Seasonal Distribution of Pressure

  • The variation of pressure from place to place and from season to season over the earth plays an important role in affecting the weather and climate.
  • Therefore pressure distribution is studied through isobar maps.
  • While drawing isobar maps, the pressures of all places are reduced to sea level to avoid the effect of altitude on air pressure.

(i) January Conditions

  • In January, with the south-ward apparent movement of the Sun, the equatorial low pressure belt shifts a little south of the mean equatorial position (see fig.).
  • Areas of lowest pressure occurs in South America, Southern Africa and Australia.
  • This is because the land tends to get hotter rapidly than water.
  • Sub-tropical high pressure cells are centered over the ocean in the southern hemisphere.
  • The belt of high pressure is interrupted by the continental land masses where the temperature is much higher.
  • They are well developed in eastern part of the ocean where cold ocean currents dominate.

Pressure and Pressure belts

  • The Aleutian and Icelandic lows disappear from the oceans while the landmasses, which developed high pressure during winter months, have extensive low pressure cells now.
  • In Asia, a low pressure develops. The subtropical highs of the northern hemisphere are more developed over the oceans – Pacific and Atlantic.
  • In the southern hemisphere, the sub-tropical high pressure belt is continuous.
  • Sub-polar low forms a continuous belt in the southern hemisphere while in northern hemisphere, there is only a faint oceanic low