All the wind

Today

• Homework in

• Friction wind

• Observing the wind

• Some special winds

All the wind

• Friction

• Resultant wind– At the surface– Above our heads

Friction WindWinds near the surface are influenced by the ground. Thisinfluence is in the form of friction. Friction acts to retard themotion of the wind -- it is always in the direction opposite thewind velocity.

H LPGFCo

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Fr

Friction acts to oppose the flow of the air. The air willslow down, reducing the Coriolis force. This results inan imbalance of forces.

Friction Force• Friction (Fr):

– Results from rubbing of air molecules against ground, trees, buildings, you.

– Greatest near and right at the surface. Usually confined to lowest 1000m (3000 ft).

– Friction retards motion, i.e.., acts backwards.

– Friction force increases with wind speed.

VFr

Friction WindThe atmosphere adjusts, to regain a balance, by turning thewind toward low pressure.

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V

Fr

A new balance is achieved when the sum of the Friction andCoriolis forces balance the horizontal pressure gradientforce.

Friction Wind

L

Winds are directed towardlow pressure.

But, the air must go somewhere!

L

•Directional convergence•Lifting of air•“Bad” Weather

Cyclonic Flow

Friction Wind

Winds are directed towardlow pressure.

But, the air must go somewhere!

•Directional divergence•Sinking of air•“Good” Weather

Anticyclonic Flow

H

H

Surface & 500 mb MapsSurface & 500 mb MapsSurface maps chart Surface maps chart pressure contours, pressure contours, highs and lows, and highs and lows, and wind direction.wind direction.

Winds blow clockwise Winds blow clockwise around highs, called around highs, called anticyclones.anticyclones.

500 mb maps reveal 500 mb maps reveal patterns that on patterns that on average are 5600 m average are 5600 m above the surface, above the surface, where westerly winds where westerly winds rise and fall across rise and fall across ridges and troughs.ridges and troughs.

Figure 9.15AFigure 9.15A

Wind Speed & Pressure ContoursWind Speed & Pressure Contours

Just as a river speeds and slows when its banks narrow and expand, Just as a river speeds and slows when its banks narrow and expand, geostrophic winds blowing within pressure contours speed as geostrophic winds blowing within pressure contours speed as contour intervals narrow, and slow as contour intervals widen.contour intervals narrow, and slow as contour intervals widen.

Figure 9.24Figure 9.24

Meridional & Zonal FlowMeridional & Zonal Flow

Wind direction and speed are indicated by lines, barbs, and flags, Wind direction and speed are indicated by lines, barbs, and flags, and appear as an archer's arrow.and appear as an archer's arrow.

Upper level winds that travel a north-south path are meridional, Upper level winds that travel a north-south path are meridional, and those traveling a west-east path are zonal.and those traveling a west-east path are zonal.

Figure 9.28Figure 9.28

Sensing Highs & LowsSensing Highs & Lows

The location of The location of high and low high and low pressure centers pressure centers are estimated by are estimated by detecting surface detecting surface wind direction and wind direction and noting pressure, noting pressure, Coriolis, and Coriolis, and friction forces.friction forces.

This figure This figure illustrates the illustrates the procedure when procedure when standing aloft and standing aloft and at the surface.at the surface.

Figure 9.32AFigure 9.32A

Vertical Air MotionVertical Air Motion

Winds converging into a low pressure center generate upward Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules.winds that remove the accumulating air molecules.

These updrafts may cause cloud formation.These updrafts may cause cloud formation.

Likewise, diverging air molecules from a high pressure area are Likewise, diverging air molecules from a high pressure area are replenished by downward winds.replenished by downward winds.

Figure 9.33Figure 9.33

Wind InstrumentsWind Instruments

Figure 10.17Figure 10.17

Wind vanes point into the wind, or toward its origin, while revolutions Wind vanes point into the wind, or toward its origin, while revolutions of a cup anemometers record wind distance and speed. Aerovanes can of a cup anemometers record wind distance and speed. Aerovanes can measure both speed and direction. Pilot balloons observed with measure both speed and direction. Pilot balloons observed with theodolites, infrared radiometers, and lidar are other wind detection theodolites, infrared radiometers, and lidar are other wind detection tools.tools.

Doppler radar detects changes in wind speed within large volumes of Doppler radar detects changes in wind speed within large volumes of atmosphere.The vertical profile may extend 16km, and can help atmosphere.The vertical profile may extend 16km, and can help identify developing storms and tornadoes. identify developing storms and tornadoes.

Figure 10.18Figure 10.18

Special winds

• There are some types of wind that occur in different places that are given names

• The most common are downslope or Katabatic winds

• These are warm (dry) winds

• Examples are– Chinook (Foehn) winds– Santa Ana winds

Katabatic WindKatabatic Wind

Figure 10.29Figure 10.29Mountain snow cover creates a thin layer of high pressure cold air that Mountain snow cover creates a thin layer of high pressure cold air that rushes into lower valleys.rushes into lower valleys.Elevated plateaus with snow cover may foster development of a thin layer of Elevated plateaus with snow cover may foster development of a thin layer of high pressure cold air.high pressure cold air.Pressure gradient winds are triggered due to lower pressure above the Pressure gradient winds are triggered due to lower pressure above the adjacent valley, pushing cold air into the lower valley.adjacent valley, pushing cold air into the lower valley.

Santa Ana WindsSanta Ana Winds

Figure 10.32Figure 10.32

Easterly winds that descend from southern California's elevated Easterly winds that descend from southern California's elevated desert plateau are compressionally heated and lowered in humidity.desert plateau are compressionally heated and lowered in humidity.

Many fires have been triggered by winds of this type.Many fires have been triggered by winds of this type.

Chinook WindsChinook Winds

Westerly winds that condense and precipitate their moisture when Westerly winds that condense and precipitate their moisture when ascending the Rockies, and then compressionally warm and dry ascending the Rockies, and then compressionally warm and dry when descending, are described as chinook winds.when descending, are described as chinook winds.

Figure 10.30Figure 10.30

Chinook Wall CloudChinook Wall Cloud

Figure 10.31Figure 10.31

Chinook wall clouds are an indicator of their presence.Chinook wall clouds are an indicator of their presence.

Summary

• A lot of wind

• Local effects

• Regional effects