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Low-Latitude Climates Chapter 9 Overview of Low-Latitude Climates The low-latitude climates lie for the most part between the Tropics of Cancer and Capricorn and represent about 50 percent of Earth’s surface. They occupy the entire equatorial zone (10° N to 10° S), most of the e tropical op ca zone o e ((10° 0 to o 15° 5 Na and d S), and a d part pa of o the e subtropical sub op ca zone (25–35° N and S). Overview of Low-Latitude Climates The low-latitude climates are characteristically warm throughout the year, and are sometimes described as lacking any winter season. Constant warmth is the distinguishing feature of these climates climates, with the 18 °C winter isotherm used to establish their poleward limits. 1 The Wet Equatorial Climate (Köppen Af) The wet equatorial climate occurs in areas where the ITCZ is nearby for most of the year. This climate is dominated byy warm,, moist maritime equatorial (mE) and maritime tropical (mT) air masses that yield heavy convectional rainfall. The wet equatorial climate is found in the latitude range 10° N to 10° S. 2 The Wet Equatorial Climate (Köppen Af) Precipitation is plentiful in all months, and the annual total often exceeds 2,500 mm. There is usually a seasonal pattern to the rainfall rainfall, with a somewhat wetter period occurring during the time of year when the ITCZ migrates into the region. Mean monthly and mean annual temperatures are typically close to 27 °C. The Monsoon and Trade Wind Coastal Climate (Köppen Am, Af) The monsoon and trade-wind coastal climate occurs in latitudes from 5° to 25° N and S. q climate,, the monsoon and tradeLike the wet equatorial wind coastal climate has abundant rainfall. However, the rainfall in monsoon and trade-wind coastal regions shows a stronger seasonal pattern (especially pronounced in monsoon climates). 3 The Monsoon and Trade Wind Coastal Climate (Köppen Am, Af) The monsoon and trade-wind coastal climate results from two somewhat different situations. p ((mT)) On tradewind coasts,, moisture-laden maritime tropical and maritime equatorial (mE) air masses produce rainfall. The trade winds move these air masses onshore across narrow coastal zones backed by highlands. As the warm, moist air passes over coastal hills and mountains, the orographic effect produces shower activity. 4 The Monsoon and Trade Wind Coastal Climate (Köppen Am, Af) Rainfall is also intensified by easterly waves, which are more frequent when the ITCZ is nearby. This trade-wind effect is limited to the east coasts of land masses because the trade winds blow from east to west. The monsoon climate occurs anywhere where there is a seasonal shift in wind direction. The Monsoon and Trade Wind Coastal Climate (Köppen Am, Af) In the monsoon and trade-wind coastal climate, the warmest temperatures occur in the high-sun season, just before the arrival of the ITCZ brings clouds and rain. The seasonall reduction Th d ti iin solar l radiation di ti caused db by th the h heavy cloud cover affects temperature through the summer. Generally there is no shortage of moisture in the wet equatorial and monsoon and trade wind coastal climates, and even during the extended dry periods that occur in the monsoon. 5 The Wet-Dry Tropical Climate (Köppen Aw, Cwa) In the monsoon and trade-wind coastal climate, the ITCZ moves into and away from the region to produce the seasonal cycles of rainfall and temperature. These cycles become stronger farther from the equator, and the monsoon and trade-wind coastal climate grades into the wet-dry tropical climate. The Wet-Dry Tropical Climate (Köppen Aw, Cwa) The wet-dry tropical climate is distinguished by very dry conditions during the low-sun season; this alternates with very wet conditions during the high-sun season. During the low low-sun sun season season, the ITCZ moves well beyond the wet-dry tropical climate region, and the dominant air masses are of the dry continental tropical (cT) type. 6 The Wet-Dry Tropical Climate (Köppen Aw, Cwa) In the high-sun season, when the ITCZ is nearby, moist mT and mE air masses dominate. A characteristic h t i ti off the th tropical t i l wet-dry t d climate li t iis itits llarge year-to-year variability in precipitation. 7 8 The Dry Tropical Climate (Köppen BWh, BSh) The dry tropical climate is associated with the subtropical high pressure cells. Here, strong subsidence and adiabatic warming inhibit condensation. Rainfall is rare and occurs only when unusual weather conditions move moist air into the region. The Dry Tropical Climate (Köppen BWh, BSh) Since skies are clear most of the time, the sun heats the surface intensely, keeping air temperatures high. During the high-sun high sun period period, heat is extreme extreme. 9 The Dry Tropical Climate (Köppen BWh, BSh) The driest areas of the dry tropical climate are centred near the Tropics of Cancer and Capricorn, with rainfall increasing toward the equator. Immediately adjacent to the dry tropical regions regions, recipitation comes during a short rainy season at the time of the year when the ITCZ is nearby. 10 Highland Climates of Low Latitudes Highland climates are cool to cold, usually moist, climates that are associated with mountainous regions and high plateaus (closely associated with the climate of the surrounding lowlands). Generally, the higher the location, the colder and wetter is its climate. Temperatures are lower since air temperatures in the atmosphere normally decrease with altitude. Rainfall increases because orographic precipitation tends to be induced when air masses ascend to higher elevations, especially on windward slopes. 11 A Look Ahead The next chapter discusses the climates of the mid- and high latitudes. The mid- and high-latitude climates are extremely diverse and include regions g that experience p very y wet, very y dry, y very y hot, and very cold conditions at least at certain times of the year. 12