Pressure Belts – 03
BASIS OF FORMATION OF PRESSURE BELTS
There are two important bases on which the pressure belts are formed. They are;
- Temperature: The equatorial low pressure and polar high pressure belts are formed due to high and low temperature respectively. So they are called as ‘Thermally formed pressure belts’.
- Dynamism: The sub tropical high and sub polar low pressure belts are formed due to movement and collision of wind system. So they are called as ‘Dynamically formed pressure belt system’.
MERIDIONAL CELL SYSTEM
The cell along with trade winds, equatorial low and sub tropical high pressure belts is called as ‘Hadley cell’,
meanwhile the cell formed by westerly wind along with sub tropical high and sub polar low pressure belt is called ‘Ferrell’s cell’.
The cell at polar formed by polar easterlies with polar high and sub polar low pressure belt is called as ‘Polar cell’ (Figure 6.15).
ITCZ – INTER TROPICAL CONVERGENT ZONE
The region where both trade wind systems meet is known as ‘Inter Tropical Convergent Zone’.
SHIFTING OF PRESSURE BELTS AND PRIMARY WIND SYSTEM
These pressure belts and primary wind systems are dynamic in character as they shift 5° north and 5° south from their position along with the apparent movement of the sun.
- As these winds approach the land, their southwesterly direction is modified by the relief and thermal low pressure over northwest India. The monsoon approaches the Indian landmass in two branches:
- The Arabian Sea branch – The monsoon winds originating over the Arabian Sea.
- The Bay of Bengal branch – The Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction.
- Another phenomenon associated with the monsoon is its tendency to have ‘breaks’ in rainfall. The monsoon rains take place only for a few days at a time. They are interspersed with rainless intervals. These breaks in monsoon are related to the movement of the monsoon trough.
Despite an overall unity in the general pattern, there are perceptible regional variations in climatic conditions within the country.
Retreating Monsoon Season
- The retreating southwest monsoon season is marked by clear skies and rise in temperature.
- The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’.
- In the second half of October, the mercury begins to fall rapidly, particularly in northern India.
- The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula. Here, October and November are the rainiest months of the year.
- The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula. These tropical cyclones are very destructive.
- A bulk of the rainfall of the Coromandel Coast is derived from these depressions and cyclones.
- Unlike the rest of the country, which receives rain in the southwest monsoon season between June and September, the northeast monsoon is crucial for farming and water security in the south
The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of high pressure over the plateau at about 9 km above sea level.
The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.