Subject: G1) When is hurricane season ? 
Contributed by Neal Dorst
 
The Atlantic hurricane season is officially from 1 June to 30 November. 
There is nothing magical in these dates, and hurricanes have occurred 
outside of these six months, but these dates were selected to encompass 
over 97% of tropical activity. June 1st has been the traditional start of 
the Atlantic hurricane season for decades. However, the end date has been 
slowly shifted outward, from October 31st to November 15th until its 
current date of November 30th.
The Atlantic basin shows a very peaked season from August through 
October, 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 then (Landsea 1993). Maximum activity is 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 G12 - "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. NHC's official 
dates for this basin are from May 15th to November 30th.
The Northwest Pacific basin has tropical cyclones occurring all year 
round regularly. There is no official definition of typhoon season for 
this reason. There is a distinct minimum in February and the first half 
of March, and 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)
Subject: G2) How does El Nio-Southern Oscillation affect tropical 
cyclone activity around the globe?
Contributed by Chris Landsea
 El Nio/Southern Oscillation (ENSO) - During El Nio events (ENSO warm 
phase), tropospheric vertical shear is increased inhibiting tropical 
cyclone genesis and intensification, primarily by causing the 200 mb (12 
km or 8 mi) westerly winds to be stronger (Gray 1984). La Nina events 
(ENSO cold phase) enhances activity. Recently, Tang and Neelin (2004) 
also identified that changes to the moist static stability can also 
contribute toward hurricane changes due to ENSO, with a drier, more 
stable environment present during El Nino events.
Reference: Tang, B. H., and J. D. Neelin, 2004: ENSO Influence on 
Atlantic hurricanes via tropospheric warming. Geophys. Res. Lett.: Vol 
31, L24204. "
The Australian/Southwest Pacific shows a pronounced shift back and forth 
of tropical cyclone activity with fewer tropical cyclones between 145 
and 165E and more from 165E eastward across the South Pacific during El 
Nio (warm ENSO) events. There is also a smaller tendency to have the 
tropical cyclones originate a bit closer to the equator. The opposite 
would be true in La Nia (cold ENSO) events. See papers by Nicholls 
(1979), Revell and Goulter (1986), Dong (1988), and Nicholls (1992).
The western portion of the Northeast Pacific basin (140W to the 
dateline) has been suggested to experience more tropical cyclone genesis 
during the El Nio year and more tropical cyclones tracking into the 
sub-region in the year following an El Nio (Schroeder and Yu 1995) , but 
this has not been completely documented yet.
The Northwest Pacific basin, similar to the Australian/Southwest Pacific 
basin, experiences a change in location of tropical cyclones without a 
total change in frequency. Pan (1981), Chan (1985), and Lander (1994) 
detailed that west of 160E there were reduced numbers of tropical 
cyclone genesis with increased formations from 160E to the dateline 
during El Nio events. The opposite occurred during La Nia events. Again 
there is also the tendency for the tropical cyclones to also form closer 
to the equator during El Nio events than average.
The eastern portion of the Northeast Pacific, the Southwest Indian, the 
Southeast Indian/Australian, and the North Indian basins have either 
shown little or a conflicting ENSO relationship and/or have not been 
looked at yet in sufficient detail.
Subject: G3) How might global warming change hurricane intensity, 
frequency, and rainfall ? 
Contributed by Chris Landsea
In November 2006 the global community of tropical cyclone researchers and 
forecasters as met at the 6th International Workshop on Tropical Cyclones 
of the World Meteorological Organization in San Jose, Costa Rica. They 
released a statement on the links between anthropogenic (human-induced) 
climate change and tropical cyclones, including hurricanes and typhoons. 
The following is a summary of their report.
There have been a number of recent high-impact tropical cyclone events 
around the globe. These include 10 landfalling tropical cyclones in Japan 
in 2004, five tropical cyclones affecting the Cook Islands in a five-week 
period in 2005, Cyclone Gafilo in Madagascar in 2004, Cyclone Larry in 
Australia in 2006, Typhoon Saomai in China in 2006, and the extremely 
active 2004 and 2005 Atlantic tropical cyclone seasons - including the 
catastrophicsocio-economic impact of Hurricane Katrina.
Some recent scientificarticles have reported a large increase in tropical 
cyclone energy, numbers, and wind-speeds in some regions during the last 
few decades in association with warmer sea surface temperatures. Other 
studies report that changes in observational techniques and 
instrumentation are responsible for these increases.
Consensus Statements by International Workshop on Tropical Cyclones-VI 
(IWTC-VI) Participants :Though there is evidence both for and against the 
existence of a detectable anthropogenic signal in the tropical cyclone 
climate record to date, no firm conclusion can be made on this point.
No individual tropical cyclone can be directly attributed to climate 
change.
The recent increase in societal impact from tropical cyclones has largely 
been caused by rising concentrations of population and infrastructure in 
coastal regions.
Tropical cyclone wind-speed monitoring has changed dramatically over the 
last few decades, leading to difficulties in determining accurate trends.
There is an observed multi-decadal variability of tropical cyclones in 
some regions whose causes, whether natural, anthropogenic or a 
combination, are currently being debated. This variability makes 
detecting any long-term trends in tropical cyclone activity difficult.
It is likely that some increase in tropical cyclone peak wind-speed and 
rainfall will occur if the climate continues to warm. Model studies and 
theory project a 3-5% increase in wind-speed per degree Celsius increase 
of tropical sea surface temperatures.
There is an inconsistency between the small changes in wind-speed 
projected by theory and modeling versus large changes reported by some 
observational studies.
Although recent climate model simulations project a decrease or no change 
in global tropical cyclone numbers in a warmer climate, there is low 
confidence in this projection. In addition, it is unknown how tropical 
cyclone tracks or areas of impact will change in the future.
Large regional variations exist in methods used to monitor tropical 
cyclones. Also, most regions have no measurements by instrumented 
aircraft. These significant limitations will continue to make detection 
of trends difficult.
If the projected rise in sea level due to global warming occurs, then the 
vulnerability to tropical cyclone storm surge flooding would increase.
A PDF version of the official report is available here.
 
Revised July 2, 2007
Subject: G4) Why do tropical cyclones occur primarily in the summer and 
autumn?
Contributed by Chris Landsea
As described in Subject G1, the primary time of year for getting tropical 
cyclones is during the summer and autumn: July-October for the Northern 
Hemisphere and December-March for the Southern Hemisphere (though there 
are differences from basin to basin). The peak in summer/autumn is due to 
having all of the necessary ingredients become most fa vorable during 
this time of year: warm ocean waters (at least 26C or 80F), a tropical 
atmosphere that can quite easily kick off convection (i.e. 
thunderstorms), low vertical shear in the troposphere, and a substantial 
amount of large-scale spin available (either through the monsoon trough 
or easterly waves).
While one would intuitively expect tropical cyclones to peak right at the 
time of maximum solar radiation (late June for the tropical Northern 
Hemisphere and late December for the tropical Southern Hemisphere), it 
takes several more weeks for the oceans to reach their warmest 
temperatures. The atmospheric circulation in the tropics also reaches its 
most pronounced (and favorable for tropical cyclones) at the same time. 
This time lag of the tropical ocean and atmospheric circulation is 
analogous to the daily cycle of surface air temperatures - they are 
warmest in mid-afternoon, yet the sun's incident radiation peaks at noon.
Subject: G5) What determines the movement of tropical cyclones?
Contributed by Chris Landsea
Tropical cyclones - to a first approximation - can be thought of as being 
steered by the surrounding environmental flow throughout the depth of the 
troposphere (from the surface to about 12 km or 8 mi). Dr. Neil Frank, 
former director of the U.S. National Hurricane Center, used the analogy 
that the movement of hurricanes is like a leaf being steered by the 
currents in the stream, except that for a hurricane the stream has no set 
boundaries.
In the tropical latitudes (typically equatorward of 20-25N or S), 
tropical cyclones usually move toward the west with a slight poleward 
component. This is because there exists an axis of high pressure called 
the subtropical ridge that extends east-west poleward of the storm. On 
the equatorward side of the subtropical ridge, general easterly winds 
prevail. However, if the subtropical ridge is weak - often times due to a 
trough in the jet stream - the tropical cyclone may turn poleward and 
then recurve back toward the east. On the poleward side of the 
subtropical ridge, westerly winds prevail thus steering the tropical 
cyclone back to the east. These westerly winds are the same ones that 
typically bring extratropical cyclones with their cold and warm fronts 
from west to east.
Many times it is difficult to tell whether a trough will allow the 
tropical cyclone to recurve back out to sea (for those folks on the 
eastern edges of continents) or whether the tropical cyclone will continue 
straight ahead and make landfall.
For more non-technical information on the movement of tropical cyclones, 
see Pielke and Pielke's "Hurricanes: Their Nature and Impacts on 
Society". For a more detailed, technical summary on the controls on 
tropical cyclone motion, see Elsberry's chapter in "Global Perspectives 
on Tropical Cyclones".
Subject: G6) Why doesn't the South Atlantic Ocean experience tropical 
cyclones?
Contributed by Chris Landsea
What never ?? Well, hardly ever.
In March, 2004 a hurricane DID form in the South Atlantic Ocean and made 
landfall in Brazil. But this still leaves the question of why hurricanes 
are so rare in the South Atlantic. Though many people might speculate 
that the sea surface temperatures are too cold, the primary reasons that 
the South Atlantic Ocean gets few tropical cyclones are that the 
tropospheric (near surface to 200mb) vertical wind shear is much too 
strong and there is typically no inter-tropical convergence zone (ITCZ) 
over the ocean (Gray 1968). Without an ITCZ to provide synoptic vorticity 
and convergence (i.e. large scale spin and thunderstorm activity) as well 
as having strong wind shear, it becomes very difficult to nearly 
impossible to have genesis of tropical cyclones.
 
In addition, the US NHC has documented the occurrence of a strong 
tropical depression/weak tropical storm that formed off the coast of 
Congo in mid-April 1991 (McAdie and Rappaport (1991)) . This 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.
 
Penn State University write up on the South Atlantic hurricane
Last updated July 13, 2005
Subject: G7) Does an active June and July mean the rest of the season 
will be busy too? 
Contributed by Stan Goldenberg
Yes and No. The vast majority of Atlantic activity takes place during 
August-September-October, the climatological peak months of the hurricane 
season. The overall number of named storms (hurricanes) occurring in June 
and July (JJ) correlates at an insignificant r = +0.13 (+0.02) versus the 
whole season activity. In fact, there is a slight negative relationship 
between early season storms (hurricanes) versus late season - August 
through November - r = -0.28 (-0.35). Thus, the overall early season 
activity, be it very active or quite calm, has little bearing on the 
season as a whole. These correlations are based on the years 1944-1994.
However, as shown in (Goldenberg 2000), if one looks only at the 
June-July Atlantic tropical storms and hurricanes occurring south of 22N 
and east of 77W (the eastern portion of the Main Development Region 
[MDR] for Atlantic hurricanes), there is a strong association with 
activity for the remainder of the year. According to the data from 
1944-1999, total overall Atlantic activity for years that had a tropical 
storm or hurricane form in this region during JJ have been at least 
average and often times above average. So it could be said that a JJ 
storm in this region is pretty much a "sufficient" (though not 
"necessary") condition for a year to produce at least average activity. 
(I.e., Not all years with average to above-average total overall activity 
have had a JJ storm in that region, but almost all years with that type 
of JJ storm produce average to above-average activity.) The formation of 
a storm in this region during June-July is taken into account when the 
August updates for the Bill Gray and NOAA seasonal forecasts are issued.
Subject: G8) Why do hurricanes hit the East coast of the U.S., but never 
the West coast?
Contributed by Chris Landsea
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.
Recently (Chenoweth and Landsea, 2004), it was re-discovered that a 
hurricane struck San Diego, California on October 2, 1858. Unprecedented 
damage was done in the city and was described as the severest gale ever 
felt to that date nor has it been matched or exceeded in severity since. 
The hurricane force winds at San Diego are the first and only documented 
instance of winds of this strength from a tropical cyclone in the 
recorded history of the state. While climate records are incomplete, 1858 
may have been an El Nino year, which would have allowed the hurricane to 
maintain intensity as it moved north along warmer than usual waters. 
Today if a Category 1 hurricane made a direct landfall in either San 
Diego or Los Angeles, damage from such a storm would likely be on the 
order of a few to several hundred million dollars. The re-discovery of 
this storm is relevant to climate change issues and the 
insurance/emergency management communities risk assessment of rare and 
extreme events in the region.
Subject: G9) How much lightning occurs in tropical cyclones?
Contributed by Chris Landsea
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).
Subject: G10) What is the 20th century hurricane record for each U.S. 
coastal county?
Contributed by Chris Landsea
The NOAA Coastal Services Center provides an on-line revision to the 
original NOAA technical memorandum by Jarrell et al. 1992. One can query 
for any U.S. coastal county and obtain a graph with hurricane strikes and 
population changes. This information lets users know how many and how 
often hurricanes have struck, what Saffir- Simpson Hurricane Scale 
category they were, and how the populations have changed during the 20th 
Century.
Subject: G11) What is my chance of being struck by a tropical storm or 
hurricane?
Contributed by Chris Landsea
The figure here 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 
1999 in the analysis and counted hits when a storm or hurricane was 
within about 100 miles (165 km). This figure is created by Todd 
Kimberlain.
For example, people living in New Orleans, Louisiana have about a 40% 
chance (the green-orange 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, Florida - 48% chance; Cape 
Hatteras, North Carolina - 48% chance; and San Juan, Puerto Rico - 42% 
chance.
For any particular location the chance that a hurricane will directly 
affect the area sometime during the whole June to November hurricane 
season is shown here. We utilized the years 1944 to 1999 in the analysis 
and counted hits when a hurricane was within about 60 miles (110 km). 
This figure is created by Todd Kimberlain. (For example, the chance for 
Miami, Florida is about 16%.)
For any particular location what the chance is that a major hurricane 
(Category 3, 4 or 5) will directly affect the area sometime during the 
whole June to November hurricane season is shown here. We utilized the 
years 1944 to 1999 in the analysis and counted hits when a hurricane was 
within about 30 miles (50 km). This figure is created by Todd Kimberlain. 
(For example, the chance for Miami, Florida is about 4%.)
Many folks are are concerned about the possible impacts that a hurricane 
could have on their vacation. If so, please check with your hotel, cruise 
company, etc. to find out how they inform their guests when a hurricane 
is coming, what actions they plan and what refund policies they have (if 
any). Keep in mind that a direct hit by a major hurricane is an extremely 
rare event and if I had a chance - for example - to go on a cruise in the 
Caribbean Sea during hurricane season, I would go without hesitation.
Subject: G12) What is my chance of having a tropical storm or hurricane 
strikeby each month?
Contributed by Chris Landsea
The following figures show that for
  All
Named 
Storms		Hurricanes	Major Hurricanes
June		June		None
July		July		None
August		August		August
September	September	September
October		October		October
November	November	None
These figures were created by Todd Kimberlain.
at any particular location what the chance that a tropical storm or 
hurricane hwill affect the area sometime during an individual month. We 
utilized the years 1944 to 1999 in the analysis and counted hits when a 
storm or hurricane was within about 100 miles (165 km). Many folks are 
are concerned about the possible impacts that a hurricane could have on 
their vacation. If so, please check with your hotel, cruise company, etc. 
to find out how they inform their guests when a hurricane is coming, what 
actions they plan and what refund policies they have (if any). Keep in 
mind that a direct hit by a major hurricane is an extremely rare event 
and if I had a chance - for example - to go on a cruise in the Caribbean 
Sea during hurricane season, I would go without hesitation.
Subject: G13) What is the average number of tropical storms and 
hurricanes to affect my town?
Contributed by Chris Landsea
The figure here (created by Todd Kimberlain) shows for any particular 
location what the average number of tropical storms and hurricanes is 
that 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).
 
Subject: G14) What is the peak number of tropical storms and hurricanes 
to affect my town?
Contributed by Chris Landsea
The figure here (created by Todd Kimberlain) shows for any particular 
location what the highest number of tropical storms and hurricanes is 
that affect the area sometime during the whole June to November hurricane 
season. Blue indicates a peak of just 1 storm, orange is 2 storms, brick 
red is 3 storms, green is 4 storms and red is 5 storms. 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).
Subject: G15) I'm vacationing in the Caribbean/the Bahamas/ Central 
America/Miami or elsewhere in the tropics during hurricane season. What's 
my chance of getting hit by a hurricane?"
Contributed by Chris Landsea
Typically, for someone visiting the tropics during June through November, 
the chance to experience (or even be threatened by) a hurricane is very 
small.
 
As an example, this figure shows the chances to have a direct hit by a 
hurricane during the month of September, which is usually the busiest 
month. If we look at Puerto Rico, the chance is 8% of experiencing a 
hurricane, if you are there for the WHOLE month. If you are there for, 
say, only a week, then the chance would be one fourth of that - or only 
about 2% chance.
To put this into perspective, if you made 50 one week trips to Puerto 
Rico in September, you would only experience a direct hit in ONE of those 
50 visits. So the chances to get impacted by a hurricane are quite small 
for relatively short trips. (And the case chosen here is the WORST 
possible, as all other locations in all other months have smaller chances 
of being hit by a hurricane.) If you would like to see chances of 
hurricane strikes in other months, see Question G12.
Despite the chance being small, one should know in advance what your 
hotel's, cruise company's, etc. policy is for guests when a hurricane is 
coming, what actions they plan and what refund policies they have (if 
any). As is described above, a direct hit by a hurricane is a very rare 
event for a short visit and if I had a chance - for example - to go on a 
cruise in the Caribbean Sea during hurricane season, I would go without 
hesitation.
Last updated August 13, 2004
Subject: G16) What is the average forward speed of a hurricane?"
Contributed by Neal Dorst
The forward speed of hurricanes is very latitude dependent. Typically, 
Atlantic hurricanes track along the western side of the subtropical ridge 
in the western Atlantic. As they recurve (turn more northerly) from their 
westward track they usually slow down. If they reach the midlatitudes, 
they can interact with upper-level troughs and pick up speed.
In the table below, the forward speed of hurricanes in the HURDAT 
database have been averaged in 5 degree latitude bins :
Forward speed of Atlantic hurricanes
averaged by 5 degree latitude bins
Latitude	     Storm Speed	No.
bin		km/hr	knt	mph	Cases
0- 5N		25.5	13.7	15.8	100
5-10N		21.8	11.8	13.6	3282
10-15N	19.6	10.6	12.2	5808
15-20N	17.7	9.5	11.0	6086
20-25N	17.7	9.5	11.0	6817
25-30N	20.1	10.9	12.5	5321
30-35N	27.6	14.9	17.2	2835
35-40N	38.5	20.8	23.9	1026
40-45N	46.7	25.2	29.0	263
45-50N	51.4	27.7	32.0	36
50-55N	51.4	27.8	32.0	15
55-60N	55.8	30.1	34.7	1
While there are many cases where the forward speed over the 6 hour 
interval in the hurricane database is zero, such as Mitch in 1998, the 
highest speed in the database is for unnamed Tropical Storm #6 in 1961. 
As it got caught up by a midlatitude trough over the midatlantic states, 
it went speeding off northeastward over Maine and New Brunswick at a 
maximum speed of 112.25 km/hr (60.57 knt or 69.75 mph). The fastest 
hurricane in the record was Emily in 1987, whose maximum speed reached 
110.48 km/hr (59.61knt or 68.65 mph) as it raced over the North Atlantic, 
before it turned extratropical.
Last updated January 16, 2007
 
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