What is a jet stream?

It is a narrow band of strong winds in the atmosphere that controls the movement of high- and low-pressure systems and associated fronts. Wind speeds can reach 380 km/h or higher in certain cases. Jet streams are usually found at 9 - 12 km (30 000 - 40 000 ft) above the surface. It owes it existence to the large temperature contrast between the polar and equatorial regions.

What is an urban heat island?

An urban heat island is an urban area which is significantly warmer than its surroundings. This occurs because roof and paving materials have a low reflectivity and absorb more of the sun's rays than natural surfaces, causing both surface temperature and overall ambient air temperature in an urban area to rise. In addition, there are fewer trees and other natural vegetation to shade buildings, block solar radiation and cool the air by evapotranspiration in urban areas. The urban heat island effect is characterized by city centres commonly having temperatures six degrees higher than that of the surrounding environment.

What is a berg wind?

A berg wind is a hot, dry wind blowing off the interior plateau of South Africa, roughly at right angles to the coast. Berg winds usually occur in winter when there is a low-pressure system along the coastal areas with a high-pressure system over the continent.

What is frost?

Frost is formed in the same way as dew, but occurs when the temperature of the air in contact with the ground is below the freezing point of water (0 ºC). The water vapour changes directly from a gas to a solid state to form tiny ice crystals. Like dew, frost forms during calm, clear nights when the ground and objects near it cool rapidly because the earth’s heat is lost to space.

In South Africa light frost can be expected when the minimum temperatures reach 3 ºC. This is because on a calm night, the ground, where the frost forms, is usually several degrees cooler than the level at which temperature is measured (about 1,2 m above the ground) and thus the ground will probably be at, or below, freezing.

What is black frost?

Black frost occurs when the temperature is below zero but there is not enough moisture in the air to produce visible frost. Its name is derived from the resulting blackened appearance of affected vegetation.

What is dew?

Dew forms when water vapour condenses on objects and solid surfaces on or near the ground because these have cooled during the night time below the dew-point temperature of the air around them. For dew to form, however, there must be moisture in the air and the night must be clear (cloudless) and fairly windless so that the earth can radiate its heat energy out into space.

What is fog?

Fog is a suspension of usually very small water particles, but also sometimes ice particles, that reduces visibility at the surface to distances less than 1 km. Although fog is not usually considered to be a cloud, it has essentially the same properties. The two phenomena differ only in extend that the base of fog is at the ground, whereas clouds are above the ground.

What is smog?

Smog is a mixture of smoke and fog which contains large amounts of soot, sulphur dioxide and other pollutants which act as nuclei around which water condenses.