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Unit 09


Unit Overview

This unit discusses atmospheric pressure and the winds that result from differences in atmospheric pressure. The main sections are:

  • Atmospheric pressure
  • Air movement in the atmosphere
  • Large- and smaller-scale wind systems
  • Local wind systems

Atmospheric pressure changes rapidly with changes in altitude: it decreases rapidly with an increase in altitude and it increases rapidly with a decrease in altitude. Atmospheric pressure also varies horizontally across the globe.

Differences in atmospheric pressure lead to a pressure gradient force. The pressure gradient force is a precursor to wind (i.e. wind flows from areas of higher pressure to areas of lower pressure). The interplay between the pressure gradient force, friction, and the Coriolis force controls the strength and direction of the resultant wind. The Coriolis force is an apparent force caused by the rotation of the Earth; it deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The intensity of this force increases with an increase in an object's velocity and latitudinal position. Friction decreases with an increase in altitude, and friction reduces the velocity of wind. Thus, friction reduces the impact of the Coriolis force. Geostrophic winds are present in the upper-troposphere, and these winds are not affected by surface friction. Consequently, these winds are substantially impacted by the Coriolis force and, therefore, blow parallel to the isobars (i.e. lines of equal pressure).

Surface winds, on the other hand, are impacted by friction more than by the Coriolis force. These winds cross isobars at an angle (i.e. get deflected slightly to the right when moving from high to low pressure in the Northern Hemisphere). Surface winds operate at different geographic scales. Examples include regional-scale Santa Ana winds and local-scale sea breezes.

Unit Objectives

  • To explain atmospheric pressure and its altitudinal variation
  • To relate atmospheric pressure to windflow at the surface and aloft
  • To apply these relationships to the operation local wind systems

Glossary of Key Terms

Anticyclone An atmospheric high-pressure cell involving the divergence of air, which subsides at and flows spirally out of the center - the isobars around an anticyclone are generally circular in shape, with their values increasing toward the center. In the Northern Hemisphere, winds flow clockwise around an anticyclone; in the Southern Hemisphere, winds flow counterclockwise around an anticyclone.
Barometer Instrument that measures atmospheric pressure; invented by Torricelli in 1643.
Chinook wind Name given to the foehn winds that affect the leeward areas of mountain zones in the western plateaus of North America.
Cold-air drainage A category of local-scale wind systems governed by the downward oozing of heavy, dense, cold air along steep slopes under the influence of gravity; produces katabatic winds (such as southeastern France's mistral) that are fed by massive pools of icy air that accumulate over such major upland regions as the Alps and the Rocky Mountains.
Coriolis force The force that, owing to the rotation of the Earth, tends to deflect all objects moving over the surface of the Earth away from their original path. In the absence of any other forces, the deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere: the higher the latitude, the stronger the deflection.
Cyclone An atmospheric low-pressure cell involving the convergence of air, which flows into and rises spirally at the center. The isobars around a cyclone are generally circular in shape, with their values decreasing toward the center. In the Northern Hemisphere, winds flow counterclockwise around a cyclone; in the Southern Hemisphere, winds flow clockwise around a cyclone.
Frictional force The drag that slows the movement of air molecules in contact with, or close to, the Earth's surface. Varies with the "roughness" of the surface; there is less friction with movement across a smooth water surface than across the ragged skyline of a city center.
Geostrophic wind A wind that results when the Coriolis and pressure-gradient forces balance themselves out; follows a relatively straight path that minimizes deflection and lies parallel to the isobars.
Isobar A line connecting all points having the identical atmospheric pressure.
Katabatic wind The winds that result from cold-air drainage; especially prominent under clear conditions where the edges of highlands plunge sharply toward lower lying terrain.
Land breeze An offshore airflow affecting a coastal zone, resulting from a nighttime pressure gradient that steers local winds from cooler (higher-pressure) land surface to the warmer (lower-pressure) sea surface.
Leeward The protected side of a topographic barrier with respect to the winds that flow across it; often refers to the area downwind from the barrier as well, which is said to be in the "shadow" of that highland zone.
Pressure The weight of a column of air at a given location, determined by the force of gravity and the composition and properties of the atmosphere at that location. Standard sea-level air pressure produces a reading of 760mm (29.92 in) on the mercury barometer; in terms of weight, it is also given as 1013.25 millibars (mb) or 14.71b per sq in.
Pressure-gradient force The difference in surface pressure over a given distance between two locations is called the pressure gradient; when that pressure gradient exists, it acts as a force that causes air to move (as wind) from the place of higher pressure to that of lower pressure.
Santa Ana wind A hot, dry,foehn-type wind that occasionally affects Southern California; its unpleasantness is heightened by the downward funneling of this airflow from the high inland desert through narrow passes in the mountains that line the Pacific coast.
Sea breeze An onshore airflow affecting a coastal zone, resulting from a daytime pressure gradient that steers local winds from the cooler (higher-pressure) sea surface onto the warmer (lower-pressure) land surface.
Wind The movement of air relative to the Earth's surface; winds are always named according to the direction from which they blow.
Wind chill temperature (WCT) index An index that tells us subjectively how cold we would feel under given combinations of wind speed and air temperature.
Windward The exposed, upwind side of a topographic barrier that faces the winds that flow across it.

Unit Outline

  • Atmospheric pressure
    • Wind is created by an imbalance of forces that act on air molecules
    • Pressure is a result of the combined weight of air molecules in a given area exerting a force
      • standard sea level air pressure is 1013 millibars
    • Atmospheric pressure and altitude
      • air pressure is highest at low altitudes, and lowest at high altitudes
  • Air movement in the atmosphere
    • Causes of atmospheric circulation
      • unequal heat energy distribution
      • rotation of Earth
      • Coriolis force is the deflective force that acts on rotating objects
      • winds are always named for the direction from which they come
    • Forces on an air molecule
      • pressure-gradient force due to gravity
      • Coriolis force greatest at poles
      • frictional force near Earth's surface slows molecules
  • Large- and smaller-scale wind systems
    • Geostrophic winds
      • follow relatively straight path, deflection minimized in upper atmosphere
      • flows parallel to isobars
    • Frictional surface winds
      • near Earth's surface, friction reduces wind speed and alters its direction
      • surface winds converge towards a cyclone (low pressure cell); air rises in center
      • surface winds diverge away from an anticyclone (high pressure cell); air subsides in center
  • Local wind systems
    • Sealland breeze systems
      • during the day - warmer lower pressure air is over land - air moves from high to low pressure from water to land
      • at night - warmer lower pressure air is over water so air flows from land to sea
    • Mountain/valley breeze systems
      • during the day - warmer lower pressure air develops on mountain slopes, triggering a valley breeze
      • at night - mountain slopes cool more rapidly creating higher pressure on slopes than in the valley and a mountain breeze results
    • Other local wind systems
      • cold air drainage on steep slopes cause katabatic winds
      • local winds created by forced passage of air across mountains
        • Chinook winds
        • Santa Ana winds

Review Questions

  1. What are katabatic winds? Santa Ana winds? Chinook winds?
  2. Describe the general formation of valley and mountain breezes.
  3. Define the terms windward and leeward and describe the role that winds play in our lives.