Summary of bathythermograph observations from the western North Atlantic : October 1940 - December 1941

The range of submarine detection is frequently limited by the refraction produced by vertical temperature gradients in the superficial layers of the ocean. In order to measure these temperature gradients and thus to permit predictions of the range, the bathythermograph was developed and is now being used on a considerable number of anti-submarine vessels, while a somewhat modified version of the instrument is being tried out on submarines. Some 6675 bathythermograph observations from the western North Atlantic have been examined in order to determine how frequently such observations should be made so that within practical limits and anti-submarine vessel may at all times know the assured range of its sound gear. The occurrence of the four basic types of refraction patterns is shown by a series of six charts. For all but one of these patterns the range can be rather quickly and easily estimated from simple tables; but when the so-called afternoon effect is encountered, which is on the average about 20% of the time, a more complete analysis is necessary. It is found that under the most unfavorable circumstances, that is, in mid-summer and near the edges of a strong current system, there is about one chance in three that the refraction pattern will chance significantly in a distance of four miles. At other times of year and in areas where horizontal variations in temperature are less pronounced a single bathythermograph observation can be considered representative of a much larger area. It is also shown that in the western North Atlantic about 92% of the time in summer and about 34% of the time in winter the assured range of submarine detection is limited by refraction to less than 2500 yards.