- RAINFALL
Three factors are needed in the atmosphere to produce rainfall. These are:
- A supply of moisture which is provided when air passes over the sea or very wet land areas;
- A supply of nuclei, which is provided in the form of dust, ice and other small particles in the air
- A mechanism to start the process of rain formation.
The first two are nearly always present in the atmosphere so that actual rainfall depends on the third factor. The production of rain depends on the fact that when air is cooled below what is called its dew point, the moisture in the air turns from vapour into small droplets a process known as condensation. The dew point is a measure of the amount of moisture in the air, the higher the moisture the higher the dew point. Moist air will condense into droplets at a higher temperature than dry air.
What causes air to cool? The answer is that air cools when it rises and the factors which cause air to rise are:
- Convection which is the air being heated directly by the sun. This warm air expands rises and cools down and condensation takes place causing rain as isolated showers or thunderstorms. This process of convection causes the build-up of cumulus thunder clouds which often rise to a great height. The air currents in these clouds are so very strong that airplanes flying through them have been battered and wrecked.
- Orographic uplift is when a stream of air meets a piece of high ground (a hill or a mountain) and the air is forced to rise. When the air mass rises it is cooled down quickly, which raises the relative humidity and rain is produced. This causes the windward side of mountains (e.g. the side facing the prevailing wind) to be wet and the other side to be dry.
Figure 1: Shows how convectional rainfall is formed, water evaporates from soil or water.
Figure 2: Shows how orographic uplift/precipitation occurs.
- Convergence is the meeting of two separate air streams, resulting in the rising of air at the point where they meet. This causes wide-spread rain over the area where the convergence has occurred. The opposite of convergence is divergence.
In South Africa, the rain at the beginning of the season (summer rainfall) is caused by convection, and falls as thunderstorms which can be patchy.
Figure 3: Shows how convergence precipitation occurs.
The atmosphere above the earth is divided into areas of high and low pressures. In the Southern Hemisphere, winds spiral inwards in a clockwise direction towards the centre of the area of low pressure. Low pressure areas are convergent and cloudy and rainy. Around high-pressure areas, the winds travel anti-clockwise outwards and away from the centre; they are divergent, free of cloud and cause fine dry weather.
Figure 4: Shows the direction of surface winds for high and low pressure cells in the Northern and Southern Hemispheres.
THE INTER-TROPICAL CONVERGENCE ZONE
This is commonly called the I.T.C.Z. The I.T.C.Z. moves north and south with the sun – during the Summer in the Northern Hemisphere it is situated over the north of Africa, and during the Summer in the Southern Hemisphere, it is over the South.
Figure 5: Shows the inter-tropical convergence zone during the summer and winter months and how it affects rainfall.
| Orography: the branch of physical geography dealing with the formation and features of mountains. |
During the rainy season movements of high and low pressure systems off the coast of South Africa cause minor fluctuations of the I.T.C.Z. giving the typical short spells of dry and wet weather which we get during the rainy season, particularly the January drought which often occurs. When pressure rises over South Africa, South Easterly winds converge with the ITCZ and rain occurs, thereafter pushing the I.T.C.Z. to the north. When the I.T.C.Z. is drawn north, a spell of fine weather occurs.
Tropical moisture or troughs are pulled down to the south which is where South Africa gets most of its summer rainfall. This is in the form of convergence. Rainfall also comes in the form of convection rain which generally starts off the summer rainfall period. South Africa also receives ‘orographic’ rainfall, but only in places of the country mainly along the escarpment. The Western Cape however has a winter rainfall pattern and is caused by cold fronts (low pressure cells) pushing up from the south.
Figure 6: Shows a cyclone off the coast of Mozambique which pushes clouds (moisture) into the North Eastern side of South Africa giving some moisture. Depending on how big the cyclone is depends on how much rainy weather South Africa experiences and how far inland it will push.
Source: sawdis1.blogspot
Figure 7: Shows a cold front (low pressure cell) pushing up from a South Westerly direction.
Source: saweatherobserver.blogspot
VARIATIONS IN RAINFALL
Factors which cause variations in rainfall over a region are:
- The carrying capacity and moisture content of the stream of air covering the region;
- The distance of the region from a reliable source of moisture e.g. the sea or large bodies of water i.e.: Gariep dam; and
- The orography of the immediate and surrounding area of the region which is the most important factor for the unequal distribution of rainfall.
The greatest rainfall occurs on the mountain slopes facing the wind, but not necessarily along the highest mountains. The lowest rainfall is downwind of the hills and in the valleys. It has been found in Canada on rolling land that valleys can get some 20% less rainfall than the surrounding plains.
The greatest alterations to rainfall patterns occur in a very high or low rainfall season, and changes in patterns that do occur can be explained by changes in prevailing winds at high altitudes.
When the prevailing wind is not at right angles to the high ground, storms tend to follow the rising ground along the windward edge of the hills and not the valley bottoms. Drier winds during the rainy season are the most likely to produce hail storms.
