Hurricane basics: how they form, how they're rated, and what drives them

A tropical cyclone is a large storm that forms over warm ocean water and has high winds and heavy rain. They are one of nature’s most powerful storms. The ingredients needed for a storm to form are:

What a hurricane needs to form.

• Warm ocean water - sea surface temperatures at least 80°F extending at least 50 meters deep, that’s why hurricanes weaken over land or cold water.
• Atmospheric instability - The surrounding air must be warm and moist, allowing it to rise rapidly and form heavy storm clouds and thunderstorms.
• Coriolis effect - the Earth's rotation must provide a rotational spin, which means the system must develop at least 5 degrees latitude.
• Low vertical wind shear - Winds high and low in the atmosphere must blow at similar speeds and directions.
• Mid-level moisture - High relative humidity in the middle troposphere is necessary to sustain thunderstorm growth.
• A starting weather trigger - such as a tropical wave or cluster of thunderstorms, is needed to begin the low-pressure circulation.

Where tropical cyclones form.

Hurricanes, typhoons, and cyclones are all the same type of storm, but have different names based on where they form. The world has seven tropical cyclone basins with around 85 tropical cyclones forming annually (WMO) over the seven basins.¹²

The basins with the highest average number of storms per year are the Northwest Pacific (26 to 28 typhoons), and the two affecting North America, North Atlantic (14 named storms)/Eastern-Central Pacific (19).³⁴

The most intense tropical cyclones for each basin are:

• Northwest Pacific - Typhoon Tip (1979) - 870 hPa (mb)⁵
• North Atlantic - Hurricane Wilma (2005) - 882 hPa (mb)⁶⁷
• Eastern/Northeast Pacific - Hurricane Patricia (2015) - 872 hPa (215 mph)⁸
• North Indian Ocean - Odisha (1999) - 912 hPa
• South-West Indian Ocean - Fantala (2016) - 910 hPa (185 mph)
• Australian region - Gwenda and Vance (1999) - 912 hPa
• South Pacific - Winston (2016) - 884 hPa (184 mph)

Hurricane seasons and classification.

The North Atlantic hurricane season goes from June 1 to November 30, when tropical or subtropical cyclones are most likely to form in the North Atlantic Ocean. The eastern Pacific hurricane season runs from May 15 to November 30 whereas the Central Pacific season begins on June 1 and ends November 30.

Storms are classified by their maximum wind speed and according to the National Hurricane Center (NHC) a hurricane is “a tropical cyclone where maximum sustained surface winds reach 74 mph (119 km/h or 64 knots) or more.”⁹ Winds less than 39 mph are classified as tropical depressions and for wind speeds 39-73 mph tropical storms.

Saffir-Simpson Hurricane Wind Scale

Hurricanes are categorized using the Saffir-Simpson Hurricane Wind Scale (SS):

• Category 1 - 74-95 mph - Very dangerous winds will cause some damage.
• Category 2 - 96-110 mph - Extremely dangerous winds will cause extensive damage.
• Category 3 - 111-129 mph - Devastating damage will occur.
• Category 4 - 130-156 mph - Catastrophic damage will occur.
• Category 5 - 157 mph or higher - Catastrophic damage will occur.

Category 3+ are categorized as major hurricanes.

Climate drivers of storm strength, track and rainfall.

The climate drivers affecting storm strength, track and rainfall include both large scale natural climate patterns and long-term global warming factors.

Natural Climate patterns include:

• El Nino-Southern Oscillation (ENSO) - El Nino creates strong vertical shear that suppresses hurricane activity whereas it increases the temperature of the Eastern Pacific waters, reducing wind shear and therefore increasing Eastern Pacific activity. La Nina reduces Atlantic wind shear therefore increasing Atlantic hurricane formation but increases Eastern Pacific shear which decreases Eastern Pacific storms.
• Atlantic Multidecadal Oscillation (AMO) - longer-period (roughly 20-40 year) cycle of Atlantic sea surface temperature that's associated with multidecadal swings between more active and less active hurricane eras.
• Pacific Decadal Oscillation (PDO) - analogous multidecadal SST pattern in the Pacific, with more direct relevance to Eastern/Central Pacific
• Madden-Julian Oscillation (MJO) - a shorter-period (30-60 day) tropical convective wave that modulates activity on sub-seasonal timescales; it can create 1-2-week windows of enhanced or suppressed tropical cyclone formation.
• Saharan Air Layer (SAL) - dry, dusty air masses off West Africa that suppress early-season Atlantic tropical wave development by introducing dry air and increased shear into developing systems.
• Sea surface temperature (SST) - warmer SSTs provide more latent heat energy for storm intensification, and the 26°C threshold is the classic minimum needed to sustain tropical cyclone formation.

Climate change effects on hurricanes

• Intensity, not necessarily frequency: the scientific consensus (including NOAA's own position, as referenced in their 2021 baseline announcement) is that climate change more likely influences storm intensity than overall frequency.¹⁰
• Rapid intensification: there's growing evidence linking warmer ocean heat content to an increased frequency of rapid intensification events (24-hour intensity jumps).¹¹ Example is Hurricane Polo (Eastern Pacific, September 2026) with a 24-hour wind speed increase from 50 mph to 160 mph, an increase of 110 mph.¹²¹³
• Rainfall rates: a warmer atmosphere holds more moisture (Clausius-Clapeyron relationship), so hurricane-associated rainfall totals are generally expected to increase.¹⁴
• Storm translation speed: some research (still debated) suggests hurricanes may be moving more slowly on average.¹⁵
• Poleward migration of peak intensity latitude: some studies suggest the latitude where storms reach peak intensity is shifting poleward.¹⁶

Tropical Storm data for all seven basins can be downloaded from https://www.ncei.noaa.gov/products/international-best-track-archive