Definition of weather patterns

Question: What is the difference between a hurricane, a cyclone, a typhoon and a tornado?

Answer: The answer to your questions comes from the National Weather Service Web site:

http://www.nws.noaa.gov/

1) What is a hurricane, typhoon, or tropical cyclone?
The terms “hurricane” and “typhoon” are regionally specific names for a strong “tropical cyclone”. A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.

Tropical cyclones with maximum sustained surface winds of less than 39 mph are called “tropical depressions” Once the tropical cyclone reaches winds of at least 39 mph they are typically called a “tropical storm” and assigned a name. If winds reach 74 mph, then they are called:

•”hurricane” (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E)

•”typhoon” (the Northwest Pacific Ocean west of the dateline)

•”severe tropical cyclone” (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E)

•”severe cyclonic storm” (the North Indian Ocean)

•”tropical cyclone” (the Southwest Indian Ocean)

2) How are tropical cyclones different from tornadoes?
While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of 100s of meters (1 meter = 39.37 inches) and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter on the scale of 100s of kilometers (1 kilometer = 39,370 inches) and is comprised of several to dozens of convective storms. Additionally, while tornadoes require substantial vertical shear of the horizontal winds (i.e. change of wind speed and/or direction with height) to provide ideal conditions for tornado genesis, tropical cyclones require very low values less than 23 mph) of tropospheric vertical shear in order to form and grow. These vertical shear values are indicative of the horizontal temperature fields for each phenomenon: tornadoes are produced in regions of large temperature gradient, while tropical cyclones are generated in regions of near zero horizontal temperature gradient. Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes have occurred). In contrast, tropical cyclones are purely an oceanic phenomena – they die out over-land due to a loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes.

3) Why do tropical cyclones spawn tornadoes?
An interesting side note is that tropical cyclones at landfall often provide the conditions necessary for tornado formation. As the tropical cyclone makes landfall and begins decaying, the winds at the surface die off quicker. This sets up a fairly strong vertical wind shear that allows for the development of tornadoes, especially on the tropical cyclone’s right side (with respect to the forward motion of the tropical cyclone). For the southern hemisphere, this would be a concern on the tropical cyclone’s left side – due to the reverse spin of southern hemisphere storms.

Linda Niemi

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