1) Seasonal distribution of rainfall and climate zones
 

The above figure (Fig.1) shows the climate zones on an idealized continent. Many of the features discussed below are based on the seasonal distribution of rainfall over the globe which we talked about in class.  You can find this animation at  http://tao.atmos.washington.edu/legates_msu/index.html (Scroll down to Analyses).
 

• The equatorial belt (often referred to as the Intertropical Convergence Zone, or ITCZ) experiences ample rain year round. It's wetter and wider about the equator on the eastern side of the continent where the trade winds carry greater amounts of moisture from the ocean onto the land. Most land areas within 5-10 degrees of the equator thus receive copious year-round due to the more or less constant presence the ITCZ (loosely defined over land). Temperatures remain nearly constant year round at around 27°C.  This dependably warm, moist environment supports the diverse ecosystems that exists within the tropical rain forests of the Amazon, equatorial Africa, and Indonesia. 

• Proceeding poleward to 15-20 degrees latitude, we encounter the monsoon climates-- so named because they are characterized by well defined wet and dry seasons.('monsoon' comes from the Arabic word for season.)  The rainy season is confined to a few months centered during summer when the equatorial rain belt extends far enough poleward to encompass the region (see figure 4-16 in textbook). The hottest weather of the year occurs just prior to the onset of the summer monsoon rains.  Winters in the monsoon belt are a few degrees cooler than summer, but they still have a tropical feel to them.  Regions with monsoon climates include India, central America, Subsaharan Africa and northern Australia. 

• Proceeding poleward to the belt centered near 30 degrees we encounter the desert climates which dominate all the western continental regions and extend westward across vast expanses of the oceans as well.  The Sahara, Arabian, Namib, Atacama, Great Sandy (Australian) and Kalahari deserts all lie within this belt (see Fig. 4-24 in textbook).  Notable exceptions to the prevailing dryness of the subtropics are the humid subtropical climates of the eastern continents, (e.g., the southeastern United States and China, the east coast of Australia, northern Argentina, Uruguay and southern Brazil). Frequent incursions of warm, moist air on the west side of the subtropical oceanic anticyclones provide the moisture for summer rainfall to these regions and winter storms passing poleward of them bring winter rains. 

• Proceeding poleward into the westerly belt, the dry summers persist up to latitudes of around 45 degrees, but extra tropical cyclones embedded in the westerlies bring winter rains that tend to be especially heavy over land on the west slopes of mountain ranges. Because of the pronounced summer dry season, the western sides of continents tend to receive most of their annual rainfall during winter. In contrast, there is year round rainfall on the eastern sides of continents.  The dry summers in the west tends to limit the extent of forests to the higher altitudes, where temperatures aren't as hot (see below). 

• Poleward of 45 degrees these seasonal contrasts in rainfall are less pronounced and they are less easy to characterize in terms of an idealized continent. 

We viewed in class an animation of temperature over the U.S. as a function of season, you can access this animation by going here: http://tao.atmos.washington.edu/pacs/legates.temp.movie.mpeg. We also looked at an animation of temperature over the Northern Hemisphere, you can access this animation by clicking here: http://tao.atmos.washington.edu/pacs/legates.temp.movie2.mpeg. 

• Latitude. Annual mean temperature decreases with latitude because at higher latitudes solar radiation strikes the earth at a more oblique angle.  The temperature contrast between equator and pole is much stronger during winter than during summer: in the region of the polar night, there isn't any solar radiation at all, whereas during summer, the long hours of daylight in the polar regions compensate for the fact that the sun is lower in the sky.  It follows that the seasonal temperature contrast is much larger in high latitudes than in low latitudes (see Fig. 4.1 on p. 57-58 of the text; and animation above).  The major climate belts discussed in Tuesday's lecture (i.e., the tropical rain areas; the tradewinds; the subtropical deserts; and the westerlies) can also be characterized in terms of the latitude ranges that they occupy.

• Land-Ocean Configuration. The higher heat capacity of oceans damps seasonal temperature range, as is clear from the third panel of Fig. 4.1 (p. 58) in the text.  Water has much higher heat capacity than soil and because it is a fluid capable of undergoing convection, it mixes heat downward through a much deeper layer.   Over land, heat is transferred downward from the surface (or upward from below) only by conduction, which is a much less efficient process than convection.  That's why temperatures in caves just 10 m below the surface stay nearly constant year round, while temperatures 100 m deep in the ocean vary with season almost as much as the sea surface temperature does.  The west coasts of continents have milder summer climates than east coasts because the circulation around the oceanic subtropical anticyclones carries cool air equatorward along their eastern flanks (e.g., along the coasts of Washington, Oregon, northern California and Chile) and warm humid air northward along their western flanks (e.g., into China, Japan and the eastern United States).  These temperature contrasts are reinforced by the wind driven ocean currents (Fig 5.3, p. 81) and by the coastal upwelling discussed in the next lecture. Prevailing onshore winds carry moisture which enhances rainfall.  For example, the heavy winter rains in the Pacific Northwest are a reflection of the strong onshore flow of moist air.  Whenever the flow  switches to offshore, the rain stops.  The onshore flow from the Gulf of Mexico is the major moisture source for the abundant year round rainfall over the central and eastern United States. Were it not for the flow off the Gulf, the Great Plains would have a semi-arid climate. 

• Mountains. Higher altitudes experiences colder climates.  There are glaciers and year round snow fields even in the equatorial belt.  In general , higher altitudes experience more of their precipitation in the form of snow.  Windward slopes of mountain ranges tend to have wetter climates, as evidenced by the widely contrasting rainfall regimes within the state of Washington (see Fig. 2 below).  You can contrast Fig. 2 with the topography map in Fig. 3 and see how precipitation follows the topography. Mountains can act as barriers, blocking cold air masses, and the flow of moisture into landlocked regions.  The Cascades often prevent bitterly cold Arctic air masses from reaching Puget Sound with full force and in summer they prevent eastern Washington from experiencing the cool 'marine pushes' that bring the Puget Sound area relief from summer heat.  The blocking effect of mountain ranges is responsible for the existence of Kara-Kum, Taklimakan, and Patagonian deserts (Fig. 4.24 in textbook). 

Fig. 2  Average annual precipitation in the state of Washington. (obtained from the Western Regional Climate Center, http://www.wrcc.dri.edu/precip.html)
 
 

Figure 4 shows how the various vegetation regimes (desert, tundra, grassland, and forests) relate to annual mean temperature and rainfall.  Since water is required to sustain life, deserts prevail for very low rainfall, regardless of how warm or cold the temperature is.  Tundra prevails for annual mean temperatures below freezing, because these conditions favor permafrost, which only supports tundra. 

By moving straight up in the diagram you can see that at any given temperature more rainfall is required to support forests than to support grasslands.  By moving from left to right on the diagram you can see that for a given amount of rainfall, forests give way to grasslands as the annual mean temperature rises and if it gets hot enough, grassland eventually gives way to desert.  These changes reflect the fact that the rate at which water evaporates at the Earth's surface increases with temperature.  Just as indoor plants need to be watered more frequently if the house is maintained at a warmer temperature, natural vegetation of a given type requires more rainfall in warmer climates than in cooler climates. 

 Last Updated:
01/24/2002