SMALL SCALE AND LOCAL WINDS

While wind is invisible, we can see its effect.

SCALES OF MOTIONS

1. Microscale - diameters a few meters or less - turbulence - eddies (paddle in water); smoke plumes
2. Mesoscale - middle scale - few kilometers to several hundred - thunderstorms; last longer than microscale.
3. Synoptic - weather map scale - hundreds to thousands of square kilometers.
4. Global - global scale.

Eddies - paddle in water - friction and turbulence. Eddy motions created by obstructions are mechanical turbulence. Usually limited to close to the ground (less than 1000 m) in the friction layer). Surface heating and instability can cause turbulence to extend higher - Thermal Turbulence.

Mechanical and thermal turbulence occur together. Depth of the mixing depends on

1. Surface Heating

2. Strong Wind Speeds

3. Rough or Hilly Landscape

EXAMPLES

Cars on a bridge - warning high winds.

Being passed by a truck.

Sand Dunes - blowing sand comes to rest behind objects, can act as an obstacle itself. Windward slope of the dune is more gentle than the leeward slope.

Snow fences, Behind the snow fence the wind speed is reduced because of friction. Used to supply moisture and insulate the ground, keep roads clear.

Shelter belts - protect soils from wind erosion.

Wind and waves - waves are created as the frictional drag of the wind transfers energy to the water. The greater the wind speed the greater the amount of energy added and the higher the wave. Eddies of air help to reinforce the up and down motion of the water. Actually depends on three things

1. Wind Speed

2. The length of time that the wind blows over the water

3. The fetch or distance of deep water over which the wind blows.

In addition to wind speed and direction, we are interested in Prevailing Wind Direction and Wind Gusts.

LOCAL WIND SYSTEMS

Thermal Circulations - circulation brought on by changes in air temperature.

Sea Breeze; Lake Breeze; Land Breeze.