CSET Requirement 2.3d: Identify the bands at specific latitudes where rainforests and deserts are distributed and the causes of this pattern.
Desert Distribution
There are four main, interlinked causes of hot deserts:
Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure called the subtropical high, is an area which receives little precipitation and has variable winds mixed with calm.The consistently warm, dry conditions of the horse latitudes also contribute to the existence of temperate deserts, such as the Sahara Desert in Africa, the southwestern United States and northern Mexico, and parts of the Middle East in the Northern Hemisphere; and the Atacama Desert the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.
A rain shadow is an area of dry land that lies on the leeward (or downwind) side of mountains. Winds carry air masses up and over the mountain range and as the air is driven upward over the mountain, falling temperatures cause the air to condensate and lose much of its moisture as precipitation. Upon reaching the leeward side of the mountain, the dry air descends and picks up any available moisture from the landscape below. The resulting profile of precipitation across the mountain is such that rainfall and moist air prevails on the windward side of a mountain range while arid, moisture-poor air prevails on the leeward side of the mountain range normally creating desert areas (Death Valley lies in the rain shadow of the Pacific Coast Ranges of California and the Sierra Nevada Mountains.)
Global currents with direction, temperature and latitudinal location also play an important role in desert formation. The Northern and Southern Hemispheres contain surface and deep ocean currents that affect global climates and in particularly in this case - the Horse Latitudes. Water from polar regions sweeps towards the equator along western coasts of continents. These cold currents cool the air, forcing it to rain over the sea, causing deserts to form at the coast. When warm air currents meets cold seas, moisture reaches the coast by fog or in the form of June Gloom in Southern California. The rainfall does not penetrate inland and forms deserts.
- The formation of the subtropical high-pressure cell (Horse Latitudes)
- The rain shadow effect in the belt of easterly trade winds.
- The effect of the cold currents off the west coast of the continents at these latitudes. (California Current; for example)
- The depositing sands of a desert along its border into the fertile land
Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure called the subtropical high, is an area which receives little precipitation and has variable winds mixed with calm.The consistently warm, dry conditions of the horse latitudes also contribute to the existence of temperate deserts, such as the Sahara Desert in Africa, the southwestern United States and northern Mexico, and parts of the Middle East in the Northern Hemisphere; and the Atacama Desert the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.
A rain shadow is an area of dry land that lies on the leeward (or downwind) side of mountains. Winds carry air masses up and over the mountain range and as the air is driven upward over the mountain, falling temperatures cause the air to condensate and lose much of its moisture as precipitation. Upon reaching the leeward side of the mountain, the dry air descends and picks up any available moisture from the landscape below. The resulting profile of precipitation across the mountain is such that rainfall and moist air prevails on the windward side of a mountain range while arid, moisture-poor air prevails on the leeward side of the mountain range normally creating desert areas (Death Valley lies in the rain shadow of the Pacific Coast Ranges of California and the Sierra Nevada Mountains.)
Global currents with direction, temperature and latitudinal location also play an important role in desert formation. The Northern and Southern Hemispheres contain surface and deep ocean currents that affect global climates and in particularly in this case - the Horse Latitudes. Water from polar regions sweeps towards the equator along western coasts of continents. These cold currents cool the air, forcing it to rain over the sea, causing deserts to form at the coast. When warm air currents meets cold seas, moisture reaches the coast by fog or in the form of June Gloom in Southern California. The rainfall does not penetrate inland and forms deserts.
Rainforest Distribution
The map to the left shows the location of the world's tropical rainforests. Rainforests cover only a small part of the earth's surface - about 6%, yet they are home to over half the species of plants and animals in the world. Tropical rainforests are located in a band around the equator (Zero degrees latitude), mostly in the area between the Tropic of Cancer (23.5° N latitude) and the Tropic of Capricorn (23.5° S latitude). This 3,000 mile (4800 km) wide band is called the "tropics." Temperatures at the equator are high. These high temperatures cause accelerated evaporation of water, which results in frequent rain in forested areas in the tropics.
In an average year in a tropical rain forest, the climate is very humid because of all the rainfall, which amounts to about 168 to 1,000 cm (66 - 390 inches) per year. The rain forest has lots of rain because it is very hot and wet. This climate is found near the equator. That means that there is more direct sunlight hitting the land and sea there than anywhere else. The sun warms the land and sea and the water evaporates into the air. The warm air can hold a lot of water vapor. As the air rises, it cools. That means it can hold less water vapor. Then as warm meets cold, condensation takes place and the vapor forms droplets, and clouds form. The clouds then produce rain. It rains more than ninety days a year and the strong sun usually shines between the storms. The water cycle repeats often along the equator. The main plants in this biome are trees. A lot of the rain that falls on the rain forest never reaches the ground. It stays on the trees because the leaves act as a shield, and some rain never gets past the trees to the smaller plants and grounds below.
In an average year in a tropical rain forest, the climate is very humid because of all the rainfall, which amounts to about 168 to 1,000 cm (66 - 390 inches) per year. The rain forest has lots of rain because it is very hot and wet. This climate is found near the equator. That means that there is more direct sunlight hitting the land and sea there than anywhere else. The sun warms the land and sea and the water evaporates into the air. The warm air can hold a lot of water vapor. As the air rises, it cools. That means it can hold less water vapor. Then as warm meets cold, condensation takes place and the vapor forms droplets, and clouds form. The clouds then produce rain. It rains more than ninety days a year and the strong sun usually shines between the storms. The water cycle repeats often along the equator. The main plants in this biome are trees. A lot of the rain that falls on the rain forest never reaches the ground. It stays on the trees because the leaves act as a shield, and some rain never gets past the trees to the smaller plants and grounds below.