The most important feature of the diurnal variations of temperature in surface water of the oceans is the fact that it is even smaller than in lakes. It is often smaller than 0.4°C. However, when the weather is fair and calm it can rise to about 1°C.

Another characteristic feature of the diurnal range is that it is confined within narrow limits. The diurnal variations tend to decrease towards higher latitudes. According to Defant, the maximum variations are found at 14.00 hours, whereas the minimum occurs at 04.00-05.00 hours.

The results from a study of the daily temperature records of the Meteor Expedition show that the average variation in the daily maximum and minimum temperatures for various areas of the South Atlantic Ocean amounts to only 26 C.

According to Meinardus, the diurnal range in the lower latitudes varies from 0.3°C to 0.4°C, whereas in the higher latitudes it can be less than this.

ADVERTISEMENTS:

An analysis of the data obtained from the ‘Meteor’ and ‘Challenger’ Expeditions shows that the diurnal range in the equatorial oceans “and seas is asymmetrical, because after the sunrise the temperature of the surface water rises quickly, but after the sunset the rate of decrease of temperature is rather slow.

On the contrary, away from the equator the diurnal range is symmetrical. As a result of the observation of the trends of diurnal ranges in various areas near the coasts of the Great Britain it was found that the range in the month of December was 0.2°C, while in May it was 0.69°C.

The Challenger Expedition made it clear, on the basis of its observations, that the amount of cloudiness and wind velocity exert important influence on the diurnal range of surface water temperature.

Clear skies favour the higher range, while the larger amount of cloudiness results in the lower range of diurnal variation. Calm weather and light breeze are favourable for the higher range, whereas the high velocity winds minimise it.

ADVERTISEMENTS:

In the oceans the diurnal range of temperature is so small that from the viewpoints of various physical and biological processes going on in the ocean water they are of little importance.

However, when the daily exchange of heat between the atmosphere and the oceans are taken into consideration, the study of the diurnal range of temperature becomes important.