Other Depressions — Page 351, Lesson 322

I want to walk you through a new topic now — Tropical Revolving Storms, often abbreviated as TRS. These are a type of thermal depression, and they are among the most powerful and destructive weather systems on the planet. Let's start with the description. A Tropical Revolving Storm is a thermal depression that develops over warm tropical oceans. The key threshold here is wind speed: a system is called a TRS when it has sustained wind speeds in excess of 33 knots. Once that sustained wind speed exceeds 63 knots, it is officially designated a tropical cyclone. So, 33 knots is the lower boundary for the TRS category, and 63 knots is the boundary for cyclone status. These storms are described as the most destructive and extensive weather phenomenon affecting our planet. To give you a comparison: a tornado may have winds that momentarily exceed those of a TRS, but a tornado's life cycle is measured in minutes. A TRS, on the other hand, can last up to about two weeks. Its physical size can match that of a polar front depression — the kind of low-pressure system you'd see in the mid-latitudes — but its intensity is far, far greater. Now, let's look at how they form. Hurricanes — which are a type of TRS — form from complexes of thunderstorms, typically on the western side of the oceans. But these thunderstorms can only grow to hurricane strength under the right conditions of both the ocean and the atmosphere. There are two main ways these thunderstorms are born. The primary way is from the Intertropical Convergence Zone, or ITCZ. At the Equator, the easterly trade winds from the Northern and Southern Hemispheres converge, creating a band of storms that circles the globe. That band is the ITCZ, and it's a breeding ground for TRS development. The second way is from equatorial easterly atmospheric waves, also called easterly waves, which originate in NW Africa. These easterly waves are the seeds for hurricanes in the North Atlantic and the Northeast Pacific. We'll look at easterly waves in more detail in Chapter 20. Now, here's the engine that powers these storms. The TRS generates its power and energy from the release of large amounts of latent heat. The storms gain moisture over warm seas. When that moist air rises and cools to condensation, it releases latent heat. That release of heat causes the air to expand, which further reduces the surface pressure at the storm's centre. Lower surface pressure creates even stronger convergence of air at the surface, which in turn causes more moist air to rise and cool to condensation, releasing even greater amounts of latent heat. It's a self-reinforcing cycle — a positive feedback loop that drives the storm's intensity. There are specific requirements for a TRS to form. First, the system must be within 5 and 25 degrees of latitude. Below 5 degrees, the Coriolis force is too small to spin the storm up. Above 25 degrees, the sea is usually too cold to provide the necessary energy. Second, ocean temperatures must be greater than 26°C. The higher the ocean temperature, the greater the pressure drop within the core of the storm. This is the reason we do not usually see TRS forming in the southern Atlantic — the sea surface temperatures there are simply too low. So, to summarise: a TRS is a thermal depression over warm tropical oceans, with sustained winds above 33 knots, and above 63 knots it becomes a tropical cyclone. It forms from thunderstorm complexes, either from the ITCZ or from easterly waves off NW Africa. It's powered by latent heat release in a feedback loop that lowers pressure and intensifies convergence. And it requires latitudes between 5 and 25 degrees, and sea surface temperatures above 26°C. That's the foundation for understanding these powerful systems.