Climate system: Desert and ice
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The large-scale patterns of atmospheric circulation cause an uneven distribution of rainfall, with more falling near the Equator and at mid-latitudes than in the subtropics or at the poles. Most deserts are located in subtropical latitudes. Although at first sight they seem very different, deserts and ice sheets have significant features in common: both tend to be very dry, with clear skies, high atmospheric pressure and high albedo – cooling the planet by reflecting sunlight.
The tropical oceans are the source of most of the world’s rainfall, two-thirds of which falls near the Equator. The easterly ‘trade winds’ in both the northern and southern tropics converge near the Equator, in a band known as the Intertropical Convergence Zone. The winds carry moisture evaporated from the sea and the intense tropical sunlight leads to intense convection, causing the humid air to rise. As the air rises it cools, so the moisture condenses on particles of dust or ice suspended in the air, forming water droplets and clouds. The high heat and humidity in this zone leads to the formation of towering thunder clouds known as cumulonimbus, which can stretch several kilometres high and hold huge quantities of water, resulting in heavy downpours during tropical storms.
Warm rising air creates an area of low pressure below. Air tends to flow from high to low pressure, so a central column of rising air pulls in more air from its surroundings. The Coriolis effect caused by the Earth’s rotation makes airflow curve round to the right in the northern hemisphere. As a result, the air flowing into a low-pressure system causes it to spin anticlockwise. Such systems spin clockwise in the southern hemisphere. Under certain conditions, several thunderstorms can start rotating around a single low-pressure area and become a tropical storm, fuelled by heat and moisture from the oceans. If ocean surface temperatures are 26 °C or more, a storm can intensify into a hurricane, with wind speeds of up to 300 km/h.
Some areas in the lower mid latitudes – such as the Mediterranean – have dry seasons. But most mid-latitude regions have temperate climates with relatively high levels of yearly rainfall. Temperatures at these latitudes aren’t as high as in the tropics, so there’s less evaporation from the oceans and consequently less moisture and convection. However, at about 60 degrees latitude the warmer mid-latitude air meets cold air flowing from the poles, creating a band known as the polar front. This boundary between contrasting temperature zones leads to rain and storminess. In the winter, cold air masses build up over the land while oceanic air masses – which are less affected by the seasons – remain warmer. This temperature contrast leads to more frequent and intense mid-latitude storms in winter.
Most of the world’s deserts are located in the subtropics, between about 20 and 30 degrees latitude in both hemispheres. These include the Sahara and Arabian Deserts in the Northern Hemisphere and the Kalahari and Great Sandy Deserts in the southern hemisphere. The subtropical latitude bands are very dry all year round as a result of the large-scale tropical convection cells. The warm air rising at the Equator loses most of its moisture as it flows north and south. By the time it sinks to the surface at about 30 degrees latitude, the air is very dry. The downward airflow also causes high pressure, preventing much cloud formation, which is why the subtropical deserts typically have clear skies.
The air in polar regions is so cold it can’t hold much moisture, leaving these regions among the driest in the world. The very low temperatures enable the little snow that does fall to accumulate and build up into thick ice sheets. But parts of the Arctic and Antarctic are classified as deserts, receiving less than 25 cm of precipitation each year – about the same amount as the Sahara. There are other similarities between polar and subtropical desert climates, notably their relatively high albedos owing to the prevalence of reflective surfaces. Fresh snow reflects about 80% of the sunlight hitting it, while older snow reflects 40–70% and sand reflects up to 40%. Both polar and subtropical deserts also typically have high air pressure and clear skies.
The northern ice cap is centred on the Arctic Ocean. There is a substantial land-based ice sheet on Greenland – more than 4 km thick in parts – but most of the Arctic is water covered with a thin layer of seasonally varying sea ice. In contrast, the Antarctic is a continent with a massive land-based ice sheet up to 4.5 km thick. Antarctica is surrounded by the Southern Ocean, large areas of which freeze over in the winter, almost doubling the ice cap’s size, before melting back to the coast each summer. Strong ‘circumpolar’ winds and ocean currents also surround the Antarctic, isolating the continent and its ice sheets from much of the outside world’s weather. These factors make the massive southern ice sheet far more stable than its northern cousin.