These two pressure belts lie at about 30°North and South of Equator and are extended up to 60º North and South. Hot air rising from the Equatorial low pressure area spreads at a high level and flows towards the Poles. Difference in the speed of rotation of the Earth also affects the masses of air moving from the Equator and the Poles. On reaching latitudes 30ºN and 30ºS, the hot air becomes cool, dense and heavy. Consequently, the ascending warm equatorial air currents descend as cool and heavy air current and thus, descent of the winds result in the contraction of their volume and ultimately cause high pressure. From these two high pressure regions of calm, two surface winds originate – the ‘Trade winds’ blow toward the Equatorial doldrums and the ‘Westerlies’ blow Pole wards.
These zones are characterized by anticyclone conditions with feeble wind, which cause atmospheric stability and aridity. This is one of the reasons for the presence of hot deserts of the world within these zones, in both the hemispheres.
The Sub Tropic High Pressure zones of calm are popularly known as ‘Horse latitudes’. In ancient times, the sailing merchant ships carrying horses had to throw out some of the horses while passing through this zone of calm, in order to lighten their ships and to conserve drinking water as well, when the ships passage was unduly prolonged.