Major storms have always been a part of earth's climate system. While in the distant past the planet has experienced hemisphere-wide storm phenomena
that would dwarf today's even most powerful tropical cyclones by orders of magnitude, climate conditions throughout the current intergalcial are ideal
to make these storms a common occurrence. Only a few other climate events can have as much impact as single hurricane or typhoon and rarely a tropical
cyclone season goes by without at least one high category storm making landfall in some part of the world.
A tropical cyclone is a rapidly-rotating storm system characterized by a low-pressure center, strong winds, and a spiral arrangement of thunderstorms
that produce heavy rain. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy from the evaporation of
water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation.
This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by
horizontal temperature contrasts. The strong rotating winds of a tropical cyclone are a result of the (partial) conservation of angular momentum
imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical
cyclones are typically between 100 and 4,000 km (62 and 2,500 mi) in diameter.
The term "tropical" refers to the geographical origin of these systems, which usually form over the tropical oceans. The term "cyclone" refers to
their cyclonic nature, with wind blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction
of circulation is due to the Coriolis force.
Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane (/ˈhʌrɨkeɪn/ or /ˈhʌrɨkən/), typhoon
/taɪˈfuːn/, tropical storm, cyclonic storm, tropical depression, and simply cyclone.
A worldwide storm-tracking and monitoring system was made possible with dawn of the satellite era and near global coverage was achieved in the early
1970's. Since then, every aspect of global (or tropical) cyclone activity has been observed, measured and analysed in unprecedented detail, which has
led to the creation of a variety of universally accepted climate indices for worldwide storminess.
The two main indices measuring cyclone activity on a global/hemispheric-scale and for each ocean basin seperately are the Global Tropical Storm and
Hurricane Frequency Index and the Global/Total Accumulated Cyclone Energy Index.
The Frequency Index is basically a simple count for all tropical storm systems above the lowest/first category (sustained maximum lifetime wind-speeds
of +34 knots) and is divided into three main categories - tropical storms, hurricanes (+64 knots) and major hurricanes (+96 knots). The index covers
the entire period of continuous satellite observation since 1970, data is available for the Globe, both hemispheres and the three main ocean
The ACE is a composite index where the sum of accumulated energy (strength/intensity/duration) for each individual storm within a season is added
together and a short/long term average of total storm energy can be calculated and analysed. The ACE also starts in 1970 and data is available for the
same sub-sets (Pacific, Atlantic, Indian ocean).
More,less or extremely normal?
Media coverage of extreme weather events has increased exponentially in recent years and everytime after a major tropical cyclone has made landfall
somewhere with often catastrophic results for the people and the region affected, the question arises as to whether or not there has also been an
increase in overall storminess or if extreme storms are getting more severe and more frequent.
A lot of people would believe or are convinced that the answer is yes. But if there has been no detectable change in overall storm activity or
intensity then the question remains if any perceived increase is simply an artifact of the flood of news reports and more media hype or if the
question and therefore the answer is generally more complex.
As to whether or not there has been an overall increase in tropical storm and major tropical cyclone frequency
, the answer is relatively
simple. If anything, the number of storms and major storms as decreased slightly over the entire 40 plus year period of satellite observations, but
since the trend is not yet significant the average number of storms/cyclones remains practically unchanged.
Last 4-decades of Global Tropical Storm and Hurricane frequency -- 12-month running sums. The top time series is the
number of TCs that reach at least tropical storm strength (maximum lifetime wind speed exceeds 34-knots). The bottom time series is the number of
hurricane strength (64-knots+) TCs.
Global Hurricane Frequency (all & major) -- 12-month running sums. The top time series is the number of global tropical
cyclones that reached at least hurricane-force (maximum lifetime wind speed exceeds 64-knots). The bottom time series is the number of global tropical
cyclones that reached major hurricane strength (96-knots+).
When broken down into the three main categories - tropical storms, hurricanes and major hurricanes - the individual trends become more clearly
The average number of tropical storms and hurricanes has remained unchanged. The number of the strongest hurricanes/typhoons (category 3+) has
increased slightly over the first half of the period with peaks in the mid 90's (peak season 1993) and after a plateau lasting until the early 2000's
with another peak in the 2004/5 season, the number has decreased again to a long term low in the last 3 seasons.
Tropical Strom Frequency - 12 month moving average -trend
Hurricane Frequency - 12 month moving average -trend
Major Hurricanes - 12 month moving average -trend
Major Hurricanes - 12 month moving average - trend last 20 years
The Global Hurricane Frequency Index can only provide a very basic overview, it's a metric to determine the quantity of worldwide storm occurrence.
It is essentially a storm counter with three main categories (all/strong/strongest).
edit on 3-12-2013 by talklikeapirat because: inHg