Abstract
Aphid density diminishes with height above the ground; the linear regression coefficient, βb, of log density on log height is an approximate expression of the vertical density profile. A low negative value of b suggests a relatively high degree of upward transport, and a high value the reverse.
From May to October 1948, aphid densities were estimated simultaneously at 6 heights from 50 to 2,000 ft over 2βhr periods during the day: lapse rates to 2,000 ft were also measured. The regression coefficient, βb, of log density on log height was inversely correlated with the mean lapse rate between months; September and October were associated with low lapse rates and steep density gradients.
Correlation coefficients of b with lapse for 2βhr periods were negative above a lapse of 10Β°F/2,000 ft, and became statistically significant in the afternoon. Below the 10Β° value no significant correlation existed, probably because low lapse rates occurred usually in the early morning when densities were low and factors other than atmospheric mixing could affect the profile.
Two examples of the change in the vertical profile from hour to hour throughout the day are given to illustrate the association of high lapse rates with highβaltitude dispersal and the limitation of upward transport when lapses are low: but biological factors also play a part in determining the form of the profile and these are briefly discussed.