These life cycles may run their course in as little as a day or last as long as a month. The longest-lasting tropical cyclone ever observed was Hurricane/Typhoon John, which existed for 31 days as it traveled a 13,000 km (8,100 mi) path from the eastern Pacific to the western Pacific and back to the central Pacific.
When does a storm receive a name? Tropical storms are given names when they display a rotating circulation pattern and wind speeds of 39 miles per hour (63 kilometers per hour). A tropical storm develops into a hurricane when wind speeds reach 74 mph (119 kph).
Names are assigned in alphabetical order each season. If the season is extremely active and the end of the alphabet is reached, storms are named after Greek alphabet letters (Alpha, Beta, Gamma, etc.). This has only happened once before, back in 2005. The sustained wind range for a tropical storm is 39 mph to 73 mph.
Hurricanes form over the warm ocean water of the tropics. When warm moist air over the water rises, it is replaced by cooler air. The cooler air will then warm and start to rise. This cycle causes huge storm clouds to form.
The oceans provide the source of energy for tropical cyclones both by direct heat transfer from their surface (known as sensible heat) and by the evaporation of water. This water is subsequently condensed within a storm system, thereby releasing latent heat energy.
The 2020 NOAA forecast calls for a likely range of 13 to 19 named storms (winds of 39 mph – 63 kph – or higher), of which six to 10 could become hurricanes (winds of 74 mph – 119 kph – or higher), including three to six major hurricanes (category 3, 4 or 5; with winds of 111 mph – 179 kph – or higher).
For tropical cyclonic storms to occur, there is a requirement that the Coriolis force must be present. Remember that the Coriolis effect is zero near the equator and increases to the north and south of the equator. Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
Tropical cyclones are like giant engines that use warm, moist air as fuel.
Classification of Tropical Cyclones
| Tropical Cyclone Classification | Maximum 10-minute mean wind near the centre |
|---|
| Tropical Depression | 41 to 62 km/h |
| Tropical Storm | 63 to 87 km/h |
| Severe Tropical Storm | 88 to 117 km/h |
| Typhoon | 118 to 149 km/h |
Tropical Depression:A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.
Only two tropical ocean basins do not support tropical cyclones, because they lack waters that are sufficiently warm. The Peru Current in the eastern South Pacific and the Benguela Current in the South Atlantic carry cool water Equatorward from higher latitudes and so deter tropical cyclone development.
Tropical cyclones, hurricanes or typhoons form when convection causes warm, moist air above the ocean to rise. They begin as a group of storms when the water gets as hot as 80 °F (27 °C) or hotter. The Coriolis effect made by the Earth's rotation causes the winds to rotate. Warm air rises quickly.
Impact of Tropical Cyclones
- Strong Winds. Of all the tropical cyclone damage agents, strong winds are perhaps the best understood.
- Rainfall and Flooding.
- Storm Surge and Waves.
- Tornadoes.
- Economic Impact.
- The Role of Meteorologists in Disaster Preparedness and Education.
- Compilation and Issue of Warnings.
- Dissemination of Warning Information.
A tropical depression is a tropical cyclone that has maximum sustained surface winds (one-minute average) of 38 mph (33 knots) or less. A tropical storm is a tropical cyclone that has maximum sustained surface winds ranging from 39-73 mph (34 to 63 knots).
Tropical cyclones on the open sea cause large waves, heavy rain, and high winds, disrupting international shipping and, at times, causing shipwrecks. Generally, after its passage, a tropical cyclone stirs up ocean water, lowering sea surface temperatures behind it.
It typically moves forward at speeds of 10-15 m.p.h., but can travel as fast as 40 m.p.h. At its very early and weak stages it is called a Tropical Depression. When the winds reach 39 m.p.h. it is called a Tropical Storm.
The circulation of high and low pressures causes the high winds and wind speed to pick up quickly, often taking meteorologists by surprise. The rapid winds are known to be the most destructive part of tropical storms, although heavy rains and thunder can accompany the wind.
The impact from cyclones extends over a wide area, with strong winds and heavy rains. However, the greatest damage to life and property is not from the wind, but from secondary events such as storm surges, flooding, landslides and tornadoes.
There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to sustain a low pressure center, a preexisting low level focus or disturbance, and low vertical wind
All of the cyclone development described thus far takes place at sea, but the entire cyclone also is blown along with the prevailing winds. Often this movement brings the storm toward land. Storm surges occur when the low barometric pressure near the center of a cyclone causes the water surface below to rise.
Hurricane or cyclone can start losing its energy and speed after hitting the land as they get energy from the warm ocean water. However, that doesn't happen so quickly. Often, the cyclone can move over to land, its wind field tends to increase. Thus, it can affect a larger area than what scientists may have calculated.
The strong winds of a tropical cyclone can cause dangerous waves that pose a significant hazard to mariners and coastal residents and visitors. When the waves break along the coast, they can produce deadly rip currents - even at large distances from the storm.
A tropical cyclone is classified as making landfall when the center of the storm moves across the coast; in strong tropical cyclones this is when the eye moves over land. When a tropical cyclone makes landfall, the eye closes in upon itself due to the weakening process, which causes surf to decrease.
In the Atlantic and the northeastern Pacific oceans, a tropical cyclone is generally referred to as a hurricane (from the name of the ancient Central American deity of wind, Huracan), in the Indian and south Pacific oceans it is called a cyclone, and in the northwestern Pacific it is called a typhoon.
When a tropical cyclone passes over land or cold water, the basic fuel that drives the storm is cut off--warm ocean water--is cut off. Passing over land will quickly weaken the storm (not because of friction as some believe, but because of the loss of the warm moisture source). Dry, cool air moves into the storm.
