Severe Weather: Thunderstorms, Tornadoes, and Tropical Cyclones
Severe weather includes thunderstorms (storms with lightning and thunder produced by cumulonimbus clouds), tornadoes (violently rotating columns of air in contact with the ground) and tropical cyclones (large, rotating storm systems that form over warm tropical oceans, called hurricanes in the Atlantic and eastern Pacific and typhoons in the western Pacific). In brief: thunderstorms need moisture, unstable air and a lifting mechanism. Tornadoes form from some of the strongest thunderstorms (supercells) and are rated on the Enhanced Fujita scale. Tropical cyclones need sea surface temperatures of at least about 26.5 °C, and they are rated on the Saffir–Simpson scale from category 1 to 5. Forecasts and warnings greatly reduce deaths from these hazards.
Severe weather causes heavy loss of life and damage each year. This guide explains how thunderstorms, tornadoes and tropical cyclones form and what hazards they bring, compares them in a table, works through examples (distance to lightning, storm timing and hurricane categories), outlines the history of forecasting and corrects common mistakes.
How do thunderstorms form?
A thunderstorm needs three things: moist air near the surface, unstable air (warm air that keeps rising because it is warmer than its surroundings), and a trigger that lifts the air (surface heating, a front or mountains). The rising air forms a cumulus cloud that grows into a cumulonimbus, which may reach 10–15 km in height. The stages are: the developing stage (updraft), the mature stage (updraft and downdraft, heavy rain, lightning, strong gusts and sometimes hail) and the dissipating stage (downdraft cuts off the supply of warm air). Lightning is a huge electrical discharge caused by the separation of charge between ice particles in the cloud. It heats the air to about 30,000 °C, which expands explosively and creates the sound of thunder. Hazards: lightning strikes, flash floods, hail, strong straight-line winds and downbursts.
What are tornadoes?
A tornado is a narrow, violently rotating column of air that extends from a thunderstorm cloud to the ground, usually visible as a funnel cloud when condensation and debris show it. Most develop from supercells, which are rotating thunderstorms, where a horizontal rotation in the wind (wind shear) is tilted by the updraft into a vertical rotation. Typical tornadoes are 50–200 m wide, last a few minutes and travel several kilometers, but violent ones can be over 1 km wide and stay on the ground for an hour. The Enhanced Fujita scale rates tornadoes from EF0 (weak, 105–137 km/h) to EF5 (over 322 km/h) by the damage they cause. The United States has the most tornadoes, on average more than 1,000 a year, especially in “Tornado Alley” in the Great Plains, but they occur on every continent except Antarctica. Waterspouts are tornadoes over water.
What are tropical cyclones?
Tropical cyclones form over oceans where the sea surface temperature is at least about 26.5 °C to a depth of about 50 m, usually between 5° and 30° latitude (not at the equator, because the Coriolis effect that gives rotation is too weak). Warm moist air rises, condenses and releases latent heat, which drives the storm, lowers the pressure at the center and draws in more air. The result is a huge spiral of thunderstorms, 200–1,000 km across, with a calm eye at the center (20–60 km wide) surrounded by the eyewall with the strongest winds and heaviest rain. They rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern. Names: hurricane (North Atlantic and northeast Pacific), typhoon (northwest Pacific), cyclone (Indian Ocean and South Pacific). The Saffir–Simpson scale uses sustained wind speeds: category 1 is 119–153 km/h, category 2 is 154–177, category 3 is 178–208, category 4 is 209–251, and category 5 is 252 km/h or more. Hazards: strong winds, torrential rain and flooding, and above all the storm surge, a rise in sea level driven by wind and low pressure that has caused the greatest loss of life. Cyclones weaken quickly over land or cool water.
How is severe weather forecast?
Forecasters use weather radar (including Doppler radar to detect rotation), satellites, surface stations, weather balloons and numerical models. For thunderstorms and tornadoes, a watch means that conditions favor them, and a warning means that one is occurring or imminent, so people should take shelter. Tropical cyclones are tracked by satellites and reconnaissance aircraft, and forecast tracks (the “cone of uncertainty”) are now much more accurate than they were decades ago, whereas forecasts of intensity changes remain harder.
Severe weather compared
| Feature | Thunderstorm | Tornado | Tropical cyclone |
|---|---|---|---|
| Typical size | 10–20 km across | 50–200 m wide | 200–1,000 km across |
| Duration | 30–60 minutes | Minutes | Days to weeks |
| Where | Worldwide, mainly warm regions | Mid-latitude plains, especially US | Tropical oceans, 5°–30° latitude |
| Main hazards | Lightning, hail, flash floods | Extreme winds | Wind, storm surge, flooding |
| Scale | — | Enhanced Fujita (EF0–EF5) | Saffir–Simpson (1–5) |
Worked example: distance, timing and categories
- Distance to lightning: sound travels at about 343 m/s, so it covers 1 km in about 3 seconds. If thunder is heard 9 seconds after the flash, the distance is 9 × 343 = 3,087 m, about 3 km (light arrives almost instantly).
- Tornado path: a tornado that stays on the ground for 40 minutes moving at 60 km/h travels 60 × (40 ÷ 60) = 40 km.
- Hurricane arrival time: a cyclone 600 km offshore moves toward the coast at 20 km/h. It reaches land in 600 ÷ 20 = 30 hours. Evacuation orders therefore have to be given more than a day in advance.
- Category: the sustained wind of 200 km/h is within the range 178–208 km/h, so it is category 3. A wind of 130 mph is 130 × 1.609 = 209 km/h, which is the lower limit of category 4.
- Eye size: a cyclone with a diameter of 600 km and an eye of 30 km has an eye that covers (30 ÷ 600)2 = 0.0025 of the area, or 0.25 percent, so most of the storm is cloud and rain.
- Water temperature: a sea surface at 24 °C is below the 26.5 °C threshold, so a cyclone moving over it would be expected to weaken, since 26.5 − 24 = 2.5 °C less than needed.
How has our understanding developed?
Ancient peoples explained storms by gods, and the first scientific studies began in the 18th century: Benjamin Franklin’s kite experiment (1752) was used to show that lightning is electrical. The Galveston hurricane of 1900 killed thousands in Texas, one of the deadliest natural disasters in United States history, and led to improved warnings. Ted Fujita introduced the Fujita scale for tornadoes in 1971, and the Saffir–Simpson scale was developed in the early 1970s. Weather satellites from the 1960s and Doppler radar from the 1990s have greatly improved the detection and tracking of severe weather.
Which mistakes are common?
Mistake 1: Thinking that the thunderstorm is over when the rain stops
Lightning can strike up to about 10 km from the storm. Wait until 30 minutes after the last thunder.
Mistake 2: Confusing hurricane, typhoon and cyclone as different phenomena
They are the same type of storm. The name depends on the ocean basin.
Mistake 3: Judging the danger of a hurricane only by its category
The storm surge and rain-induced flooding often cause most of the deaths, and they depend on the size, speed and local geography, not only on the wind speed.
Mistake 4: Opening windows during a tornado warning
It does not equalize the pressure to protect a building. Take shelter in the lowest, most interior room instead.