Waves In The Atmosphere

the process of propagation of periodic or nearly periodic motions that are superimposed on the general transfer of air.

In addition to elastic, longitudinal acoustic, and blast waves in the atmosphere, there are several types of atmospheric waves of different origin and character and much longer wavelengths, which can be considered as only roughly periodic. Among these are the waves that develop at the boundary between two air layers moving at different velocities and having different densities and temperatures. On the crests, where there is an ascending air movement, the air is cooled, the water vapor contained in it condenses, and clouds are formed. In the troughs, where there is a descending current, the air is heated and becomes unsaturated, and the sky between crests remains clear. All this leads to the appearance of a series of wavy clouds.

A similar process takes place in the so-called mountain waves that develop in the flow past mountains, hills, and so on. The oscillatory movements continue for a fairly long time after a given volume of air has passed a mountainous obstacle. Short waves of this kind are widespread. They affect the flight of aircraft, often producing bumpy air conditions. The amplitude and wavelength of this kind of wave increase with increasing difference in the velocities of the moving masses and decreasing differences in the densities and temperatures. The wavelength varies from hundreds of meters to dozens of kilometers; the amplitude reaches 1-2 km. The velocities of ascending movements—for example, in the crests of mountain waves—may reach several m/sec; glider pilots take ad-vantage of this feature.

In addition to the short atmospheric waves, where the particles are oscillating in the vertical plane, large-scale waves having wavelengths in the hundreds and thousands of km also exist in the atmosphere; in this case the oscillations take place mainly in a horizontal direction. First, there are the cyclonic waves that occur at atmospheric fronts—that is, at the boundaries between air masses with different temperatures. When these waves become unstable, they result in the formation of cyclones. So-called long waves also exist. In the middle latitudes of the atmosphere the prevailing westerly flow is wavy; the length of these waves is on the order of several thousand km, so that several wavelengths (three to six) are laid out around the circumference of the globe. One of the reasons for their occurrence is the difference in the temperature conditions over the continents and oceans. The cyclonic and long atmospheric waves control the weather conditions over large areas; studying them is of primary importance in weather forecasting.

Other types of waves exist in the atmosphere: waves in the tropopause, which are changes in the height of the tropopause during atmospheric movements of cyclones and anticyclones; tidal waves, which are due to the attraction of the moon and the sun; and seismic waves, which are associated with earthquakes and falling meteorites.