With moderate or light winds and a clear sky the diurnal heating which occurs near the sea surface can cause a serious reduction in the range of submarine detection, especially on shallow targets. This has usually been called the "afternoon effect", although as will be noticed below the ranges often remain short long after sun down. The heating of surface waters which causes such sharp downward refraction can of course be noted on a bathythermograph record, provided pen vibration does not confuse the upper part of the trace. Unfortunately it is the upper 20 or 30 feet of a bathythermograph curve which in the case of ships moving faster than 12 knots is often somewhat difficult to read with sufficient certainty. Moreover, in planning a days operations it is clearly desirable to know in advance how much reduction in range may be expected from diurnal warming. Unfortunately it has turned out that five, more or less independent variables are involved. Listed in the order of their importance these are as follows: the altitude of the sun, the degree of cloud coverage, the strength of the wind, the difference in temperature between air and water, and the humidity of the air. It was at first thought that wind and cloud observations alone would be sufficient in most cases for a rough prediction of the seriousness of diurnal warming to echo ranging conditions. Thus it has been previously reported that with winds of force 4 or greater it can be expected that turbulence will prevent thermal stability from developing at depths critical to sound ranging, while with lighter winds ranges will be more or less reduced in the afternoon, except during cloudy weather. But the problem is considerably more complex than this and such simplification is not always justified.
Office of Scientific Research and Development National Defense Research Committee Division C - Section 4. OSRD No., Section No. C4=sr31-137