Weather with Heather: Tropical Storms
Information Provided by Captain Belle; Edited by 1/C Evan Bowen
When I heard that Captain Belle was training the seniors in tropical storms, I jumped at the opportunity to learn from him. Captain Belle is definitely the most knowledgeable person onboard the ship when it comes to meteorology, as he teaches that class back at school. Tropical storms were of special interest to me after seeing the effects of Hurricane Erma in Tortola.
A tropical storm is a warm-core, non-frontal low-pressure system, differentiating it from the frontal systems that drive our weather on shore. They develop over warm tropical waters which fuel the convective systems which produce towering clouds and thunderstorms. The defining feature, however, is the distinct cyclonic (counter-clockwise) rotation of the storm; this is actually reversed in the southern hemisphere. As the characteristics may vary with region, so do the names. They are known as hurricanes in the Atlantic and eastern Pacific, where the U.S. is responsible for forecasting; they are typhoons in the western Pacific, simply tropical cyclones in the Indian Ocean, and Willy-Willies in Australia.
Tropical storm’s strength is derived from release of stored energy from the evaporation of the warm tropical water. Tropical storms can actually produce up to 600 trillion watts of energy. The center (eye) is warmer than the rest of the storm, meaning that the storm has a warm core. The storms we see on shore, typical low-pressure systems, are powered by cold air masses, making them cold core.
Tropical storms form when latent heat is released by thunderstorms; warmer air rises and the moisture condenses, thus further warming the air. The surface pressure then drops, and air converges and rises, lowering the surface pressure further, bringing more warm moisture to further the heating. The pressure gradient steepens and more fuel is added. This self-fulfilling cycle is what produces the massive winds which hurricanes are known for. Once it is large enough, the rotation of the earth—the Coriolis effect—causes the storm to rotate.
The development of the storm is tracked by the National Hurricane Center in Miami through different stages. A tropical disturbance becomes a tropical depression, each identified by a number. Once the storm produces sustained wind speeds of over 34 knots (39 mph), it is a tropical storm and receives an official name. If a named storm produces winds consistently over 64 knots (74 mph), it has earned the title of hurricane. These winds can produce waves in excess of 45 feet in height, as dangerous to the shore as they are to ships.
Until next time,