Measuring General Winds

Wind is air in motion relative to the earths surface. Its principal characteristics are its direction, speed, and gustiness or turbulence. Wind direction and speed are usually measured and expressed quantitatively, while in field practice turbulence is ordinarily expressed in qualitative or relative terms. Ordinarily only the horizontal components of direction and speed are measured and reported, and this is adequate for most purposes. In fire weather, however, we should remember that winds can also have an appreciable vertical component which will influence fire behavior, particularly in mountainous topography.

At weather stations making regular weather observations, surface wind direction is determined by a wind vane mounted on a mast and pointing into the wind. The direction can be determined visually or, with more elaborate instruments, it can be indicated on a dial or recorded on a chart.

Wind direction is ordinarily expressed as the direction from which the wind blows. Thus, a north wind blows from the north toward the south, a northeast wind from the northeast, and so on around the points of the compass. Direction is also described in degrees of azimuth from north--a northeast wind is 45°, a south wind 180°, and a northwest wind 315°.

The method of describing the direction of both surface winds and winds aloft, by the direction from which the wind blows, is ordinarily very practical. In mountain country, though, surface wind direction with respect to the topography is often more important in fire control and provides a better description of local winds than the compass direction. Here it is common to express the wind direction as the direction toward which the wind is headed. Thus, an upslope or upcanyon wind is actually headed up the slope or up the canyon. Wind is described as blowing along the slopes, through the passes, or across the ridges. Similarly, "offshore" or "onshore" are used to describe the directions toward which land and sea breezes are blowing.

Surface wind speeds are measured with anemometers. Many types of anemometers are in use, but the most common is the cup anemometer. It indicates either the air speed at any given instant or the miles of air that pass the instrument in a given time period. The latter gives an average wind for the selected time period. Normally, a 2-minute average is used. The standard height at which wind speed is measured is 20 feet above open ground.

In the United States, wind speed is usually measured in miles per hour or knots (nautical miles per hour). One knot is 1.15 miles per hour. Weather Bureau and military weather agencies use knots for both surface and upper winds, while miles per hour is still in common use in many other agencies and operations, including fire weather.

The direction and speed of winds aloft are determined most commonly by tracking an ascending, gas-filled balloon from the surface up through the atmosphere.

The simplest system employs a pilot balloon followed visually with a theodolite. If a constant rate of rise of the balloon is assumed, periodic readings of elevation and azimuth angles with the theodolite allow computation of average wind direction and speed between balloon positions. Errors are introduced when the ascent rate is not constant because of vertical air currents. If a radiosonde unit (which transmits temperature, moisture, and pressure data during ascent) is added to the balloon, the height of the balloon at the time of each reading can be calculated fairly accurately, and the computed winds are more accurate.

The most refined of present systems have the further addition of a self-tracking, radio direction-finding unit that measures elevation and azimuth angles, and slant range from the observing station to the balloon. This unit known as a rawinsonde, yields quite accurate upper-air information. All of these methods furnish wind soundings for meteorological use and interpretation.

The speed and direction of upper winds are sampled at regular intervals each day at selected weather stations across the continent. These stations are often more than 100 miles apart. Although winds aloft tend to be more uniform than surface winds, there are exceptions. The wind structure over an area some distance from a sampling station may differ considerably from that indicated by the nearest sounding.