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When is hurricane season?
While the Atlantic hurricane season is "officially" from 1 June to 30 November, the Atlantic basin shows a very peaked season with 78% of the tropical storm days, 87% of the minor (Saffir-Simpson Scale categories 1 and 2 - see Subject D1) hurricane days, and 96% of the major (Saffir-Simpson categories 3, 4 and 5) hurricane days occurring in August through October (Landsea 1993). Peak activityis in early to mid September. Once in a few years there may be a tropical cyclone occurring "out of season" - primarily in May or December. (For more detailed information, see Subject G13 - "What is my chance of having a tropical storm or hurricane strike by each month?")
The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September.
The Northwest Pacific basin has tropical cyclones occurring all year round regularly though there is a distinct minimum in February and the first half of March. The main season goes from July to November with a peak in late August/early September.
The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds [76 mph]) occur almost exclusively from April to June and late September to early December .
The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/early November, reaching a double peak in activity - one in mid-January and one in mid-February to early March, and then ending in May.
The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin's lull. The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in late February/early March, and then fades out in early May. Globally, September is the most active month and May is the least active month. (Neumann 1993)
Are we getting stronger and more frequent hurricanes, typhoons, and
tropical cyclones in the last several years?
Globally, no. However, for the Atlantic basin we have seen an increase in the number of strong hurricanes since 1995. As can be seen in section E9, we have had a record 33 hurricanes in the four years of 1995 to 1999 (accurate records for the Atlantic are thought to begin around 1944). The extreme impacts from Hurricanes Marilyn (1995), Opal (1995), Fran (1996), Georges (1998) and Mitch (1998) in the United States and throughout the Caribbean attest to the high amounts of Atlantic hurricane activity lately.
As discussed in the previous section, it is highly unlikely that global warming has (or will) contribute to a drastic change in the number or intensity of hurricanes. We have not observed a long-term increase in the intensity or frequency of Atlantic hurricanes. Actually, 1991-1994 marked the four quietest years on record (back to the mid-1940s) with just less than 4 hurricanes per year. Instead of seeing a long-term trend up or down, we do see a quasi-cyclic multi-decade regime that alternates between active and quiet phases for major Atlantic hurricanes on the scale of 25-40 years each (Gray 1990; Landsea 1993; Landsea et al. 1996). The quiet decades of the 1970s to the early 1990s for major Atlantic hurricanes were likely due to changes in the Atlantic Ocean sea surface temperature structure with cooler than usual waters in the North Atlantic. The reverse situation of a warm North Atlantic was present during the active late-1920s through the 1960s (Gray et al. 1997). It is quite possible that the extreme activity since 1995 marks the start of another active period that may last a total of 25-40 years. More research is needed to better understand these hurricane "cycles".
For the region near Australia (105-160E, south of the equator), Nicholls (1992) identified a downward trend in the numbers of tropical cyclones, primarily from the mid-1980s onward. However, a portion of this trend is likely artificial as the forecasters in the region no longer classify weak systems as "cyclones" if the systems do not possess the traditional tropical cyclone inner-core structure, but have the band of maximum winds well-removed from the center (Nicholls et al. 1998). These changes in methodology around the mid-1980s have been prompted by improved access to and interpretation of digital satellite data, the installation of coastal and off-shore radar, and an increased understanding of the differentiation of tropical cyclones from other type of tropical weather systems. By considering only the moderate and intense tropical cyclones, this artificial bias in the cyclone record can be overcome. Even with the removal of this bias in the weak Australian tropical cyclones that the frequency of the remaining moderate and strong tropical cyclones has been reduced substantially over the years 1969/70-1995/96. Nicholls et al. (1998) attribute the decrease in moderate cyclones to the occurrence of more frequent El Nino occurrences during the 1980s and 1990s.
For the Northwest Pacific basin, Chan and Shi (1996) found that both the frequency of typhoons and the total number of tropical storms and typhoons have been increasing since about 1980. However, the increase was preceded by a nearly identical magnitude of decrease from about 1960 to 1980. It is unknown currently what has caused these decadal-scale changes in the Northwest Pacific typhoons.
For the remaining basins based upon data from the late 1960s onwards, the Northeast Pacific has experienced a significant upward trend in tropical cyclone frequency, the North Indian a significant downward trend, and no appreciable long-term variation was observed in the Southwest Indian and Southwest Pacific (east of 160E) for the total number of tropical storm strength cyclones (from Neumann 1993). However, whether these represent longer term (> 30 years) or shorter term (on the scale of ten years) variability is completely unknown because of the lack of a long, reliable record.
Why doesn't the South Atlantic Ocean experience tropical cyclones?
However, in rare occasions it may be possible to have tropical cyclones form in the South Atlantic. In McAdie and Rappaport (1991), the US National Hurricane Center documented the occurrence of a strong tropical depression/weak tropical storm that formed off the coast of Congo in mid-April 1991. The storm lasted about five days and drifted toward the west-southwest into the central South Atlantic. So far, there has not been a systematic study as to the conditions that accompanied this rare event.
Why do hurricanes hit the East coast of the U.S., but never
the West coast?
Hurricanes form both in the Atlantic basin (i.e. the Atlantic Ocean, Gulf of Mexico and Caribbean Sea) to the east of the continental U.S. and in the Northeast Pacific basin to the west of the U.S. However, the ones in the Northeast Pacific almost never hit the U.S., while the ones in the Atlantic basin strike the U.S. mainland just less than twice a year on average. There are two main reasons. The first is that hurricanes tend to move toward the west-northwest after they form in the tropical and subtropical latitudes. In the Atlantic, such a motion often brings the hurricane into the vicinity of the U.S. east coast. In the Northeast Pacific, a west-northwest track takes those hurricanes farther off-shore, well away from the U.S. west coast. In addition to the general track, a second factor is the difference in water temperatures along the U.S. east and west coasts. Along the U.S. east coast, the Gulf Stream provides a source of warm (> 80 F or 26.5 C) waters to help maintain the hurricane. However, along the U.S. west coast, the ocean temperatures rarely get above the lower 70s, even in the midst of summer. Such relatively cool temperatures are not energetic enough to sustain a hurricane's strength. So for the occasional Northeast Pacific hurricane that does track back toward the U.S. west coast, the cooler waters can quickly reduce the strength of the storm.
How much lightning occurs in tropical cyclones?
Surprisingly, not much lightning occurs in the inner core (within about 100 km or 60 mi) of the tropical cyclone center. Only around a dozen or less cloud-to-ground strikes per hour occur around the eyewall of the storm, in strong contrast to an overland mid-latitude mesoscale convective complex which may be observed to have lightning flash rates of greater than 1000 per hour maintained for several hours.
Hurricane Andrew's eyewall had less than 10 strikes per hour from the time it was over the Bahamas until after it made landfall along Louisiana, with several hours with no cloud-to-ground lightning at all (Molinari et al. 1994). However, lightning can be more common in the outer cores of the storms (beyond around 100 km or 60 mi) with flash rates on the order of 100s per hour.
This lack of inner core lightning is due to the relative weak nature of the eyewall thunderstorms. Because of the lack of surface heating over the ocean ocean and the "warm core" nature of the tropical cyclones, there is less buoyancy available to support the updrafts. Weaker updrafts lack the super-cooled water (e.g. water with a temperature less than 0° C or 32° F) that is crucial in charging up a thunderstorm by the interaction of ice crystals in the presence of liquid water (Black and Hallett 1986). The more common outer core lightning occurs in conjunction with the presence of convectively-active rainbands (Samsury and Orville 1994).
One of the exciting possibilities that recent lightning studies have suggested is that changes in the inner core strikes - though the number of strikes is usually quite low - may provide a useful forecast tool for intensification of tropical cyclones. Black (1975) suggested that bursts of inner core convection which are accompanied by increases in electrical activity may indicate that the tropical cyclone will soon commence a deepening in intensity. Analyses of Hurricanes Diana (1984), Florence (1988) and Andrew (1992), as well as an unnamed tropical storm in 1987 indicate that this is often true (Lyons and Keen 1994 and Molinari et al. 1994).
What is the chance of my town will be hit by a tropical
storm or hurricane ?
The figure here, created by Todd Kimberlain, shows for any particular location what the chance is that a tropical storm or hurricane will affect the area sometime during the whole June to November hurricane season. We utilized the years 1944 to 1997 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km).
For example, people living in New Orleans, LA have about a 40% chance (the olive green color) per year of experiencing a strike by a tropical storm or hurricane. For the U.S., the locations that have the highest chances are the following: Miami, FL - 48% chance; Cape Hatteras, NC - 48% chance; and San Juan, PR - 42% chance.