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Smog
Semi-arid climate
A climate of low rainfall characterised by irregular and unpredictable rainy seasons.
Semi-arid regions often form on the edges of deserts or in rain- shadow basins. They are more risky than deserts because, from time
to time, there is enough rainfall
to support farming, which provides hope and encouragement to farmers. As a result, they may over-develop their land. Then, when the weather changes to a period of less rain (often described as ‘when the rains fail’), catastrophe may follow.
The Sahel of Africa is a region
of semi-arid climate with a large population where disasters due to rain failures are common. The Great Plains region of North America is also a semi-arid region. The disaster that produced the Dust Bowl was due to a period of low rainfall and over-development.
Settled weather
When the sky is clear and sunshine (in warm weather) or fog (in cold weather) is most likely. (See also: Anticyclone.)
Siberian high
The high positioned over north and central Asia in winter.
Simoom
The Arabian equivalent of the Harmattan wind. Simoom means ‘poison wind’.
Sirocco
A dry, hot wind from the Sahara, which can blow dust and sand across the Mediterranean. It is equivalent to the khamsin. It is pulled north into the warm sector of depressions moving over the Mediterranean Sea during autumn and spring. Air temperatures may exceed 40°C and approach 50°C.
Sleet
A form of freezing rain. Sleet is made of small pellets of ice that form when raindrops fall through a layer of air whose temperature is below freezing.
Smog
A fog or haze intensified by atmospheric pollution. Smog affected by sunlight is called photochemical smog. The term was first applied to the fogs
that plagued industrial cities such as Pittsburgh and London in the coal-burning era of the 19th and early 20th centuries. It is now used for the photochemical smogs that develop over cities such as Los Angeles and Mexico City. These cities have sunny weather and lie in basins that block most winds. The main polluters are motor vehicles.
In December the Sun is overhead just over 22°S of the equator, and it warms the Southern Hemisphere much more strongly than the Northern Hemisphere. As a result, the equatorial low is centred about 10°S of the equator, and all of the weather belts adjust accordingly, squashing together in the south and stretching out in the north.
Overhead (high-angled Sun)
Sun
Low-angled Sun
Mid-latitude westerly winds Polar high Subtropical highs
Warm air from the tropics meets cold air from the poles in this region of the mid-latitudes. Here, the weather is very changeable, with periods of cloud and rain mixed with settled weather.
In this region of subtropical highs the air sinks throughout the year, and little rain falls. This is where most deserts are found.
Cold air sinks over the pole and moves back to the equator.
Equatorial low
Subtropical highs
Low-angled Sun
Mid-latitude westerly winds Polar high
Rising moist air over tropical oceans causes thunderstorms nearly every day.
This region is so cold that little snow falls. However, snow that does fall rarely melts, and so snow and ice build up to cover the surface.
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