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LONG AND SHORT WAVES

Waves are classified according to their length, amplitude, and speed. Wavelength is the measured distance (in degrees longitude) between successive waves. The measurement is usually taken from trough to trough, ridge to ridge, or from any point on one wave to the same corresponding point on the next wave. The amplitude is one-half of the wave’s total range, which is measured in degrees latitude from the peak of the ridge to the base of the trough. Figure 8-3-1 illustrates the measure-ment of wavelength and amplitude on a long wave. Also, note the short wave on the long wave. The speed of waves is usually governed by their length. The longer the waves, the slower they move, and vice versa.

Long Waves

A significant feature of the westerlies in both hemispheres, long waves vary in length from 50° to 120° longitude, have large amplitudes, and are


Figure 8-3-1.—Illustration of a long and short wave and the measurement of length and amplitude of a long wave.

slow moving. In the overall hemispheric pattern there are normally four or five long waves in existence at any one time. However, there are times when there are as many as seven or as few as three. The pattern is a persistent feature, and waves do not appear or disappear rapidly. A change in the number of waves in the pattern is significant. The fewer the number, the more progressive are the weather patterns at the surface. The greater the number, the more stagnant the weather patterns. It is during these periods that prolonged good or bad weather affects a region. The number of waves and pattern changes are often discussed at briefings for meteorologists. New long waves form from short waves or a changing synoptic situation, and their develop-ment is associated with the development of new intense circulations at lower levels.

Because some short waves have large ampli-tudes, it is often difficult to distinguish them from long waves. Also, it is virtually impossible to identify wave types (long or short) on a single chart. A series of charts over a 3- to 5-day period is best for this. This is usually enough time for most short waves to move through the slower long-wave pattern, thereby distinguishing between the two types. The long waves have a normal movement at 40°N of about 2° longitude per day in the spring to less than 1° per day during the fall. They can also become stationary or even retrogress.

Because a long wave’s amplitude increases with height within the troposphere (greater at 300 mb than at lower levels) the long-wave pattern is best identified at the 300-mb level. Here, the wave contours are approaching their maximum ampli-tude, and the overall pattern is smooth (no short wave distortion). The increase in amplitude with height also distinguishes long waves from short waves. Short waves often disappear with height and may not be detectable above 500 mb. This is attributable to the temperature patterns associated with the two types of waves. With long waves, the troughs are cold and the ridges warm; the opposite holds true for short waves.

Synoptically, long waves are related to a number of weather occurrences. Figure 8-3-2 illustrates many of the relationships. Simply stated, the weather between the long-wave trough and the downstream ridge (trough sector) is more apt to be bad than the weather between the trough and the upstream ridge (ridge sector).


Figure 8-3-2.—Long waves and related weather. 

Short Waves

Superimposed on the long-wave contours of a given upper-air chart, say 500-mb, are numerous short waves. Ten or more short waves are present in the hemisphere most of the time. They have shorter wavelengths and smaller amplitudes and move faster than long waves. They move in the same direction as the current in which they are embedded. Their eastward motion is very near that of the 700-mb flow. Their normal movement is on the order of 8° longitude per day in summer and 12° per day in winter. Short waves are progressive and never retrograde. Their troughs are warm and their ridges cold; therefore, they do not extend to great heights and are most predominant in the lower half of the troposphere (500 mb and below).

Short waves have a great effect on long waves. They dampen (flatten) long-wave ridges as they move across them, and at the same time, the short wave is weakened. As a short wave approaches a long-wave trough, the short wave strengthens and the long-wave trough intensifies. This latter occurrence often results in the formation of a surface low-pressure system (cyclogenesis). The location of short waves coincides with the small closed height fall centers (troughs) and height rise centers (ridges) of a 700- or 500-mb time-differential chart.

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