Other Depressions — Page 351, Lesson 325

I want to walk you through the conditions needed for a tropical revolving storm — a TRS — to form and sustain itself, then how it moves, and finally the stages it goes through from birth to decay. First, the ocean has to supply continuous energy. There must be a sufficient depth of warm water — specifically 200 to 300 feet deep — in the ocean to provide that continual energy source. If the warm water layer is too shallow, the storm would quickly drain the energy from the ocean and cease to develop. So the depth of that warm water is a critical limiting factor. Second, there must be very little wind shear within the atmosphere. Wind shear means a change in wind speed or direction with height. If there is significant shear, the storm would topple — the vertical structure would get tilted and disrupted. Shear also has another effect: it increases the area over which latent heat is released. Latent heat is the heat released when water vapour condenses into cloud droplets. If that release is spread over a larger area, its effect on intensifying the storm is reduced. So low shear is essential for the storm to strengthen. Now let's look at movement. The path of a TRS depends greatly on the wind belt in which it is located. Since most TRSs originate from the tropics, they are initially driven westwards by the easterly trade wind belt at around 10 to 20 knots. That's their first steering mechanism. Eventually the storms move away from the Equator. As they do, they increase in strength because the Coriolis force increases. The Coriolis force is the apparent deflection caused by the Earth's rotation, and it's weaker near the Equator and stronger at higher latitudes. More Coriolis force means better spin-up of the storm. At higher latitudes, the subtropical highs and the prevailing westerlies take over and drive the TRS eastwards. But by this stage the storms have moved to latitudes where the seas are now too cold to feed energy into the storm, and they will eventually die. Also, if at any time the storm goes over land, the influx of moisture is cut off — and again, the storm will die. So two things kill it: cold sea surface temperatures, or moving over land. Now the stages of development. TRSs evolve through a life cycle, from birth to death, much like that of a thunderstorm. The stages are based on the organization of the storm and the sustained wind speeds they create. Importantly, not all of the stages will eventually evolve into a full TRS. Stage 1 is called a Tropical Depression. This is designated when we see the first appearance of a lowered pressure and organized cyclonic circulation in the centre of the thunderstorm complex. Cyclonic circulation means the air is spinning in the same direction as the Earth's rotation — counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. On a surface pressure chart, you will see at least one closed isobar — a closed loop of equal pressure — around the system. Stage 2 is called a Tropical Storm. A tropical depression is upgraded to a tropical storm when the sustained wind speeds exceed 33 knots. It is at this stage that the system is assigned a name. Stage 3 is called a Tropical Cyclone. The system is designated a tropical cyclone, hurricane, or typhoon — the name depends on the location — when sustained wind speeds are greater than 63 knots. At this stage there is a pronounced rotation around a central core, which will eventually form what we call the "eye." Let me describe the eye in detail, because it's one of the most recognizable features of a TRS. The eye is found at the centre, with a typical diameter of 20 to 50 kilometres. A tightening of the eye is a useful guide that the storm is increasing in strength — if the eye gets smaller, the storm is getting more intense. Inside the eye we find the lowest surface pressures and the calmest conditions. Here's how it forms: as air is forced up and outward from the storm, some of that air returns down the centre. That descending air undergoes adiabatic heating — it warms as it compresses. This heating evaporates clouds, creating the familiar clear column of air that distinguishes the eye itself. There's an important temperature detail here. The air descending in the eye has cooled at the Saturated Adiabatic Lapse Rate — the SALR — while it was in the clouds of the eye wall. But now that air is dry, so as it descends it warms at the Dry Adiabatic Lapse Rate — the DALR. The result is that the air in the eye is warmer than the air in the eye wall surrounding it. That's a key characteristic of a mature tropical cyclone.