The height-versus-time presentation of radar echoes (used with radars of 1.25- and 0.86-centimeter wave-length) is a record of the precipitation and of some of the clouds passing directly over the radar. Such a record corresponds to a “surface of observation” in the atmosphere. For non-precipitating clouds, the shape of the surface depends upon the upper-wind patterns; for precipitating clouds, the surface is a vertical plane.
Many of the recorded precipitation echoes have a marked slope, due to the effect of wind shear on falling precipitation particles. The magnitude of slope appears to be determined mainly by the speed of travel of the precipitation column, the wind shear, and the fall speed of the particles; lesser influences are the change of wind direction with height, horizontal dimension of the column, and the path of the column relative to the radar position. The horizontal distribution of particle sizes may be very important, but not enough is known about this factor to permit an estimate of its effect on the shapes of echo boundaries.
In practical use of this radar system for weather observation, it does not appear that any purpose would be served by making measurements of echo slope. Probably the chief value of the system lies in the qualitative information it furnishes about the current physical processes of cloud and precipitation. An important question arises as to the size of geographical area adequately represented by the height-time observations.