Seasonal Distribution of Pressure
Like temperature, seasonal distribution of pressure is studied with reference to two extreme months January and July. In the month of January, the sun’s rays are almost vertical on the Tropic of Capricorn in southern hemisphere. Hence, high temperature and low pressure conditions prevail slightly to the south equator. Winds from the sub-tropical high pressure belts blow towards the equatorial low pressure belt forming Inter Tropical Convergence Zone (ITCZ). The lowest pressure pockets exist on the land masses of South America, South Africa and Australia, because land masses become much hotter than the adjoining oceans. Sub-tropical high pressure belt of the southern hemisphere is broken over the continents and remains conned to the oceans only. Its development is maximum in the eastern hemisphere.
The horizontal movement of air is called wind. Wind is caused by the horizontal differences in atmospheric pressure. Wind velocity depends upon pressure gradient. The rate of change of pressure per unit distance is called pressure gradient force. The greater the pressure gradient and its force, higher will be wind speed. The direction of winds is influenced by the rotation and relief of the earth. If the earth were stationary and had a uniform surface, air would low directly from high pressure areas to low pressure areas. But the reality is that earth is neither stationary nor uniform in its surface configuration. The rotation of the earth causes deflection in the path followed by winds. The force which causes this deflection is called Coriolis effect or Coriolis force after a French mathematician Gaspard de Coriolis, who demonstrated it in 1844.
Under the influence of Coriolis force, winds get defeccted to their right in northern hemisphere and to their left in the southern hemisphere. This concept was developed by W. Ferrel in 1855 and is known as Ferrel’s Law. This phenomenon can also be understood by Buys Ballot’s Law put forward by Buys Ballot in 1857. According to this law, “If an observer stands with his back to the wind in the northern hemisphere, pressure is lower on his left hand than on his right; in the southern hemisphere, pressure is lower on his right.” It is noteworthy that Coriolis force changes wind direction but not its speed. Coriolis force defects winds to a different extent on different latitudes. Its effect is zero at the equator. In other words, there is no deflection in the wind direction at the equator. This deflection is maximum at the poles. If we assume that there is 100% deflection at the poles then the deflection at 30° and 60° latitude will be 50% and 86.7% respectively.
Following three types of winds are normally recognised on the surface of the earth:
- Planetary winds
- Seasonal or Periodic winds
- Local winds
These are also known as permanent or prevailing winds. Planetary winds blow over vast areas of continents and oceans throughout the year in a particular direction. They blow regularly from high pressure belts to low pressure belts (Fig.3.8). Hence, planetary winds are divided into three categories according to pressure belts and temperature zones:
The Trade Wind
The winds blowing from the sub-tropical high pressure areas (30° N and 30°S) towards the equatorial low pressure belt are the extremely steady winds known as the trade winds. e name trade comes from the German word trade meaning ‘track’. To blow trade means to blow steadily in the same direction and in a constant course. Its nomenclature has nothing to do with the English word trade, which means business or commerce. Trade winds are deflcted to the right in the northern hemisphere and to the left in the southern hemisphere. Thus, they low as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere. That is why they are known as ‘easterlies’ also.
Since they travel from high latitude to low latitude areas, they become gradually hot and dry and do not cause much rainfall. However they cause heavy rainfall on the eastern margins of the continents because they get moisture after blowing over the oceans. These winds converge near the equator and Inter Tropical Convergence Zone (ITCZ) is formed. Here the winds rise and cause heavy rainfall.