Atmospheric Hazards: Tropical Cyclones and Tornadoes

Tropical cyclones are large, rotating storm systems that form over warm tropical oceans and bring very strong winds, torrential rain and storm surges, and are called hurricanes, typhoons or cyclones depending on the region, while tornadoes are narrow, violently rotating columns of air that extend from a thunderstorm to the ground. In brief: tropical cyclones need sea temperatures of at least about 26.5 °C, a location more than about 5° from the equator and low wind shear, and they are rated from category 1 to 5 on the Saffir-Simpson scale. Tornadoes last minutes and affect paths only a few hundred meters wide, but wind speeds can exceed 300 km/h, and they are rated EF0 to EF5.

These atmospheric hazards cause some of the greatest losses from natural events each year. This guide explains how tropical cyclones and tornadoes form, their structure and impacts, compares them in a table, works through calculations, outlines the history of forecasting and warning and corrects common mistakes.

How do tropical cyclones form?

A tropical cyclone starts as a cluster of thunderstorms over warm ocean water. Several conditions are needed.

• Warm water: a sea surface temperature of at least about 26.5 °C to a depth of 50 m or more, which supplies heat and moisture as fuel.
• Distance from the equator: at least about 5° of latitude, so the Coriolis effect is strong enough to spin the system. The storm rotates anticlockwise in the Northern Hemisphere and clockwise in the Southern.
• Low vertical wind shear: little change in wind with height, so that the storm’s tall structure is not torn apart.
• Humid air and a disturbance: moisture through the lower and middle atmosphere and a starting disturbance, such as a tropical wave.

Warm moist air rises, condenses and releases latent heat, which warms the air and lowers pressure at the surface, so more air is drawn in. A storm is called a tropical storm when sustained winds reach 63 km/h and a hurricane (Atlantic and northeastern Pacific), typhoon (northwestern Pacific) or cyclone (Indian Ocean and South Pacific) at 119 km/h. Storms weaken when they move over cold water or over land, because they lose their heat source and gain friction. Tropical cyclones do not form in the South Atlantic and southeastern Pacific under normal conditions, because the water is too cool and shear is high.

What are the structure and impacts of a tropical cyclone?

A mature cyclone has a calm eye some 20 to 60 km wide with sinking air and light winds, surrounded by the eyewall, a ring of towering clouds where the strongest winds and heaviest rain occur, and spiral rainbands. The whole system can be hundreds of kilometers wide.

• Storm surge: a rise in sea level caused by the low pressure and the winds pushing water onshore, which can reach several meters and is usually the greatest cause of deaths. The surge is worst where the coast is low and funnel-shaped, as in Bangladesh, and is added to the tide.
• Wind: destroys buildings, trees and power lines. Damage rises steeply with wind speed.
• Heavy rain and flooding: a slow-moving storm can drop a meter of rain, causing river floods and landslides far inland, as Hurricane Harvey did in Texas in 2017.
• Tornadoes: cyclones sometimes spawn tornadoes when they make landfall.

Notable cases include the Bhola cyclone of 1970 in Bangladesh, which killed hundreds of thousands, Hurricane Katrina in 2005, which flooded New Orleans and killed around 1,800 people, and Typhoon Haiyan in 2013, which killed over 6,000 people in the Philippines. Warmer oceans are expected to increase the share of the most intense storms and the heavy rain they bring, although the total number of storms may not rise.

How do tornadoes form and what do they do?

The strongest tornadoes come from supercell thunderstorms, which are large, long-lived storms with a rotating updraft called a mesocyclone. They form where warm, humid air near the ground lies under cooler, drier air aloft, and where winds change in speed and direction with height, giving the air a horizontal spin. The updraft tilts this spin into the vertical, and a funnel can extend down to the ground. The United States has the most tornadoes of any country, concentrated in the Great Plains, in an area called “Tornado Alley,” most often in spring; others occur in Bangladesh, Argentina, Europe and elsewhere. Tornadoes are typically a few tens to a few hundred meters wide and travel along paths of a few kilometers, but the strongest may be over 1 km wide and stay on the ground for more than 100 km, as with the Tri-State Tornado of 1925, which killed 695 people.

The Enhanced Fujita (EF) scale rates tornadoes by damage and estimated wind speed: EF0 (65–85 mph, about 105–137 km/h), EF1 (86–110 mph), EF2 (111–135 mph), EF3 (136–165 mph), EF4 (166–200 mph) and EF5 (over 200 mph, above about 322 km/h). Most tornadoes are EF0 or EF1, and violent ones (EF4–EF5) are rare but account for a large share of deaths. Warnings rely on Doppler radar, and the lead time is typically several minutes to tens of minutes.

Tropical cyclones and tornadoes compared

Worked example: scales, speed and surge

1. Saffir-Simpson category: a hurricane with sustained winds of 230 km/h is in category 4 (209–251 km/h). A tropical storm with 90 km/h is below hurricane strength (below 119 km/h).
2. Unit conversion: 1 mph = 1.609 km/h. An EF3 tornado with winds of 150 mph has winds of 150 × 1.609 = 241 km/h.
3. Wind and damage: the force of the wind on a structure is roughly proportional to the square of wind speed. If winds double from 100 to 200 km/h, the force is multiplied by 2² = 4.
4. Storm surge from pressure: the central pressure of a cyclone is 940 hPa, compared with a normal of 1,013 hPa. The difference is 73 hPa, and sea level rises about 1 cm for each 1 hPa drop (the “inverse barometer” effect), so about 0.73 m, with wind driving more water onshore. Together, the surge can reach 3 to 5 m.
5. Evacuation time: a cyclone moving at 20 km/h is 400 km offshore. The time to landfall is 400 ÷ 20 = 20 hours. If evacuating the coast takes 12 hours, authorities must order it at least 12 hours before the storm arrives, and gale-force winds begin earlier, so in practice they must begin about 24 to 36 hours ahead.
6. Tornado path area: a tornado with an average width of 300 m and a track length of 25 km damages 0.3 × 25 = 7.5 km².

How has forecasting developed?

Before satellites, ships and islands gave only limited notice of cyclones, and storm surges killed many people. The 1900 Galveston hurricane in Texas killed an estimated 6,000 to 12,000 people, which is still the deadliest natural disaster in US history. Weather satellites from the 1960s allowed the tracking of storms over open oceans, and aircraft “hurricane hunters” measure storm structure. The Saffir-Simpson scale was developed in the early 1970s, and the Fujita scale for tornadoes in 1971, with the enhanced version in 2007. Computer forecasting has improved the accuracy of track forecasts: the average errors of 3-day forecasts have fallen by more than half since the 1990s. Intensity forecasts have improved more slowly. In Bangladesh, a cyclone preparedness program with volunteers and shelters has sharply reduced loss of life.

Which mistakes are common?

Mistake 1: Judging the danger of a cyclone by its category alone

Storm surge and rain, which are not part of the Saffir-Simpson wind scale, cause most of the deaths. Lower-category storms can be deadly.

Mistake 2: Thinking that the eye means the storm is over

The calm eye is followed by the other side of the eyewall with winds from the opposite direction.

Mistake 3: Opening windows to protect a house from a tornado

Opening windows does not equalize pressure and wastes time. Follow official advice instead: go to a strong shelter, a basement or an interior room on the lowest floor.

Mistake 4: Confusing hurricanes, typhoons and cyclones

They are the same type of storm in different regions. The name depends on the ocean basin.