World Biomes. Deserts. Global Air Circulation. Climate.
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Dry desert climates are formed by high-pressure zones in which cold air descends. Then the descending air becomes warm but, instead of releasing rain, the heat from the ground evaporates the water before it can come down as rain. The ground is super hot because the sun's rays beat down on it directly overhead. Not a lot of atmosphere to protect it from radiant energy.
Approximately 0.25 cm of rain falls in dry deserts per year. The average annual temperature of these miles of hot sand is 18° C.
The latitude range is 15-28° north and south of the equator. Their global range covers about 1/5 of the earth, including the world's great deserts: Sahara, Sonora, Thar, Kalahari and the Great Australian.
There is poor development of horizons, with accumulation of calcium carbonate at or near the surface. Sparse vegetative cover and tiny leaves results in little humus and soils typically have a light gray colour.
Plants of the Dry Desert have adapted to the lack of water by using dew for moisture and taking in water through their leaves and stems.
Shrubs are the dominant growth form of deserts. They may be evergreen or deciduous; typically have small leaves; and frequently have spines or thorns and/or aromatic oils. Shallow but extensive root systems procure rainwater from well beyond the canopy of the shrub whenever it does rain.
Another growthform adapted to desert conditions is the ephemeral. This is an especially short-lived annual plant that completes its life cycle in two-three weeks. The seeds are encased in a waterproof coating that prevents desiccation for years if necessary.
Perennial plants with underground bulbs store nutrients and water in underground tissues and also remain dormant most of the year. They can sprout rapidly after sufficient rains and replenish their underground stores.
Behavioral adaptations such as being active during night and staying the shade during the heat of day are common.
Morphological adaptations. The better to radiate body heat to the environment from warm-blooded animals, body sizes are small. Bodies are light coloured to reflect sunlight and help prevent the absorption of heat form the environment.
Rarer, but important, are physiological adaptations such as dormancy during summer, the absence of sweat glands, the concentration of urine, localized deposits of fat in tails or humps; and salt glands to secrete salt without loosing fluids.
Reptiles with their waterproof skin, production of uric acid instead of urine, hard-shelled eggs, and ability to gain body heat directly from the sun and to retreat to shade or underground to avoid heat are exceptionally well adapted to drylands.