Local Winds - Ch. 9

thermal highs/lows

regions of surface high and low atmospheric pressure created as the atmosphere either cools or warms

• shallow, weakening with height, and are maintained by local surface heating/cooling

Thermal low = most warming, rising air

Thermal high = most cooling, sinking air

Example:

In thermal low, atmospheric pressure is lowest in the center of the warm thermal low, and in warm air above isobars spread apart and it changes into a high

land/sea breeze (thermal low)

Uneven heating rates of land/water cause mesoscale coastal winds - large temperature differences exist

• During the day, thermal low over land, thermal high over water

  • effect of this pressure distribution is a sea breeze that blows at the surface from the sea towards the land

• At night, land cools more quickly than the water (high over land, low over water)

  • surface wind reverses and becomes a land breeze, flowing from the land toward the water

mountain/valley breeze (thermal low)

Day (valley breeze): sunlight warms valley walls, warming air - heating air (less dense) rises upslope

Night (mountain breeze): flow reverses - slopes cool, chilling the air - condensed air glides downslope into the valley

katabatic winds (thermal lows)

rush downslope at hurricane speeds

• ideal setting is a plateau surrounded by mountains, where horizontal PGF is strong enough to cause these winds

• cold wind despite adiabatic warming

chinook wind (driven by westerlies)

warm, dry downslope wind descending Eastern slope of rockies or other leeward sides of mountains

• occur when strong westerly winds aloft flow over a north-south-trending mountain range

  • produces trough of low pressure on the mountain’s eastern side, tends to force air downslope

  • as it rises, cools at the moist adiabatic rate but as it descends, warms at the dry adiabatic lapse rate (compressional heating)

santa ana winds (thermal lows)

warm, dry wind that blows downhill from east/northeast into southern California

• Develops as region of high pressure over Basin, anticyclone forces it downslope at exceptional speed from high plateau

• Compressional heating as it rushes downhill