***** Here is a hurricane question ***** Look inside...

[OtherHD]

On average, how many tropical depressions form in the Atlantic each year?

EDIT: It is a 30-year average based on 1967-1997.

[rainstorm]

13

[wxman57]

Is that TDs, TSs, and Hurricanes? Or do you not count storms that go from wave to TS, bypassing TD stage?

[OtherHD]

It does include storms that bypassed the TD stage, as well as TSs and hurricanes.

[southerngale]

15

[Stormsfury]

Several sites would indicate that no less than 25 tropical depressions actually form. Here's an excerpt from that site found by clicking HERE

Frequencies and development of Atlantic tropical cyclones

In an average year, more than one hundred hurricane seedlings, tropical disturbances with hurricane potential, are observed in the Atlantic, the Gulf of Mexico and the Caribbean; but less than 25 obtain an organized cyclonic circulation and develop into a tropical depression. Of these tropical depressions fewer than ten reach the tropical storm stage and only about six mature into hurricanes. This is not at all surprising, taken the different sources of origin and the various, known but also still unknown or not completely understood, atmospheric conditions which together determine the possibility of development. Predominantly the tropical troposphere (the lower level of the atmosphere where most vertical mixing and weather occur), potentially unstable as it is, maintains a delicate balance in its dynamics, even in seemingly ideal convective situations.
Satellite imagery has confirmed that some North Atlantic tropical cyclones classically develop from tropical waves which regularly move off the coast of Africa near 15 degrees North latitude. These systems are embedded in the deep easterly trade wind current and may travel several thousands of kilometers with little change in structure. But where the waves are destabilized by intense convection or by some external force - for example, a high level wind regime that promote greater organization of the circulation in the wave by acting as a sort of exhaust mechanism to compensate for the low level convergence - they may curl inward. The vertical circulation accelerates, and a vortex develops that sometimes reaches hurricane intensities.
Some tropical cyclones originate from the Intertropical Convergence Zone (ITCZ), the quasi-permanent equatorial region of low pressure which follows the sun. Generally, the ITCZ moves from a position near the Equator in February to its extreme limit near about twelve degrees north latitude in August; however, its day-to-day surface position varies greatly. As it shifts northward, the influence of the rotating globe - the Coriolis force - is great enough to permit a circulation to develop that can evolve into the tight, violent eddy (individual currents in a moving fluid) of a tropical cyclone. Weather satellites also have confirmed that some tropical cyclones may develop in connection with old polar troughs or upper level cold lows and have initial baroclinic (cold core) circulation.
In recent years, these latter systems have been designated as subtropical cyclones during the period they exhibit cold core characteristics. Although there is no full understanding of what triggers off a hurricane, it seems that some starter mechanism - an intruding polar trough, a tropical wave, an eddy from an active ITCZ - stimulates an area of continued deep convection, vertical air motion. Further development may occur when, for instance simultaneously:

a) depending on high low-level temperature and moisture content, enough water vapor in the ascending moist columns condenses (releasing large amounts of heat energy to drive the wind system),

b) the vertical wind shear, the difference in airflow in the usually two layer structure of the tropical troposphere, remains below certain limits, to prevent ventilation of energy over a large area and, thus, allowing the vertical circulation to acquire greater organization,

c) the high altitude wind system supports divergence, carrying the vertical transport of exhaust air well away from the disturbance before it can sink to lower levels again,

d) depending on the northward latitude of the disturbance, the influence of the rotating globe - the Coriolis force - is great enough to permit the development of a cyclonic circulation.

The horizontal form of the disturbance then becomes the familiar cyclonic spiral, in which the movement of low and mid level air is counter clockwise, an embryo hurricane.

It is also believed that planetary wind systems, displaced northward, set up an essential large-scale flow which supports the budding storm, and that the development of a hurricane is often preceded by high-level warming and low- level inflow, in some balance that is not fully understood.

Back to Table of Contents

Classification of Atlantic tropical cyclones

Tropical cyclones are technically defined as non frontal low pressure synoptic scale*)systems that develop over tropical or subtropical waters and have definite organized circulation. Further classification depends upon the wind speed near the center of the system. The terms tropical depression, tropical storm, or hurricane are assigned depending upon whether the sustained surface winds near the center of the system are, respectively, 61 km/h, 62 to 117 km/h, 118 km/h. More complete definitions are given in the table. Tropical cyclones are not archived (or named) unless they reach at least tropical storm strength.
The term sustained wind refers to the wind averaged over one minute. Shorter period gusts (or lulls) in the wind may be considerably higher (or lower) than the sustained wind.
Although the wind criteria defining the various stages of tropical cyclones are rather rigidly defined, the maximum sustained wind, however, often must be inferred from indirect evidence and a figure is subjectively assigned by the responsible analyst after considering all available information.
These operational constraints should be kept in mind. The extra tropical stages of the cyclone tracks indicate that modification of the tropical circulation was started by movement of the cyclone into a non tropical environment. In this situation, the size of the circulation usually expands, the speed of the maximum wind decreases, and the distribution of winds, rainfall and temperature around the center become increasingly asymmetric. While these characteristic features develop some tropical features such as; a small area of strong often hurricane force winds near the center, the remnants of an eye and extremely heavy rainfall, may be retained for a considerable time. There are no wind speed criteria associated with the term extra tropical. Usually wind speeds near the center of a storm gradually subside. In some cases however, Renton- suffocation of the system may occur when mechanisms conducive to extra tropical development offset the loss of the tropical energy source. If over land, these mechanisms may offset the dissipative effects of the increase in surface friction.
Subtropical cyclones are defined as non frontal low pressure systems comprising initially baroclinic (cold-core) circulations developing over subtropical water. Many of these eventually develop into purely tropical (warm core) systems, but others remain as subtropical.
Depending upon wind speed, two classes of subtropical cyclones are recognized subtropical depressions and subtropical storms. The former have maximum sustained surface winds of 62 km/h and the latter 62 km/h. There is no upper wind speed limit associated with subtropical storms as there is with tropical storms. However, experience has shown that when and if surface winds in subtropical storms do reach or exceed 118 km/h, the system typically takes on sufficient tropical characteristics to be formally designated as a hurricane. Only in rare cases, such systems do associate themselves with hurricane force winds without attaining sufficient tropical characteristics. In this case, the term subtropical storm is retained.

*) Synoptic scale refers to large-scale weather systems as distinguished from local systems, such as thunderstorms. On rare occasions subtropical systems have evolved from tropical systems.

[wxman57]

On average, how many tropical depressions form in the Atlantic each year?

EDIT: It is a 30-year average based on 1967-1997.

Also, are you asking how many actually "form", or how many are actually recognized by the NHC and classified? Many TDs go unclassified by the NHC each season.

[Stormsfury]

On average, how many tropical depressions form in the Atlantic each year?

EDIT: It is a 30-year average based on 1967-1997.

Also, are you asking how many actually "form", or how many are actually recognized by the NHC and classified? Many TDs go unclassified by the NHC each season.

wxman57, look at the findings above that I posted and see if you agree with those numbers featured. I found those numbers a little shocking but not really surprised either.

If we assumed classified systems, the number would obviously be a lot lower than the 25 suggested by several sites including the one I linked above, both classified and unclassified. On the order of classified systems, we're looking at roughly 14-17 (considering the active periods and inactive periods). This year, we've seen 19 tropical depressions (classifiied). However, unless one has all the number of tropical depressions tracked by year (which I don't) and classified T.D.'s aren't kept in the final storm records, it's very hard to judge averages.

[OtherHD]

Well, these aren't my guidelines that I'm going by. I just got this info from another website. So I can't really say if they're counting classified systems or what.

And SF, the website that I got the info from (I will post it later if nobody guesses) does have a tally of tropical depressions that formed each year. Looking on that list back to 1991, I can say with certainty that the list only included classified systems. Prior to that, I do not know.

[Stormsfury]

Well, these aren't my guidelines that I'm going by. I just got this info from another website. So I can't really say if they're counting classified systems or what.

And SF, the website that I got the info from (I will post it later if nobody guesses) does have a tally of tropical depressions that formed each year. Looking on that list back to 1991, I can say with certainty that the list only included classified systems. Prior to that, I do not know.

Ah ok. Then my preliminary estimate (assuming only classified tropical depressions) would stand at 17. But assuming that either 1) Some depressions were not classified in the earlier years of the 30 year average and/or 2) Reanalysis of the seasons found some rather uneventful systems later deemed of enough organization .. my uneducated guess would be what we've seen this year ... that would be 19.

SF