220 RADIO WAVE PROPAGATION EXPERIMENTS 
on the ground is available, vapor pressure gradients 
follow evaporation processes and are maximum at the 
time of the maximum temperature at about sunrise. 
This diurnal course characterizes conditions over the 
sea, cloudy days over the land, and winter or rainy 
seasons over the continent. 
On clear days over the continent the vertical vapor 
pressure gradient is minimum at about sunrise, 
reaches a maximum in midmorning, and lowers to a 
secondary minimum at the time of the maximum tem- 
perature (in desert regions this is the principal maxi- 
mum shortly after sunset). Evaporation and mixing 
with drier air aloft govern this course. At night the 
soil absorbs moisture from the air causing a decrease 
in vapor pressure gradient. 
The seasonal and diurnal variation of vapor pres- 
sure gradient is illustrated in Figure 11. Maximum 
vapor pressure gradients are noted during April, the 
hottest time of year, and minimum in January. The 
oceanic type is represented by the curve for July in 
the rainy season. 
Vertical'M Gradient 
Over the sea both temperature and humidity gradi- 
ents aid in causing a maximum of trapping during the 
day and a minimum during the night. The effect, 
however, is probably small. 
Over the continent, a minimum of trapping will 
exist in midafternoon. Thereafter both temperature 
and humidity factors will cause a rise in the vertical 
M gradient to a maximum shortly after sunset. From 
oes 
eee 
ie 
eo 
o” 
VAPOR PRESSURE IN MILLIBARS. 
= 
/ 
Gi 
be 
fa 
a 
skal 
TIME (IST) 
Ficure 11. Hourly vapor pressure difference, 6 to 46 ft 
at Calcutta. 
that time to sunrise a decrease in the M gradient will 
occur. However, the height of the inversion continues 
to.grow until sunrise, tending to cause an increase in 
the height of the duct. Whether a maximum or mini- 
mum of trapping will occur at sunrise will depend 
on whether the increase in the height of the inversion 
balances the decrease in vertical humidity gradient. 
It is probable that the humidity factor is the more 
important since the small magnitude of the tempera- 
ture increase in the upper portions of the inversion 
will seldom be sufficient to cause a decrease in M with 
height. After sunrise rising humidity gradients, par- 
tially balanced by falling temperature gradients, will 
cause a small maximum of M gradient at midmorning. 
Thereafter the M gradient will decrease to the after- 
noon minimum. 
The maximum and minimum decrease of M from 
6 ft to 46 ft, based on mean temperature and humidity 
data at Calcutta, India, are given in Table 12. 
In July the morning minimum and afternoon maxi- 
mum with small amplitude illustrate the oceanic type. 
The other months illustrate the continental type. Ac- 
cording to the table a maximum of trapping in India 
should occur in April just prior to the rainy season. 
TaBLeE 12. Decrease of M from 6 to 46 ft (Calcutta). 
A.M, P.M. 
Month Min Max Min Max 
Jan 1 4 1 6 
April 7 9 5 16 
July 2 56 6 
Oct 1 7 3 11 
DETERMINING FLUCTUATIONS IN RE- 
FRACTIVE INDEX NEAR LAND OR SEA! © 
Tn connection with the rapid fluctuation or scintil- 
lation frequently observed in microwave reception, 
questions arise concerning turbulent atmospheric 
fluctuation at fixed points along the transmission 
path, particularly fluctuations in refractive index. 
Rapid measurement of both temperature and humidity 
so as to give a direct determination of fluctuation in 
refractive index is difficult. The purpose of this paper 
is to suggest that in certain cases the measurement 
of temperature fluctuation alone can give a good in- 
direct estimate of fluctuation in the modified index. 
The basic principle underlying this suggestion is 
that, if two initial kinds of air are mixed in different 
proportions, for all possible mixtures a fixed relation 
exists between any two properties conservative for 
adiabatic changes. 
To illustrate this, consider a diagram with poten- 
tial temperature and specific humidity as coordinates. 
Two initial kinds of air would be represented by two 
iBy R. B. Montgomery, Radiation Laboratory, MIT. 
