AIR in motion is called WIND
- Stagnant air in contact with a water surface eventually assumes the vapor pressure of the surface so that no evaporation takes place.
- It is also important in the production of precipitation
Measurement of Wind
- Wind has both speed and direction. The wind direction is the direction from which it is blowing. Direction is usually expressed in terms of 16 compass points (N, NNE, NE, ENE, etc.) for surface winds, and for winds aloft in degrees from north, measured clockwise. Wind speed is usually given in miles per hour, meters per see and, or knots (1 m/sec = 2.2 mph and 1 knot = 1 1/7 mph, approximately).
- Wind speed is measured by instruments called anemometers.
- The three- or four-cup anemometer with a vertical axis of rotation is most commonly used for official observations. It tends to register too high a mean speed in a variable wind because the cups accelerate faster than they lose speed. Vertical currents (turbulence) tend to rotate the cups and cause over registration of horizontal speeds.
- Propeller anemometer with a horizontal axis of rotation.
- Pressure-tube anemometers, of which the Dines is the best known, operate on the Pitot-tube principle.
Geographic variation of Wind
- The winds are generally variable since most of the country is affected by migratory pressure systems.
- In winter there is a tendency for surface winds to blow from the colder interior of land masses toward the warmer oceans
- Wind speeds are reduced and directions deflected in the lower layers of the atmosphere because of friction produced by trees, buildings, and other obstacles.
Time Variation of Wind
- Wind speeds are highest and most variable in winter, whereas middle and late summer is the calmest period of the year.
- In winter westerly winds prevail over theUnited Statesup to at least 20,000 ft, except near theGulf of Mexicowhere there is a tendency for south-easterly winds up to about 5000 ft.
- At about 2000 ft above the ground, the maximum occurs at night and the minimum in the daytime.