ATMOSPHERIC TURBULENCE

irregular air movement in which the wind constantly varies in speed and direction. Turbulence is important because it churns and mixes the atmosphere and causes water vapour, smoke, and other substances, as well as energy, to become distributed at all elevations. Atmospheric turbulence near the Earth's surface differs from that at higher levels. Within a few hundred metres of the surface, turbulence has a marked diurnal variation, reaching a maximum about midday. When solar radiation heats up the surface, which in turn warms the air, the warm, light air rises, and cooler, denser air descends to replace it. That movement of air, together with disturbances around surface obstacles, makes low-level winds extremely irregular. At night, the surface cools rapidly, chilling the air near the ground; when that air becomes cooler than the air above it, a stable temperature inversion is created. Under that condition, the wind's speed and gustiness both decrease sharply. When the sky is overcast, the low-level air temperature varies much less between day and night, and turbulence remains nearly constant. At altitudes of several thousand metres or more, the frictional effect of surface topography on the wind is greatly reduced, and the small-scale turbulence that is characteristic of the lower atmosphere is absent. Although upper-level winds usually are relatively regular, they sometimes become turbulent enough to have an effect on aviation. See also clear-air turbulence.