The preceding sections have dealt with only the broadscalewind systems that make up the global circulation and influencethe climatic pattern of the world. Many kinds oflesser winds, however, are of considerable significance toweather and climate at a more localized scale. Such windsare the result of local pressure gradients that develop inresponse to topographic configurations in the immediatearea, sometimes in conjunction with wider circulationconditions.
Sea and Land Breezes
A common local wind system along tropical coastlinesand to a lesser extent during the summer in midlatitudecoastal areas is the cycle of sea breezes during the dayand land breezes at night. (As is usual withwinds, the name tells the direction from which the windcomes: a sea breeze blows from sea to land, and a landbreeze blows from land to sea.) This is essentially a convectionalcirculation caused by the differential warming ofland and water surfaces. The land warms up rapidly duringthe day, warming the air above by conduction and reradiation.
This warming causes the air to expand and rise,creating low pressure that attractssurface breezes fromover the adjacent water body. Because the onshore flow isrelativelycool and moist, it holds down daytime temperaturesin the coastal zone and provides moisture for afternoonshowers. Sea breezes are sometimes strong, but theyrarely are influential for more than 15 to 30 kilometers (10to 20 miles) inland.
The reverse flow at night is normally considerablyweaker than the daytime wind. The land and the air aboveit cools more quickly than the adjacent water body, producingrelatively higher pressure over land. Thus, air flowsoffshore in a land breeze.
Valley and Mountain Breezes
Another notable daily cycle of airflow is characteristic ofmany hill and mountain areas. During the day, conductionand reradiation from the land surface cause air near themountain slopes to warm more than air over the valleyfloor.
The warmed air rises, creating a low-pressurearea, and then cooler air from the valley floorflows upslope from the high-pressure area to the low-pressurearea. This upslope flow is called a valley breeze.
The rising air often causes clouds to form around the peaks,and afternoon showers are common in the high countryas a result. After dark, the pattern is reversed. The mountain
slopes lose warmth rapidly through radiation, whichchills the adjacent air, causing it to slip downslope as amountain breeze.
Valley breezes are particularly prominent in summer,when solar warming is most intense. Mountain breezesare often weakly developed in summer and are likely tobe more prominent in winter. Indeed, a frequent winterphenomenon in areas of even gentle slope is cold airdrainage, which is simply the nighttime sliding of cold airdownslope to collect in the lowest spots; this is a modifiedform of mountain breeze.