One storm, several names
A tropical cyclone is a large, rotating storm system that forms over warm tropical oceans. The same type of storm is called a hurricane in the Atlantic and the northeastern Pacific, a typhoon in the northwestern Pacific, and a cyclone in much of the Indian Ocean and South Pacific. The physics is the same in every case; only the regional name changes.
The basic ingredients
Forecasters and textbooks usually list a handful of conditions that need to come together:
- Warm ocean water. Sea-surface temperatures of about 26.5 degrees Celsius (around 80 degrees Fahrenheit) or higher, extending down to a depth of tens of meters, are typically needed. Warm water supplies heat and moisture.
- Moist, unstable air. Humid air in the lower and middle atmosphere lets thunderstorms grow tall and keep going.
- Low vertical wind shear. If winds change strongly in speed or direction between the surface and the upper atmosphere, a developing storm can be tilted and torn apart. Light shear lets it stack vertically.
- A starting disturbance. Hurricanes usually grow from an existing cluster of thunderstorms, often a wave of low pressure drifting off a landmass such as Africa.
- Enough distance from the equator. The Coriolis effect, caused by Earth’s rotation, is too weak right at the equator to spin up a storm. Tropical cyclones rarely form within about 5 degrees of latitude of it.
All of these usually need to be present at once, which is why many disturbances never become hurricanes.
How the storm feeds itself
A hurricane works somewhat like a heat engine. Warm ocean water evaporates into the air above it. When that moist air rises and the vapor condenses into cloud droplets, the vapor releases the heat it absorbed during evaporation. This is one of the practical consequences of changes between the states of matter, and it warms the surrounding air, making it rise further.
As air rises near the center, surface pressure falls. Surrounding air flows inward toward the low pressure and picks up more moisture from the sea on the way. Because of Earth’s rotation, the inflowing air is deflected and begins to circle. In the Northern Hemisphere the circulation is counterclockwise; in the Southern Hemisphere it is clockwise. The lower the central pressure falls, the faster the winds tend to blow, and the faster the winds blow, the more evaporation they drive. This loop of feedbacks is what lets a storm intensify, as long as conditions stay favorable.
Stages of development
Meteorologists use standard labels for stages, based largely on the maximum sustained wind speed:
- Tropical disturbance: a disorganized cluster of showers and thunderstorms.
- Tropical depression: a closed circulation with sustained winds up to about 38 miles per hour.
- Tropical storm: winds of roughly 39 to 73 miles per hour. At this stage the storm is given a name.
- Hurricane: winds of at least 74 miles per hour.
Once a storm is a hurricane, the Saffir-Simpson scale rates it from Category 1 to Category 5 by wind speed, where Category 5 means sustained winds of 157 miles per hour or more. The scale describes wind only. Rainfall and flooding can be severe in any category.
Anatomy of a mature hurricane
Near the center is the eye, often a region of relatively light winds and clearer skies, typically tens of kilometers across. Around it is the eyewall, a ring of towering thunderstorms where the strongest winds and heaviest rain usually occur. Spiral rainbands extend outward for many kilometers. Air sinks in the eye, rises in the eyewall, and flows outward near the top of the storm.
What weakens a hurricane
Storms lose strength when they move over cooler water or across land, because they lose their supply of heat and moisture, and friction with the ground disrupts the circulation. Strong wind shear or dry air drawn into the core can also weaken them. A weakening storm can still produce dangerous flooding from heavy rain after it has moved well inland.
Why storm surge is such a threat
Much of the loss of life in historic hurricanes has come from storm surge, the rise in sea level that the storm’s winds and low pressure push onto the coast. When it coincides with high ocean tides, the water can reach even farther inland. Wind, surge and inland flooding are all separate hazards, which is why official guidance from local authorities during an approaching storm should always be followed.
Hurricanes and a changing climate
Scientists generally expect that warmer oceans will tend to increase the intensity and rainfall of the strongest storms, while it remains less clear whether the total number of storms will change. Research here is active and some details are uncertain. A broader look at the underlying physics of heat trapping is in the explanation of the greenhouse effect.
Predicting the track and the strength
Forecasters are usually better at predicting where a storm will go than how strong it will become, since intensity depends on small-scale processes inside the storm that are hard to capture. Satellites, aircraft and ocean buoys supply data for the computer models behind modern weather forecasting. Even so, forecasts of both track and strength carry uncertainty, which is why they are shown as a cone or range rather than a single line.