Global Patterns of Atmospheric Heating and Circulation
Global patterns of atmospheric heating and circulation influence the climate of particular locations at given times. One of the causes of air circulations on the earth’s surface has been the uneven irradiation of the earth’s surface by the sun. Due to the shape of the earth, much more solar radiation is received at the equatorial region and the area bound by the tropics than at the Polar Regions. Tropical region is the region bound by latitudes 30 degrees North and 30 degrees South while Polar Regions lie beyond latitude 60 degrees North and 60 degrees South. Also, the tropical region emit relatively less energy than they receive while at the polar regions, more energy is given off than it is received from the sun. Thus, the tropical region has an excess amount of energy while the Polar Regions have a deficit. Consequently, the difference in temperature causes air movement for a redistribution of the energy across the earth’s surface (Gabler, 2009).
Atmospheric circulation has been described using two models. In 1735, George Hadley, an English meteorologist described a single-cell model, which describes movements of cold and warm air. According to the model, warm air from low latitudes moves to the high latitudes and cold air from the high latitudes moves towards low latitudes. The phenomenon has been said to occur due to stronger heating at the equator. It causes air to rise and move towards the poles and then descends and returns to the equator.
Another circulation mode has been associated with the deflection of winds due to rotation of the earth causing winds to move from east to west. The occurrence known as Coriolis Effect is caused by difference in speed of rotation of earth’s surface at various latitudes. The speed is higher towards the equator and decreases towards the Polar Regions. At high latitudes, deflections are greater. Winds are deflected to the left of their initial path in the Southern Hemisphere and to the right in the Southern Hemisphere (Gabler, 2009).
Another model describing atmospheric circulation is the three-cell model. It sets apart the circulations into three cells in each hemisphere. The regions between latitude 60 degrees and 90 degrees to the north and south called the polar cell, Hadley cell bordered by latitudes 30 degrees to the north and south and Ferrel cell between 30 degrees and 60 degrees both north and south. Global winds develop as a result of differences in pressure caused by disparity in heat in the tropics and the Polar Regions. The high temperature causes warm air to rise and flow either to the cooler north and south to about 30 degrees before descending and flowing back to the tropics. Rising air creates low pressure at the equator and at 30 degrees the descending cold air creates high pressure. The region is thermally direct and the trade winds circulate creating the Hadley cell and form the Inter-Tropical-Convergence Zone (ITCZ).
In the Ferrel cell, there are westerly winds and at 60 degrees there is warm rising air and at 30 degrees latitude there is air sinking. The Coriolis Effect deflects the air to the north. The convergence of the air masses at 60 degrees and at low altitude causes air to rise and some air to flow back to 30 degrees latitude forming the Ferrel cell. However disruptions occur due to jet streams and earth depressions. The poles are thermally direct and form the Polar cell. Warm air spreads to the poles. In the North Pole air sinks and flows south where it is warmed by the earth’s surface at 60 degrees. The air spreads out as easterly winds. Atmospheric circulation forms an important heat distribution mechanism on earth (U.K meteorological Office, 2016).
What mechanisms produce high precipitation in the tropics?
Precipitation refers to “adiabatic cooling of ascending moist air masses” leading to formation of solid or liquid water droplets in the atmosphere (Oliver, 2005, 577). High precipitation occurs in zones where there is continual and rapid ascension of air masses. In the tropics, there is convergence of trade winds in the ITCZ which ascend due to the low pressure. Also, the evaporation rates are high. Thus, moist air rises as it cools. On the other hand, warm air is able to condense and hold more water leading to high precipitation (Schumacher, 2003).
What mechanisms produce high precipitation at temperate latitude?
It is at the temperate latitudes where convergence of moist subtropical air and cold polar air causes forced condensation. Also, the region receives the most intense sun radiation per unit area, thus, increased evaporation. Besides, the low pressure system causes a rise in more air for precipitation. Clouds build up due to the rising air that picks up moisture from the dry land at lower latitudes (Schumacher, 2003).
What mechanisms produce low precipitation in the tropics?
Mountainous regions in the tropical zone affect precipitation. High mountain ranges block the warm prevailing winds causing non-latitudinal differences in precipitation. Precipitation in this case occurs due to the flow of moist air over the mountains. The precipitation is largely due to storms caused by tropical cyclones, frontal systems or the convective clouds. Due to vertical differences in temperature and water vapor content, convective storms arise (Schumacher, 2003). A horizontal difference in atmospheric temperature leads to frontal systems and the variations in heat stored in the oceans cause tropical cyclones. Low precipitation occur when the mountainous terrain slow down warm prevailing winds and occasionally the trade winds. The slowed warm air condenses in the windward side leaving the leeward side with low precipitation (Houze, 2012).
Use what you know about atmospheric circulation and seasonal changes in the sun’s orientation to earth to explain the highly seasonal rainfall in the tropical dry forest and tropical savanna biomes.
Tropical dry forests and tropical savannas are found within the tropics, about 10 to 25 degrees North and South of the equator and are characterized by wet and dry seasons. Tropical savannas are covered with grassland and a few scattered trees while tropical dry forests have more trees that are green during rainy seasons and dry with loss of leaves in dry seasons. The spherical shape of the earth and the tilting on own its axis determines the intensity of sun’s rays reaching a given surface as well as the absorbed thermal energy. At the equator, the rays reach perpendicularly to the surface and hence the tropics are always warmer to the temperate regions. Towards the poles, the rays strike at an angle and energy spreads to a much larger surface area; therefore, the regions are cooler. As the earth revolves round the sun, insolation remains nearly constant at the equator but changes for higher latitudes (Ritter, 2006).
Although the regions of the two biomes are warm throughout the year, precipitation occurs seasonally in about five months per year. The phenomenon is caused by migration of winds causing the shifting of Inter Tropical Convergence Zone (ITCZ). When the sun is overhead and there is low equatorial pressure, it causes a warm and moist air to rise and high precipitation occurs. The ITCZ shifts between Northern and Southern hemispheres causing seasonal variations in pressure and hence precipitation. Thus in the dry season, ITCZ moves to the other side of the equator and warm and dry trade winds are flowing to the equator. The sun is also low and precipitation leading to a dry season (Ritter, 2006).
Gabler, R. E., Petersen, J. F., & Sack. DPhysical geography Belmont, CA: Brooks/Cole, Cengage Learning.
Houze, R. A. Orographic effects on precipitating clouds [Abstract]. Rev. Geophys. Reviews of Geophysics, 50(1), 1-47 (2012).
Oliver, J. E. Encyclopedia of world climatology. Dordrecht, Netherlands: Springer. Google Scholar(2005).
Ritter, M. E. The Physical Environment: an Introduction to Physical Geography.
Schumacher, C. Tropical precipitation in relation to the large-scale circulation (Doctoral dissertation, University of Washington).(2003).