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Physical Geography Lecture 11 Tropical and Humid Subtropical Climates and Weather  Tropical weather involves only one air mass  Subtropical weather involves two air masses ­especially in winter  Thunderstorms occur frequently in the tropics and humid subtropics  Tornadoes occur frequently in the subtropics  Tropical Cyclones ­Powerful regional scale storms that form over warm tropical ocean surfaces ­Hurricanes, Typhoons, Cyclones ­High winds and associated storm surges, heavy rains, landslides and riverine flooding are hazardous ­Transfer surplus heat and moisture to the higher latitudes ­Help to redress global energy imbalances ­Replenish freshwater supply  Stages of a thunderstorm 1. Early 2. Mature 3. Dissipating Tropical Cyclones  Form in Trade Wind Zones ­between 8 degrees to 20 degrees N and S ­do not form along the equator ­coriolis effect is too weak  Warm tropical Ocean Surfaces  Water temperatures at least 27 degrees C ( 80 degrees F) to depth of 200 ft  Sources of energy ­sensible heat from ocean ­latent heat of condensation ­degrades once it makes landfall or moves into colder waters ­cut off from energy supply ­convergence at surface ­winds stimulate evaporation ­adds moisture to the atmosphere Monsoon  Term is derived from an Arabic word for a wind that reverses direction seasonally  A true monsoon experiences a 180 degree reversal  Operates on a regional scale (second­order circulation)  Summer Monsoon associated with RAIN  Winter Monsoon is usually DRY in most locations  Most well developed in Asia due to large land mass ­other land masses also experience monsoon like winds and weather patterns Humid Tropical Climates  Humid: have annual moisture surpluses ­precipitation exceeds evapotranspiration  Occupy almost all of the land area from 25 degrees N or S of the equator  Interrupted by mountains and zones of aridity  Temperatures are generally high  Annual moisture surplus ­region of convergence ­atmosphere is unstable ­precipitation is convectional in nature  General movement of air is from east to west  2 main types of humid tropical climates ­tropical rain forest climates ­tropical monsoon climates ­tropical savanna climates Lecture 12 Conditions Favorable to Development of Dry Climates  Locations on leeward side of mountains ­rainshadows  Presence of subsiding air masses  Prevailing wind direction from land to water  Locations off the path of moisture laden winds  Cold currents offshore Characteristics of Dry Climates  Evaporation exceeds precipitation  Skies are clear with sunshine  Daily temperature ranges are excessive due to lack of cloud cover  Insolation proceed rapidly during the day causing temperatures to be high  Ground radiation proceeds rapidly at night causing temperatures to drop rapidly  Most precipitation is convectional  Humidity is low  Natural vegetation is sparse and scanty  Fog is common Distribution and Influence of Topography on Dry Climates Mountains in Deserts  Mountains may have relatively moist climates in desert regions due to: ­orographic lifting and precipitation ­lower temps leading to lower evapotranspiration rates ­may contain relict plant and animal populations isolated due to climate change (refugia) ­ex: dwarf crocodiles in sahara deserts