Salient features of the pressure distribution in January



Because of the apparent movement of the sun towards the Tropic of Capricorn, the equatorial trough of low pressure is displaced well to the south of the equator. This shifting of low pressure belt is well marked over the warm continents of the southern hemisphere.

January being one of the summer months in the southern hemisphere, the continents of Australia, Africa and South America are relatively warmer, and there are distinct low pressure cells over them.

The sub-tropical belt of high pressure in the northern hemisphere is located well to the north, especially over the cold continents. In fact, the isolated cells of high pressure are united to form a continuous belt.

The high pressure cells over the Pacific and Atlantic oceans become weaker in summer. High pressure exists over North America and Eurasia. In the northern hemisphere the north-eastern part of Siberia in Asia becomes the coldest region on the earth's surface with highest pressures.

During the summer of the southern hemisphere, the sub-tropical high pressure belt lies at latitudes 30°-40°S, with centers over the pacific, Atlantic, and Indian oceans.

In the northern hemisphere, because of large continental areas the sub-polar low pressure belt is represented by individual oceanic cells and their continuity is broken.

There are two low pressure cells found over the north Atlantic and the north Pacific oceans, and are known as the Icelandic Low and Aleutian Low respectively.

In the southern hemisphere, since there are no land masses in the high latitudes, there is a well-developed circumpolar belt of low pressure.

The air-pressures observed over the coastal regions of the Antarctica and the southern oceans clearly indicate that beyond latitude 65"S, there is a gradual increase in atmospheric pressure. The south-easterly winds from the continental land mass of Antarctica confirm the above assumption.

In the east-central part of Eurasia a very extensive and deep high-pressure cell is established. North America being smaller in size than the vast land mass of Eurasia, the high-pressure cell over there is not so well developed.