Exploring the Unknown - NASA's History Office

ty will determine the departure from an adiabatic lapse rate, while the degree of cloudiness
(moisture) will help an analyst to decide whether to use the moist or the dry adiabatic
lapse rate as the limiting one.)
Vertically developed cloud will also aid in determining the temperature gradient of
the surrounding area. This is true because of the fact that the vertical shear, as indicated
[18] by the slope of towering cumulus and cumulonimbus clouds, orients the direction of
the higher and lower temperatures in the areas. This method is employed by taking the
direction of vertical shear as being from the low levels toward the high levels. If one then
faces in the direction of shear in the northern hemisphere, the lower temperature will be
on the observer’s left and the higher on the observer’s right (see Fig. 3).
This relationship holds for the northern hemisphere. In the southern hemisphere the
directions of decreasing and increasing temperature in relations to vertical shear are
reversed.
Pressure
Fig. 3—Vertical wind shear—temperature gradient relationship
It is apparent that no quantitative values of pressure are forthcoming from this analysis.
Furthermore, it is virtually impossible even to make a quantitative estimate other than
to state whether the area is thought to be under the influence of a high- or a low-pressure
system. Charts of average pressures for various times of the year in different areas of the
world are available. Using these and the weather situation at the time, trends of pressure
may be established. This information when applied in conjunction with known weather
may be a very useful tool for forecasting purposes. Little work has been attempted on this
subject in this pilot study, and the above should serve only as a possible starting point for
any detailed research along these lines.
C. F. Brooks 15 points out some further pressure information that may be obtained
from clouds. He says, in effect, that, since in the presence of any constant vertical shear
the cumulus clouds will tend to lean or slope (the amount of departure from the vertical
being a resultant of the vertical velocity and the rate of change of wind velocity with
height), any cloud that has a uniform rate of vertical growth and a 90˚ slope throughout
is an indication of the “uniformity of wind velocity in all layers pierced.” This indicates a
decrease of horizontal pressure gradient with height. (This can be shown very simply by
an examination of the geostrophic wind equation
V g = 1 ( p) 1,
p ( n) λ
where V g = the geostrophic wind velocity
p = density o air
p/ n = horizontal pressure gradient
λ = Coriolis parameter.
EXPLORING THE UNKNOWN 199
[19] It can be seen that since λ, which depends on the sine of the latitude, will remain
constant and p decreases with height, p/ n must also decrease for V g , to remain constant.)
This decrease turns out to be very small when actual values are used. In the case of
a uniformly growing cumulus that slopes in its lower layers and then straightens or even
15. C. F. Brooks, “Clouds in Aerology and Forecasting,” B. Amer. Meteorol. Soc., Vol. 22, November, 1941,
pp. 335–345.