persitent, large area of high pressure develops. Winds blow clockwise, spiralling away from the high pressure cell, bringing very dry conditions to the Indian subcontinent, with only light rain evident along the southern most tip of India
Why and What this means:
The overhead moves southwards - therefore the ITCZand equatorial jet stream move just south of the equator. so the low pressure is replaced by a large area of high rpessure as the region cools. Subsidence from the Siberian high pressure cell surpresses uplift. As the air sinks towards the Ganges plain, it warms further, making precipitation less likely.
Bombay recieves 100mm of rain in these 8 months, as opposed to 2000mm in the 4 summer monsoon months
Wet Season (Summer)
A persistent south-westerly wind is driven by a warm air mass and this results in low pressure forming over Southern Asia as it is warmed - centred above the Ganges Valley in Northern India. Air from the high pressure zone over the Indian and Western Tropical Pacific Ocean travels northwards toward the low pressure over the land, bringing torrential rain
The overhead sun shifts towards the tropic of cancer, pulling the ITCZ north with it (up to 30dN). The intense heating of the region of Northern India, Pakistan and Central Asia produces a large area of low pressure. The monsoon climate tends to have its highest temperature just before the rainy period (as this is when the intense heating occurs so is where moisture is picked up - when it condenses into clouds, radiation is absorbed/reflected). Once the season starts, clouds block incoming solar radiation to reduce monthly temepratures. As this warm air rises, it draws warm moist equatorial and tropical maritime air from the Indian Ocean. As the air crosses the geographical equator, it is diverted to the North East due to the Coriolis effect; they become south westerly winds.
As the winds pass the western Ghats, bordering the Arabian sea, and the Himalayas, it delivers a substantial level of precipitation - Cherrapunji recieves 13,000mm of rain in these four months. This orographic and conventional uplift is exacerbated by latent heat released by condensation which creates more instability and uplift