Page:Handbook of Meteorology.djvu/147

 but the effects cannot always be forecast. These movements are commonly grouped under the generic name of turbulence, and much of the impairment of visibility is due to them. Air in convectional equilibrium as to temperature and humidity may be clear and transparent at one time; a mixing process may cause it to be opaque a few minutes later. The change may be due wholly to turbulence.

Turbulence.—Sudden and local movements of the air are due usually to changes of temperature. A change of tem- perature produces a change in pressure; a flow of air results, and the flow—that is, the wind—continues until equilibrium is restored. But the moving of the mass of air does not always produce a mixing; indeed, the plane of contact where it meets another body of air differing in temperature and humidity sometimes is sharply defined. The friction between the two masses frequently causes the condensed vapor to be rolled into long windrows or billow clouds. The airman has learned that visibility is impaired in this plane of contact and, that in passing from one mass to another, he is likely to experience a sharp bump.

As a matter of fact, most of the turbulence which results from the mixing of air begins at the ground. The “skin friction” of the wind dragging over water reduces its velocity along the plane of contact about one-third; over the ground the reduction is roughly twice as great. The drag rolls great sheets of air into volutes. These, as they are pushed upward, bend into fantastic shapes, but continue to rotate upon many axes of many angles. Sometimes a volute bends into a ring, and the ring itself rotates on a constantly changing diameter in irregular librations. The movement, however, is upward as well as onward.

Now, this process of mixing is wholly different from the ordinary convectional movements of the air. A knowledge thereof is important to the marine pilot because it is the chief cause of sea fog along the Atlantic steamship lanes. Thus, a warm, moist wind blows into a region of the drift of a cold current. The chilling of the water vapor quickly condenses it to fog, and the churning movements of turbulence carry the process of condensation higher and higher.