1. Describe global patterns of atmospheric heating and circulation.
What mechanisms produce high precipitation in the tropics?
What mechanisms produce high precipitation at temperate latitudes? What mechanisms
produce low precipitation in the tropics?
Global patterns of atmospheric heating and circulation are based on the three-cell model, which
stipulates that there are three cells between the Earth’s poles and equator that regulate this
behavior. These three cells are the polar cell, which operates between the latitudes of 60 degrees
and 90 degrees, the Ferrel cell, which operates between the latitudes of 30 degrees and 60
degrees, and the Hadley cell, which operates between the latitudes of 0 degrees and 30 degrees.
The Hadley cell, which is responsible for trade winds, begins with warm air rising at the equator
in low pressure areas moving up to 30 degrees latitude until it descends in high pressure areas. In
the polar cell, which is responsible for the polar easterlies, at lower latitudes, warm air rises and
moves to higher altitudes, where the cooled air descends due to high pressure. The Ferrel cell is
more complicated, and has warm air rising at higher latitudes near the polar cell and cooling as it
moves to lower latitudes and descending in high pressure areas in these lower latitudes near the
Hadley cell.
High precipitation in the tropics is caused by warm air from both hemispheres converging in the
Inter-tropical Convergence Zone and the rise of this air due to low pressure, which causes
convection in the atmosphere.
In temperate zones, high precipitation occurs as a result of a number of factors, including warm
fronts, cold fronts, stationary fronts, and low pressure systems. In tropical zone, low precipitation
can occur as a result of warm, prevailing winds being impeded by mountain ranges since the
mountains force the moisture in those winds to come down on the mountainside that is blocking
the winds as rain. This leaves the other side with low precipitation.
2. 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. (Hint: Why does the rainy season in these biomes come during the
warmer months?)
Global pressure systems have a significant role in the geographic distribution of precipitation.
The global pressure system affects the relationship between pressure patterns and the distribution
of precipitation. The biomes of tropical dry forests and tropical savannas correspond to their
climatic zones. The geographical location of the tropical dry forest, up north and down south of
the equator causes that the tropical dry forest to experience more climate variation than the
tropical rain forest. The tropical dry forest has dry and wet seasons however; it rains less than in
the rainforest. The tropical savannas also have alternating dry and wet seasons like the tropical
dry forests except that they are dominated by grasslands.
The geographical area that receives most of the direct daylight is the Inter-Tropical Convergence
Zone (ITCZ), the ITCZ receives direct sunlight causing the air to become unsteady rise, and
causing the pressure to drop, which makes the ITCZ an area of low pressure because of the high
temperatures and unsteadiness in the air once the air rises cools and condense form clouds, and
causes precipitations. The moving of the inter-tropical convergence zone (ITCZ) affects seasonal
precipitation patterns, because the ITCZ is always in an area where the sun posses directly
overhead, in the summertime is posses directly overhead the tropics. The moving of the intertropical convergence zone (ITCZ) affects seasonal precipitation patterns, in the areas of tropical
climate where the sun is not directly overhead the areas as of high pressure and are not
advantageous to cloud production; therefore, they cannot produce rain.
The areas were the sun is not overhead are call doldrums, and constitutes the north and the south
of the Inter-Tropical Convergence Zone, since they move seasonally parallel the doldrums are
accountable for the dry season in the tropics. This explains the phenomenon of the excessive rain
in the tropical rainforest, and tropical monsoon in Africa because they are close to the equator,
therefore, close to the ITCZ during most of the year favoring the formation of clouds and
precipitations.
Because of its geographical location on either side of the rainforest the wet and dry forests are
under the ITCZ for longer periods of time than the savannahs that are located in the northern
hemisphere north of the dry forests and in the southern hemisphere the south of the dry forests.
The Savanna climate is considered the opposite of the monsoon and rain forest typical weather,
which is the Tropical Wet/Dry climate. The raining in the Savanna climate is also seasonal but
much less, and during the low-sun season it experiences dry conditions. The geographical
location is one of the aspects that influence rain in savannahs and dry forests; there are other
factors that may affect rain in those areas such as rain shadows and ocean currents.