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SUBTOPICS FOR ALL THINGS HURRICANE
Types of Storms
What is a Tropical Cyclone, Tropical Disturbance, Tropical Depression, Tropical Storm, Hurricane, and Typhoon?
A tropical cyclone is a generic term for a low-pressure system that formed over tropical waters (25°S to 25°N) with thunderstorm activity near the center of its closed, cyclonic winds. Tropical cyclones derive their energy from vertical temperature differences and are symmetrical and warm core.
If it lacks a closed circulation it is called a tropical disturbance. If it has a closed circulation but under 39 mph (34 knots, or 17 meters per second) maximum sustained surface winds, it is called a tropical depression. When winds exceed that threshold it becomes a tropical storm and is given a name. Once winds exceed 74 mph (64 knots, 33 meters per second) it will be designated a hurricane (in the Atlantic or East Pacific Oceans) or a typhoon (in the northern West Pacific).
Tropical Disturbances -> Tropical Depressions -> Tropical Storms -> Hurricane or Typhoon.
What is the difference between a Sub-tropical Cyclone, an Extra-tropical Cyclone, and a Post-tropical Cyclone?The Sub-tropical in Sub-tropical cyclone refers to the latitudes 25°N to 35°N (or °S). However, the term refers to cyclones whose characteristics are neither fully tropical nor extratropical. They are either asymmetrical with a warm core or symmetrical with a cold core. Sub-Tropical cyclones can transform into Tropical or Extra-tropical storms depending on conditions.
The Extra-tropical in Extra-tropical cyclone refers to the latitudes 35°N to 65°N (or °S). However, the term refers to cyclones that get their energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones are low-pressure systems generally associated with cold fronts, warm fronts, and occluded fronts. They are asymmetrical and have a cold core.
A Post-Tropical Cyclone is a former tropical cyclone that no longer possesses sufficient characteristics to be considered a tropical cyclone, such as convection at its center. Post-tropical cyclones can continue producing heavy rains and high winds. Former tropical cyclones that have become fully extra-tropical, sub-tropical, or remnant lows, are three classes of post-tropical cyclones.
Neutercane is a term no longer in use. It referred to small (<100 miles in diameter) Sub-tropical low-pressure systems that are short-lived.
What is a Hurricane?
Once winds from the tropical cyclone reach 74 mph (64 knots, 33 meters per second) it is classified as a Hurricane in the Atlantic Basin. However, around the world there are different names for this storm. For instance, a storm of the same magnitude in the Northwest Pacific is called a Typhoon.
What is the Saffir-Simpson Scale?The Saffir-Simpson Scale classifies hurricane-strength tropical cyclones into five categories (1-5) based on maximum sustained wind speed. Major Hurricanes (also called Intense Hurricanes) fall into categories 3, 4, and 5 on the Saffir-Simpson Scale. A Super-Typhoon reaches category 4 or 5 on the Saffir-Simpson Scale.
Anatomy & Lifecycle of a Tropical Storm
How Do Tropical Cyclones Form?In order for a tropical cyclone to form, several atmospheric and marine conditions must be met.
Temperature & Humidity: Ocean waters should be 80° Fahrenheit at the surface and warm for a depth of 150 feet, because warm ocean waters fuel the heat engines of tropical cyclones. They also need an atmosphere which cools fast enough with increasing height so that the difference between the top and bottom of the atmosphere can create thunderstorm conditions. A moist mid-troposphere (3 miles high) because dry air ingested into thunderstorms at mid-level can kill the circulation.
Spin & Location: The Coriolis force is an apparent force that deflects movement to the right coming from the Northern hemisphere and to the left coming from the Southern hemisphere. The force is greatest at the poles and zero at the equator, so the storm must be at least 300 miles from the equator in order for the Coriolis force to create the spin. This force causes hurricanes in the Northern hemisphere rotate counter-clockwise, and in the southern hemisphere to rotate clockwise. This spin may play some role in helping tropical cyclone to organize. (As a side note: the Coriolis force is not strong enough to affect small containers such as in sinks and toilets. The notion that the water flushes the other way in the opposite hemisphere is a myth.)
Wind: Low vertical wind shear (the change of wind speed and direction with height) between the surface and the upper troposphere favors the thunderstorm formation which provides the energy for tropical cyclones. Too much wind shear will disrupt or weaken the convection.
What Causes Tropical Cyclones and What Affects Their Formation?In addition to hurricane-favorable conditions such as temperature and humidity, many repeating atmospheric phenomenon contribute to causing and intensifying tropical cyclones. For example, African Easterly Waves (AEW) are winds in the lower troposphere (ocean surface to 3 miles above) that originate and travel from Africa at speeds of about 3mph westward as a result of the African Easterly Jet. These winds are seen from April until November. About 85% of intense hurricanes and about 60% of smaller storms have their or