By Agencies

A rare sub-tropical depression has formed over open water.

It may have formed in open water, well away from any human habitation, but Sub-Tropical Depression One has already made a name for itself.

It shows on satellite imagery as a swirling mass of thunderstorms, located about 1,170km to the westsouthwest of The Azores. It has sustained winds of 55 kilometres an hour and it is heading towards the north at 22km/h.

Such low pressure systems are usually confined to the hurricane season, which runs from June 1 to November 30. This depression seems to have beaten the odds, becoming only the fourth such depression recorded in the Atlantic, according to NOAA's Historical Hurricane Track website.

The formation of these tropical or sub-tropical lows usually requires sea surface temperatures (SSTs) of at least 26.5C. Currently, SSTs in the region are only 20C, and the disruptive effects of wind shear (the change of wind speed and direction with height) are quite high.

This depression seems to have sustained itself because of the very cold air extending through the atmosphere, which has supplied the atmospheric instability for thunderstorm formation, required for One's classification.

Only one of the previous April depressions - Tropical Storm Ana in 2003 - went on to become a named storm.

It is possible for systems to develop into full-blown storms outside the hurricane season. This happened in 2016, with Hurricane Alex in January and Tropical Storm Bonnie in May.

There are many state and private organisations that issue predictions of the coming hurricane season. The general consensus seems to be that activity will be slightly below average, with a total of 11 to 12 named storms, four to six hurricanes, and between zero and three major hurricanes.

It is unlikely that Sub-Tropical Depression One is going to cause any revision of the forecasts. If SSTs had been well above average, then that might have been a different matter. But seasonal hurricane forecasting is still in its relative infancy, so forecasters will be closely monitoring the situation in the Atlantic in the weeks ahead.