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Basic Definitions

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 (Holland 1993).

Tropical cyclones with maximum sustained surface winds of less than 34 kt (39 mph, 17 m/s) are called tropical depressions. Once the tropical cyclone reaches winds of at least 34 kt (39 mph, 17 m/s) they are typically called a tropical storm and assigned a name. If winds reach 64 kt (74 mph,33 m/s), they are called:

  • hurricane in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E
  • typhoon in the Northwest Pacific Ocean west of the dateline
  • severe tropical cyclone in the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E
  • severe cyclonic storm in the North Indian Ocean
  • tropical cyclone in the Southwest Indian Ocean

2. What is a tropical disturbance, a tropical depression, or a tropical storm ?

These are terms used to describe the progressive levels of organized disturbed weather in the tropics that are of less than hurricane status.

  • Tropical Disturbance
    A discrete tropical weather system of apparently organized convection - generally 200 to 600 km (100 to 300 nmi) in diameter - originating in the tropics or subtropics, having a non-frontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. Disturbances associated with perturbations in the wind field and progressing through the tropics from east to west are also known as easterly waves.
  • Tropical Depression
    A tropical cyclone in which the maximum sustained wind speed (using the U.S. 1 minute average standard) is 33 kt (38 mph, 17 m/s). Depressions have a closed circulation.
  • Tropical Storm
    A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1 minute average standard) ranges from 34 kt (39 mph,17.5 m/s) to 63 kt (73 mph, 32.5 m/s). The convection in tropical storms is usually more concentrated near the center with outer rainfall organizing into distinct bands.
  • Hurricane
    When winds in a tropical cyclone equal or exceed 64 kt (74 mph, 33 m/s) it is called a hurricane (in the Atlantic and eastern and central Pacific Oceans). Hurricanes are further designated by categories on the Saffir-Simpson scale. Hurricanes in categories 3, 4, 5 are known as Major or Intense Hurricanes.

The wind speed mentioned here are for those measured or estimated as the top speed sustained for one minute at 10 meters above the surface. Peak gusts would be on the order of 10-25% higher.

3. What is a super-typhoon ? What is a major hurricane ? What is an intense hurricane ?

Major hurricane is a term utilized by the National Hurricane Center and Central Pacific Hurricane Center for hurricanes that reach maximum sustained 1-minute surface winds of at least 96 kt, (50 m/s, 111 mph). This is the equivalent of category 3, 4 and 5 on the Saffir-Simpson scale.

Intense hurricane is an unofficial term, but is often used in the scientific literature. It is the same as major hurricane.

4. What is an easterly wave ?

Grpahic showing the Azore High in the Atlantic

It has been recognized since at least the 1930s (Dunn 1940) that lower tropospheric (from the ocean surface to about 5 km (3 miles) with a maximum at 3 km (2 miles)) westward traveling disturbances often serve as the "seedling" circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. Riehl (1945) helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee (1972) who documented that the waves were being generated by an instability of the African easterly jet. (This instability - known as baroclinic-barotropic instability - is the are where the value of the potential vorticity begins to decrease toward the north.) The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert in contrast with substantially cooler temperatures along the Gulf of Guinea coast.

The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km (1200 to 1500 miles) typically (Burpee 1974). One should keep in mind that the waves can be more correctly thought of as the convectively active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year.

While only about 60% of the Atlantic tropical storms and minor hurricanes ( Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves (Landsea 1993). It has been suggested that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995).

It is currently completely unknown how easterly waves change from year to year in both intensity and location and how these might relate to the activity in the Atlantic and East Pacific.

5. What is a Cape Verde hurricane ?

Graphic showing the Subtropical Ridge in the Atlantic

Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000 km or 600 miles or so) to the Cape Verde Islands and then become hurricanes before reaching the Caribbean. Typically these occur in August and September, but in rare years (like 1995) there may be some in late July and/or early October. The numbers range from none up to around five per year - with an average of around 2.

6. What is a sub-tropical cyclone ?

A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50°N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often these storms have a radius of maximum winds which is farther out (on the order of 100-200 km [60-125 miles] from the center) than what is observed for purely tropical systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about (64 kt (74 mph, 33 m/s).

Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Florence in November 1994 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973).

Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds:

  • less than 18 m/s (34 kts, 39 mph) - subtropical depression
  • greater than or equal to 18 m/s (34 kts, 39 mph) - subtropical storm

Prior to 2002 subtropical storms were not given names, but the National Hurricane Center in Miami issued forecasts and warnings similar to those for tropical cyclones. Now they are given names from the tropical cyclone list. For more information see Penn State University's article on the Subtropical Cyclones. Forecasts are not issued for subtropical cyclones in the Pacific basin.

7. What is an extra-tropical cyclone ?

Graphic comparing tropical and extratropical cyclones

The top schematics show horzontal maps of the surface temperature and wind fields associated with a tropical cyclone (left) and an extratropical cyclone (right). Colors indicate temperature (blue 15C=59F, blue green 20C=68F, green 25C=77F). Solid lines indicate surface windspeeds - 34 kt=39 mph=63 kph and 64 kt=74 mph=117 kph . The bottom schematics show vertical maps of the pressure surfaces and circulation at the surface and tropopause.

An extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones (also known as mid-latitude or baroclinic storms) are low pressure systems with associated cold fronts, warm fronts, and occluded fronts.

Tropical cyclones, in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics (Holland 1993, Merrill 1993).

Structurally, tropical cyclones have their strongest winds near the earth's surface , while extra-tropical cyclones have their strongest winds near the tropopause - about 8 miles (12 km) up. These differences are due to the tropical cyclone being warm-core in the troposphere (below the tropopause) and the extra-tropical cyclone being warm-core in the stratosphere (above the tropopause) and cold-core in the troposphere. Warm-core refers to being relatively warmer than the environment at the same pressure surface (pressure surfaces are another way to represent height or altitude).

Often, a tropical cyclone will transform into an extra-tropical cyclone as it recurves poleward and to the east. Occassionally, an extra-tropical cyclone will lose its frontal features, develop convection near the center of the storm and transform into a full-fledged tropical cyclone. Such a process is most common in the North Atlantic and Northwest Pacific basins. The transformation of tropical cyclone into an extra-tropical cyclone (and vice versa) is currently one of the most challenging forecast problems (i.e., Jones et al. 2003).

Jones, S.C., Harr, P.A., Abraham, J., Bosart, L.F., Bowyer, P.J., Evans, J.L., Hanley, D.E., Hanstrum, B.N., Hart, R.E., Lalaurette, F., Sinclair, M.R., Smith, R.K., Thorncroft, C. 2003: The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions. Weather and Forecasting, 18, 1052-1092.

Merrill, R. T., (1993): "Tropical Cyclone Structure" - Chapter 2, Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland. Web version of Guide

8. What is storm surge and how is it different from tidal surge ?

Storm surge is the onshore rush of sea or lake water caused by the high winds associated with a landfalling cyclone and secondarily by the low pressure of the storm.

Tidal surge is often misused to describe storm surge, but storm surge is independent of the usual tidal ebb and flow. In some inlets, such as the Bay of Fundy, rapid changes in sea level due to the tides will cause a tidal bore or surge to move in to or out of the inlet. This surge occurs independently of the present weather.

9. What is a CDO ?

Satellite picture of a Central Dense Overcast storm

CDO is an acronym that stands for central dense overcast. This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rain bands. Before the tropical cyclone reaches hurricane strength (33 m/s, 64 kts, 74mph), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDO's are indicative of favorable, low vertical shear environments.

10. What is a TUTT ?

Streamline analysis depicting a Tropical Upper Tropospheric Trough

A TUTT is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an upper cold low. TUTTs are different than mid-latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere. For a more detailed discussion on TUTTs see the article by Fitzpatrick et al. (1995).

11. What is the Saharan Air Layer (SAL) ? How does it affect tropical cyclones ?

Satellite pictures of the Saharan Air Layer

The Saharan Air Layer (SAL) is a mass of very dry, dusty air which forms over the Sahara Desert during the late spring, summer, and early fall and usually moves out over the tropical Atlantic Ocean. The SAL usually extends between 5,000-20,000 ft (1500-6000 m) in the atmosphere and is associated with large amounts of mineral dust, dry air (~50% less moisture than a typical tropical sounding), and strong winds (~25-55 mph or ~10-25 m/s).

The SAL has been shown to have significant negative impact on tropical cyclone intensity. Its dry air can act to weaken a tropical cyclone by inhibiting updrafts in the storm, while its strong winds can substantially increase the vertical wind shear in and around the storm environment. It is not yet clear what effect the SAL's dust has on tropical cyclone intensity, though some studies have suggested that it too may have a negative impact on intensification.

The SAL can cover an area the size of the continental U.S. and has been tracked as far west as the Caribbean Sea, Central America, and the Gulf of Mexico. Real-time satellite imagery for tracking the SAL can be found here.


Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor. Soc., vol. 85 no. 3, 353-365.

12. What is the eye ? How is it formed and maintained ? What is the eyewall ? What are spiral bands ?

Graphical 3-D representation of a hurricane

The eye is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft - the eye temperature may be 10°C [18°F] warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only 0-2°C [0-3°F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988).

The eye is surrounded by the eyewall, the roughly circular ring of deep convection which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate and occasionally strong updrafts and downdrafts. The eye's warm temperatures are due to compressional warming of the subsiding air. Most soundings taken within the eye show a low-level layer which is relatively moist, with an inversion above suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km [ 1-2 mi] of the surface.

The exact mechanism by which the eye forms remains somewhat controversial. One idea suggests that the eye forms as a result of the downward directed pressure gradient associated with the weakening and radial spreading of the tangential wind field with height (Smith, 1980). Another hypothesis suggests that the eye is formed when latent heat release in the eyewall occurs, forcing subsidence in the storm's center (Shapiro and Willoughby, 1982). It is possible that these hypotheses are not inconsistent with one another. In either case, as the air subsides, it is compressed and warms relative to air at the same level outside the eye and thereby becomes locally buoyant. This upward buoyancy approximately balances the downward directed pressure gradient so that the actual subsidence is produced by a small residual force.

Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall convection. Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are sometimes called spiral bands. Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eyewall form (Willoughby 1979, 1990a, 1995). Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use more research to ascertain which mechanism is primary.

Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centered at the circulation center of the storm (Willoughby et al. 1982,Willoughby 1990a).

Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.

13. What is a moat in a hurricane ?

Radar image showing the moat in a stormNOAA/HRD

The term moat usually refers to the region between the eyewall and an outer rainband, such as a secondary eyewall rainband (see the image above). The moat is the relatively light rain region between the rainband and the eyewall.

14. How do tropical cyclones form ?

Graphic showing the pieces that contribute to tropical cyclone formation

To undergo tropical cyclogenesis, there are several favorable precursor environmental conditions that must be in place (Gray 1968,1979) :

  1. Warm ocean waters (of at least 26.5%deg;C [80°F]) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]). Warm waters are necessary to fuel the heat engine of the tropical cyclone.
  2. An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.
  3. Relatively moist layers near the mid-troposphere (5 km [3 mi]). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.
  4. A minimum distance of at least 500 km [300 mi] from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
  5. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.
  6. Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.

Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes [MCC]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages:

  • stage 1 occurs when the MCC produces a mesoscale vortex
  • stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds

15. Why do tropical cyclones require 80°F (26.5°C) ocean temperatures to form ?

Tropical cyclones can be thought of as engines that require warm, moist air as fuel (Emanuel 1987). This warm, moist air cools as it rises in convective clouds (thunderstorms) in the rainbands and eyewall of the hurricane The water vapor in the cloud condenses into water droplets releasing the latent heat which originally evaporated the water. This latent heat provides the energy to drive the tropical cyclone circulation, though actually very little of the heat released is utilized by the storm to lower its surface pressure and increase the wind speeds.

In 1948 Erik Palmen observed that tropical cyclones required ocean temperatures of at least 80°F (26.5°C) for their formation and growth. Later work (e.g., Gray 1979) also pointed out the need for this warm water to be present through a relatively deep layer (~150 ft, 50 m) of the ocean. This 80°F value is tied to the instability of the atmosphere in the tropical and subtropical latitutes. Above this temperature deep convection can occur, but below this value the atmosphere is too stable and little to no thunderstorm activity can be found (Graham and Barnett 1987).


Graham, N. E., and T. P. Barnett, 1987: Sea surface temperature, surface wind divergence, and convection over tropical oceans. Science, No.238, pp. 657-659.

Gray, W.M. 1979 : "Hurricanes: Their formation, structure and likely role in the tropical circulation" Meteorology Over Tropical Oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc., James Glaisher House, Grenville Place, Bracknell, Berkshire, RG12 1BX, pp.155-218.

Palmen, E. H., 1948: On the formation and structure of tropical cyclones. Geophysica , Univ. of Helsinki, Vol. 3, 1948, pp. 26-38.

16. What is UTC ?

UTC stands for Universal Time Coordinated, what used to be called Greenwich Mean Time (GMT) and Zulu Time (Z). This is the time at the Prime Meridian (0° Longitude) given in hours and minutes on a 24 hour clock. For example, 1350 UTC is 13 hours and 50 minutes after midnight or 1:50 PM at the Prime Meridian.

The Greenwich Royal Observatory at Greenwich, England (at 0° Longitude) was where naval chronometers (clocks) were set, a critical instrument for calculating longitude. This is why GMT became the standard for world time. Meteorologists have used UTC or GMT times for over a century to ensure that observations taken around the globe are taken simultaneously.

Local Time Zone Time Adjustment in Hours
Atlantic Daylight Time (ADT) -3
Atlantic Standard Time (AST)
Eastern Daylight Time (EDT)
Eastern Daylight Time (EDT)
Central Daylight Time (CDT)
Central Standard Time (CST)
Mountain Daylight Time (MDT)
Mountain Standard Time (MST)
Pacific Daylight Time (PDT)
Pacific Standard Time (PST)
Alaskan Daylight Time (ADT)
Alaskan Standard Time (ASA) -9
Hawaiian Standard Time (HST) -10
New Zealand Standard Time (NZT)
International Date Line Time (IDLE)
Guam Standard Time (GST)
Eastern Australian Standard Time (EAST)
Japan Standard Time (JST)
China Coast Time (CCT)
West Australia Standard Time (WAST)
Russian Time Zone 5 (ZP5)
Russian Time Zone 4 (ZP4)
Russian Time Zone 3 (ZP3)
Russian Time Zone 2(ZP2)
Bagdad Time (BT)
Eastern European Time (EET)
Russian Time Zone 1(ZP1)
Central European Time (CET)
French Winter Time (FWT)
Middle European Time (MET)
Swedish Winter Time (SWT)
Middle European Winter Time (MEWT)
Western European Time (WET)
Greenwich Mean Time (GMT)

17. How do I tell at what time a satellite picture was taken ?

A satellite image of a storm, highlighting the location of the time stamp.A satellite image of a storm, highlighting the location of the time stamp.

On most satellite pictures and radar images the time will be given. If it's not in local time then it will usually be given as UTC, GMT, or Z time. To convert this to your local time it is necessary to subtract the appropriate number of hours for the Western Hemisphere or add the correct number of hours for the Eastern Hemisphere.

18. How do I convert from mph to knots (or m/s), from inches of mercury to mb (or hPa), or convert degrees of latitude to miles (or kilometers) ?

For winds:
1 mile per hour = 0.869 international nautical mile per hour (knot)
1 mile per hour = 1.609 kilometers per hour
1 mile per hour = 0.4470 meter per second
1 knot = 1.852 kilometers per hour
1 knot = 0.5144 meter per second
1 meter per second = 3.6 kilometers per hour

For pressure:
1 inch of mercury = 25.4 mm of mercury = 33.86 millibars = 33.86 hectoPascals

For Distance:
1 foot = 0.3048 meter
1 international nautical mile = 1.1508 statute miles = 1.852 kilometers = .99933 U.S nautical mile (obsolete)
1° latitude = 69.047 statute miles = 60 nautical miles = 111.12 kilometers
For longitude the conversion is the same as latitude except the value is multiplied by the cosine of the latitude.