important one. It is an established fact that wind gustiness is very 
greatly reduced under certain conditions--especially when low level 
temperatures inversions at night eliminate the two types of turbulence-- 
that due to rising convection currents and that due to mechanical fric- 
tion with irregularities on the earth's surface. Temperature inver- 
sions frequently occur soon after sunset and persist until after sun- 
rise; they occur most frequently when a clear sky and little or no wind 
permit rapid cooling of the earth's surface through radiation. After 
the surface temperature has dropped to or telow the temperature of the 
air immediately above it, convection currents must cease. The inver- 
Sion may build up to several hundred feet above the earth's surface. 
Although the existence of an inversion frequently cannot be detected 
by an observer on the group » studies conducted at Brookhaven National 
Laboratory on long Island =/have shown the conditions under which they 
are likely to occur. If we consider only those mornings when the wind 
was  m.epsh. or less, the sky clear (or with fog) and the temperature 
low (under 0 F.) we find that in every case the amount of winnowing 
was above the average. With similar evening conditions, but with 
temperature under 50°, seven out of eight observations () different 
evenings) were normal or above, and the eighth one was only 18 percent 
below the average. 
Altogether there were 5 evenings and 5 mornings with winnowing at 
or above average. Four of these mornings and 3 of the evenings are dis- 
cussed in the preceding paragraph. The other three cases may also have 
been under inversion conditions, as the wind was 1 to 2 miles an hour, 
the temperature within (or within one degree of) the above arbitrary 
limits, and the sky from 50 to 100 percent overcast. While inversions 
are most frequent under clear skies, they occur to a lesser extent under | 
partly cloudy and cloudy conditions. 
In some instances snipe have been observed winnowing under very 
turbulent conditions, as upon the approach of a thunder shower; so 
turbulence, while it may limit the amount of winnowing, will not always 
stop it entirely. Winnowing is frequently but irregularly noted during 
the daytime and is more prevalent during calm, cloudy weather than other- 
wise. This also suggests a correlation between stability of the air and 
amount of winnowing. Further investigations will be required to test the 
importance of gustiness as related to winnowing, but present indications 
suggest the advisability of conducting winnowing counts under inversion 
or isothermal conditions. 
1 | | : | 
Singer, Irving A., and Maynard E. Smith. Relation of gustiness 
to other meteorological parameters. Journal of Meteorology 10 (2): 121. 
126, Apr. 1953. . 
65 
