Weather Facts

What is a Sub-Tropical Storm?

Sub-Tropical Storms are actually a fairly new designation of ocean storms that have some tropical characteristics, but are not truly tropical cyclones. Years ago, they werenít recognized by the National Hurricane Center (NHC). Lows were either extra-tropical or tropical cyclones. However, there was strong pressure on the NHC from several meteorologists to designate certain storm systems as sub-tropical since although they werenít truly tropical in nature, they did have some characteristics of a tropical cyclone and they caused more problems than extra-tropical storms. What makes a storm tropical? It has to be a ìwarm coreî system. That is, temperatures warm as you head towards the center of the storm. The processes that create the clouds, winds, and rain in a warm core system are different than an extra-tropical storm, which is your everyday cold season low pressure center. Warm core systems donít have fronts and donít need a difference in air masses to cause the pressure to lower or precipitation to form. Warm core systems depend on the warmth of the ocean for their energy. The warm, moist air evaporates from the ocean. Since this air is lighter and more buoyant, it rises into the atmosphere and condenses into clouds as it cools. The condensation process gives off latent heat which further accelerates the rising air. Sub-tropical means that the storms have the same processes that tropical storms do to create the lowering pressures and the rainfall, but they also have some of the processes that the cold core systems have, such as fronts and baroclinic processes due to the different air masses.