Hurricane Climatology

An old adage explains the difference between climate and weather; "Climate is what you expect, weather is what you get." Climatology is the analysis of general weather patterns over a long period of time. For example, The World Meteorological Organization (WMO) specifies the period for climate analysis to be thirty years. Weather, on the other hand, is comprised of the meteorological conditions that an area is currently experiencing. Weather can, and frequently does, vary markedly from climatology.

Scientists at the NHC have studied the historical record and have prepared a climatological chart for each month of the hurricane season that depicts the most likely area for tropical cyclone development and the most likely path that a hurricane would follow. The NHC offers the following disclaimer; "hurricanes can originate in different locations and travel much different paths from the average." A case in point was Vince, which in mid-October of 2005, became the first known tropical cyclone to strike the Iberian Peninsula. Vince defied climatology by forming far to the east and following a most unconventional path.

A variety of atmospheric variables influence the development and path of tropical cyclones, and these variables change as our hemisphere transitions from summer to winter. Below is a series of charts for the months of August, September, October and November that provide the NHC's climatological analysis of origin and anticipated path, mean sea surface temperature, and mean sea level barometric pressure.

Chart Name	Description
Tropical Cyclone Formation	The graphic shows the regions likely to produce a tropical cyclone and its anticipated path. The area of highest likelihood of formation is shaded in orange.
Sea Surface Temperature	A chart of mean sea surface temperature for the specified month. Tropical cyclones require a sea surface temperature of at least 28°C for development. The region representing this temperature is shaded in light orange and outlined in red.
Sea Level Pressure	The chart of mean sea level pressure for the specified month. The prominent feature on the chart is the subtropical high which plays an important role in steering tropical cyclones. The area characterized by a pressure reading of 1020mb is outlined in red (known as an isobar).

A review of the time series show how the region of formation expands both eastward and westward as August gives way to September. Also note the region of warm water that expands ever so slightly to the east and the contraction of the area enclosed by the 1020mb isobar on the chart of sea level pressure. It is the retreat of the North Atlantic subtropical high that allows a cyclone an opportunity for a more northeasterly path. As the season progresses into October, the region of formation contracts considerably in response to sea surface cooling in the Caribbean basin. Both the retreat of the subtropical high and surface cooling in the Caribbean basin are the result of the Sun's southward progression into the Southern Hemisphere. The continued retreat of the subtropical high combined with the strengthening westerly winds diminishes the possibility for landfall in the Gulf of Mexico while increasing the chances for a Florida landfall. Finally, as the season draws to a close in November, the areas of formation, sufficiently warm water and the subtropical high have contracted considerably. Hurricane formation isn't impossible but the environment is far less supportive.

August


Tropical Cyclone Formation	Sea Surface Temperature	Sea Level Pressure

While there are tropical cyclones such as Vince that defy climatological convention, Wilma's formation and subsequent track was eerily consistent with the NHC's climatological analysis. Note how Wilma, a late October hurricane, formed exactly where expected and headed to the northwest before moving northeasterly across Florida and up the East Coast.

HURRICANE CLIMATOLOGY