The distribution of atmospheric pressure across the latitudes is termed global horizontal distribution of pressure. Its main feature is its zonal character known as pressure belts.

All air movements have their roots in pressure differentials in the atmosphere, called pressure gradients. Systematic differences in the Earth’s land temperature affect air pressure, and significant patterns of pressure that persist over time are called pressure belts, or wind belts. Wind belts depend on temperature, so temperature changes can move the belts and also change wind patterns.

Body:

The horizontal distribution of air pressure across the latitudes is characterized by high or low-pressure belts. These pressure belts are:

• The Equatorial Low-Pressure Belt
• The Sub-Tropic High-Pressure Belts
• The Sub-Polar Low-Pressure Belts
• The Polar High-Pressure Belts

Reasons for season shifting of pressure belts:

•  In the absence of the revolution of the earth around the sun in about 365 days the global pressure belts would have been permanent and stationary at their places.
• However, the relative position of the earth with the sun changes within a year due to earth’s revolution and thus the position of all the pressure belts except the polar high pressure belts changes with the northward and southward migration of the sun.
• Due to the inclination of the Earth on its axis, there are differences in the heating of the continents, oceans and as a result, the pressure conditions in January and July vary greatly.
• On 21 June, where the sun is overhead on the tropic of Cancer then the pressure belt shifts 5° northwards and on 22 December when it shines overhead on the Tropic of Capricorn, they shift 5° southwards.
• The pressure belts remain balanced in both the hemispheres when the sun shines vertically over the equator on 21st March and 23rd September.

Impact on formation of various climatic regions across the globe:

• The shifting of the pressure belts causes seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres.
  • In this region the Mediterranean type of climate is experienced because of shifting of permanent belts southwards and northwards with the overhead position of the sun.
  • During winters Westerlies prevail and cause rain.
  • During summers dry Trade Winds blow offshore and are unable to give rainfall in these regions. These seasonal changes in the relative positions of the pressure and wind belts intro­duce the following typical climatic conditions:
  • This belt extends upto 40° latitudes in the northern hemisphere at the time of summer solstice and in the southern hemisphere at the time of winter solstice. Thus, the western parts of the continents within the zone of 30°-40° latitudes do not receive rainfall during summer season.
  • Mediterranean regions are characterized by dry summers and wet winters and a typical Mediterranean type of climate is produced due to shifting in pressure belts.
• The regions lying between 60°-70° latitudes are characterized by two types of winds in a year because of shifting of pressure and wind belts.
  • With the northward migration of the sun at the time of summer solstice the polar easterlies are weakened during north­ern summer because the westerlies extend over these areas due to northward (poleward) shifting of sub-polar low pressure belt
  • While the situation is quite opposite in the southern hemisphere because the polar easterlies extend over much of the areas of the westerlies due to equator-ward shifting of sub-polar low pressure belt.
  • The situation is reversed at the time of winter solstice when there is southward migration of the sun.
  • Consequently, a typical climate characterized by wet summers through westerlies and associated cy­clones and dry winters due to polar easterlies is pro­duced.
• Monsoon climate:
  • Monsoon climate is the result of the shifting of pressure and wind belts. Due to northward migration of the sun in the northern hemisphere at the time of summer solstice the north intertropical convergence (NITC) is extended upto 30°N latitude over Indian subcontinent, south-east Asia and parts of Africa. Thus, the equatorial westerlies are also extended over the aforesaid regions.
  • These equatorial westerlies, in fact, become the south-west or summer monsoons. These south-west monsoon winds bring much rains because they come from over the ocean and are asso­ciated with tropical atmospheric storms (cyclones). The NITC is withdrawn from over the Indian subcon­tinent and south-east Asia because of southward shift­ing of pressure and wind belts due to southward migra­tion of the sun at the time of winter solstice.
  • Thus, north-east trades are re-established over the aforesaid areas. These north-east trades, in fact, are north-east or winter monsoons. Since they come from over the lands, and hence they are dry.