DYNAMICS OF THE ATMOSPHERE 
isolated result and a complete picture of the variation 
with height does not exist at present. During the inver- 
sion period the gradient shows large long-period fluctua- 
tions unlike those met with in daytime. 
There is much less information concerning tempera- 
ture gradients over the ocean, but data have been 
given by Wiist [80], Johnson [29], Montgomery [35], 
Craig [10], Emmons [16], and Sverdrup [71]. Because of 
the relatively small effects of insolation and long-wave 
radiation on the sea, the gradients are much less than 
those found over land and diurnal effects are hardly 
noticeable, except in shallow landlocked waters. 
The exploration of the mean temperature field near 
the ground is thus virtually complete except in one 
important respect. It has now become clear that radia- 
tion, especially in the longer wave lengths, is quite as 
important as convection in the problem of heat transfer, 
and systematic simultaneous records of both tempera- 
ture gradient and radiative flux are needed to complete 
the picture of the thermal structure of the lower 
498 
TURBULENCE 
Experimental investigations into atmospheric turbu- 
lence may be conveniently divided into: (1) those 
dealing with localized effects, usually in shallow layers 
near the ground, in which the consequences of the 
PHYSICAL FEATURES OF ATMOSPHERIC 
earth’s rotation may be disregarded; (2) those relating 
to larger-scale processes, usually in the so-called “‘fric- 
tion layer” or “planetary boundary-layer” (Lettau), 
extending from the surface to the level at which the 
geostrophic velocity is attained (c. 500 m); and (8) 
those concerned with the atmosphere as a whole. 
The Temperature Field. The most direct manifesta- 
tion of atmospheric turbulence is the presence of fluctua- 
tions in the wind, but in view of the controlling influence 
of temperature gradient, it is convenient to begin by 
discussing the temperature field near the ground. Ac- 
curate continuous observations of temperature differ- 
ences between various heights extending from 2.5 cm 
to 87.7 m have been tabulated and analysed for southern 
England by Johnson [28], Best [2], Johnson and Hey- : : : ; 
aoa 130), and by aoe a be Ismailia, eh The Velocity Field. Following the lead given by the 
theory of the turbulent boundary layer in aerodynam- 
while Ramdas [48] has given mean values of air tem- © y 
perature at various levels from 2.5 em to 10.6 m at 1¢S,1t has now been shown conclusively by many workers 
atmosphere. 
that the profile of mean velocity near the ground in 
conditions of small temperature gradient is adequately 
represented by a logarithmic law (e.g., of the type 
proposed by Rossby and Montgomery, equation (7)), 
provided that the vegetation cover is not too high. For 
profiles measured above long grass it is necessary to use 
(z 2 & ar d) ? 
an empirical modification of the equation, namely 
1 ib ie = = 
20 
u — 
Tk 
where d is a zero-plane displacement, of the order of the 
depth of the layer of air trapped among the plants. 
Ux. 
maximum- and minimum-temperature epochs at Poona, 
Model has used the same equation for the wind profile 
India. From these investigations there emerges the 
now familiar picture of the temperature field in the 
lowest 100 m. In clear weather there is a well-marked 
diurnal variation of temperature gradient, with super- 
adiabatic lapse rates during the hours of daylight and 
pronounced inversions at night. With overcast skies, 
and especially with strong winds, the gradient remains 
close to the adiabatic lapse rate both day and night. 
The most significant feature of the work referred to 
over the sea. 
above is that which emerges from the detailed investi- 
gations of Sheppard [61], Paeschke [38], Deacon [12] 
and others. In conditions of small temperature gradi- 
ent, air flow near the ground is identical with that 
observed in the turbulent boundary layer of a fully 
The gradient of temperature near the ground is 
ferences in scale. Sheppard has shown that the Karman 
subject to such large variations with locality and season, 
time of day and height above the surface that it would 
be misleading to attempt to give representative values 
here. Very large gradients, as high as thousands of 
times the adiabatic lapse rate, are a persistent feature 
of the temperature field in the first few centimetres 
above the ground, especially in summer, but the sharp 
rough surface in the laboratory, despite the great dif- 
constant / has much the same value as in wind-tunnel 
work, while Deacon has provided good evidence that 
the aerodynamical theory of the roughness length is 
equally appropriate for natural surfaces. 
The real difficulties and complexities of the meteoro- 
logical problem begin to appear when the temperature 
gradient differs from the adiabatic lapse rate. The 
general high level of turbulence during the superadia- 
batic lapse-rate period promotes a free exchange of 
momentum between higher and lower levels, while the 
reverse holds during inversions. Thus, in general, the 
velocity gradient exhibits a diurnal variation, being 
small in daytime and large at night, but this is not all. 
Thornthwaite and Kaser [78] and, more recently, Dea- 
curvature in the temperature-height curve is confined 
to the first few metres. Sheppard [60] states that on the 
con [12] have shown that for nonadiabatic gradients 
average the daytime fall of temperature is roughly 
proportional to the logarithm of the height, but more 
detailed investigations by Deacon [12] indicate that, 
in general, the gradient of temperature is inversely 
proportional to a power of the height over the first 
17 m, the index being greater than unity during lapse 
conditions and less than unity in inversions. 
In warm weather the temperature of the air near the 
ground shows rapid fluctuations of considerable ampli- 
tude, as much as several degrees centigrade. Schmidt 
[58] gives observations by Robitzsch which show that 
these oscillations have a well-marked diurnal variation 
in phase with the temperature gradient, the fluctua- 
tions tending to die away as the lapse rate approaches 
the adiabatic value. Sutton [64], from an examination 
of records obtained in clear warm weather, has found 
that in these conditions the oscillations decrease as 
24 over the range 7 m S z S 45 m, but this is an 
