Chapter 12 Notes


  • Winds occur on all scales, but small scale winds are driven by pressure gradient force and are slowed by the effects of friction.
  • Viscosity is the friction in a fluid, such as air.  Higher viscosity means higher friction.
    • Molecular Viscosity- friction at the smallest scales when molecules bump into each other, occurs near boundaries
    • Eddy Viscosity- friction that arises through Eddies, which are human-sized swirls of air that interact with the larger-scale wind and help slow it down.  Eddies arise when wind blows over obstacles such as trees or buildings, or through daytime heating from the sun, or from the atmosphere itself.
  • Turbulence is the irregular, almost random, pattern of wind.


Small Scale Winds going from East to West Across the US


  • A Coastal Front is the boundary between two air masses along a coastline that acts as a smaller-scale version of a stationary front.  Common in Northeast US.
  • Cold Air Damming is the stubborn entrenchment of cold air that in pinned against high mountains.  Classic case of shallow, cold air.
  • Gravity Waves are atmospheric waves with an alternating small scale pattern of high and low pressure maintained by the help of gravity.  They are sometimes visible when the rising air in the crests in the waves becomes saturated and forms parallel lines of clouds.




  • Lakes Breezes are winds that blow onshore during the day around the Great Lakes.  They are similar to sea breezes with cool water with high pressure at the surface and warmer land with low pressure at the surface forming a slope of pressure gradient force from the water to the land.
  • A Derecho is an hours long windstorm associated with a line of severe thunderstorms that can leave significant damage.  It is a result of straight line winds.  A thunderstorm’s cold downdrafts can drag down high speed air from aloft.
  • A Bow Echo can result form a derecho when high winds push the thunderstorms outward, causing it to bend or “bow”.
  • Derechos are generally confined to the eastern two thirds of the US.
  • A Blue Norther is the fierce north wind behind a fast racing cold front in west Texas.  It can also be called a Norte or a Buran in other parts of the world.
  • Dust Storms are weather conditions characterized by strong winds and dust filled air over and extensive area.
  • A Heatburst is when the temperature rises drastically throughout the night and conditions becomes very dry with winds howling.  It can be explained by adiabatic warming and static stability.  They are most common in late spring and early summer in the Great Plains.
  • Microbursts are similar to heatbursts in how the air splashes against the ground and rapidly pushes outward.  A microburst is a strong, localized downdraft less than 4 km in diameter that sometimes develops underneath a thunderstorm as a result of evaporative cooling.
  • A Chinook is a dry, warm wind in western North America on the lee side of the Rocky Mountains.  The Chinook speeds the melting and evaporation of snow.




  • Valley Breeze- Develops during the day when the thin air above the high mountainsides warms quickly and the warm air rises and creates local low pressure along the slopes.  Air from the lower valley moves in to replace it, creating an upslope breeze that becomes strongest around noon.
  • Mountain Breeze- Develops at night when the high mountain slopes cool very quickly and this cold, dense air forms a local high-pressure area.  The pressure gradient then drives a gentle breeze down the slope into the valley that is strongest just before sunrise.
  • Mountain and Valley Breezes are generally very gentle.
  • Katabatic Winds are more violent relatives of mountain breezes and occur all over the world with gusts sometimes exceeding 100 mph.
  • Boulder Windstorms are downslope winds associated with mountain gravity waves that can be very strong.
  • Dust Devils are thin, rotating columns of air in the desert Southwest that develop from intense daytime heating.
  • Lenticular Clouds are clouds in the shape of a lends that sometimes resemble spaceships.  Usually form downwind.
  • Santa Ana Winds form when the pressure gradient caused by an anticyclone (high pressure) over the Rockies, in combination with friction, forces already dry air from the mountainous West down the Coast Range of northern California or down the San Gabriel mountains in southern California and all the way down out into the ocean.  Can cause roaring fires.