| description abstract | SYNOPSIS This article is based on the free-air observations by means of Marvin-Hargraves kites begun at Drexel, Nebr., in October, 1915. Average annual temperature gradients in the lower air over any region are dependent largely on the intensity and persistence of the winter temperature inversions for in summer surface heating produces local convection which results in nearly uniform vertical temperature gradients over wide areas. The gradient at any time is dependent largely on wind direction at different levels, especially in winter. The winter vertical temperature gradients over Drexel are the result of the successive cold and warm-wave conditions. In the cold wave the cold air arrives first near the surface and gradually affects higher levels. Near the ground turbulence and heating by day may maintain a thin sheet in which there is a lapse in temperature, but the general condition is one of inversion. Near the center of the anticyclone dynamic warming by the compression of the descending air is sometimes in evidence aloft. Then a shift of wind to south will raise the temperature at some distance above the surface several hours before the slower-moving surface wind arrives. The degree of diurnal change of surface wind depends on the vertical temperature gradient at well as on the relative velocities of the upper and lower winds, convectional interchange being greater when the gradients are steepest. Gradient winds computed from sea-level isobaric charts frequently do not agree at all closely with those observed a few hundred meters above the surface at Drexel (itself at 396 m.), for, aside from arbitrary assumptions in sea-level reductions the diverse temperatures of the air over neighboring regions may make the slopes of the isobaric surfaces even at moderate heights differ considerably from those indicated by the sea-level charts.-C. F. B. | |
