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<br />terrain and (b) location in relation to the prevailing paths of stann <br />centers. Extratropical cyclones occur in the zone of prevailing westerly <br />winds, generally between 30 and 60 degrees of latitude and derive II>>st <br />of their energy fl'Oll aperature differences in the vicinity of the <br />polar front that separates polar and tropical air masses. Tropical <br />cyclones (hurricanes) occur in the zone of prevailing easterly winds, <br />generally between 10 and 20 degrees of latitude. and derive most of their <br />energy from moisture evaporation in tropical seas. Th~ usually terminate <br />when they travel over land for any substantial distance. and. of course. <br />they frequently move into the zone of prevailing westerlies. where th~ <br />usually dissipate after a few days of intense activity. <br />The severity of stonn activity, in the case of extra-tropical <br />cyclones can be great over the entire zone of prevailing westerlies. but <br />the amount of storm precipitation depends on the amount of moisture <br />available. This is usually greater nearer the equator and nearer the <br />oceanic sources of lI>>isture in the warm air mass. MaxilllUm moisture over <br />the ocean is that corresponding to the lI>>ist adiabat that intersects <br />sea-level pressure at ocean-surface temperatures. as indicated on an <br />adiabatic diagra.. If the lOisture must travel great overland distances. <br />it is subject to substantial depletion through precipitation on the way. <br />If .auntain barriers intervene. only moisture that can be retained when <br />the air ..sses are lifted over the mountains can be carried beyond theM. <br />In the case of tropical cyclones. storm potential is greatest along <br />the coasts in the general paths of the cyclones and as much as 1.000 <br />kilometers or more inland. Again. rainfall amounts are diminished <br />substantially as the distance from the ocean increases. Since tropical <br />hurricanes originate in the zones of prevailing easterly winds. associ- <br />ated rainfall potential is greatest along the east coasts and coasts <br />toWard the equator in latitude zones below 45 degrees. <br />In the case of convective rainfall. amounts are greatest where <br />precipitable moisture in the air Misses is greatest. They occur under <br /> <br />2-04 <br />