Atmospheric pressure and temperature are mainly responsible for our weather systems. Atmospheric pressure is the pressure that the air exerts on the surface of the earth and everything on it. It changes with altitude and temperature. The air pressure in Johannesburg is less than that at the coast. When the air is heated it expands, becomes less dense than the surrounding air and rises. In an area where this happens the atmospheric pressure drops. This difference in atmospheric pressure in adjacent areas results in the formation of two pressure systems, namely depressions and anti-cyclones.

Depressions. A depression is an area of low pressure surrounded by relatively high pressure. The isobars (lines of equal pressure) around it are circular or oval shaped. The horizontal circulation of wind around the centre of the low is clockwise and inwards in the southern hemisphere. The lower the central pressure and the closer the isobars, the stronger the wind accompanied by adverse weather. Lows move from west to east. They are generally associated with cloudy, unsettled weather and strong winds.

Depression in the southern hemisphere.

Anti-cyclones. An anti-cyclone is an area of relatively high pressure surrounded by areas of much lower pressure. The isobars around the centre are more or less circular. The horizontal circulation of wind is anti-clockwise and outwards from the centre. In general, anti-cyclones are usually associated with light winds and fine weather. They move very slowly but like depressions in an easterly direction. The weather associated with an anti-cyclone is often fine and sunny though at times the weather can become gloomy and the sky overcast. Winds are usually light or moderate in strength and rain is unlikely. Radiation fog often forms over land at night and affects harbours and coastal areas.

Anti-cyclone in southern hemisphere.