How Hurricanes Form and How to Prepare: A Science-Based Guide

Every year, forecasters at the U.S. National Oceanic and Atmospheric Administration (NOAA) and its National Hurricane Center track dozens of tropical disturbances that form over warm seas. Only some of them grow into hurricanes. The difference comes down to a specific set of atmospheric and oceanic conditions that meteorologists have studied for decades.

The ingredients a hurricane needs

According to NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML), tropical cyclones require six favorable conditions to form. Ocean water must be at least 26.5°C (80°F) through a sufficient depth, providing the heat energy that drives the storm. The atmosphere must cool quickly enough with height to be unstable, allowing moist air to rise freely. Mid-level air must be relatively humid, since dry air chokes off the thunderstorm activity a cyclone depends on.

The system also needs to form at least 500 kilometers from the equator, so the Coriolis force—created by Earth's rotation—can make the air spin around a center rather than simply flowing straight into the low-pressure area. A pre-existing disturbance with some spin and surface convergence, such as a tropical wave, is also necessary; AOML notes that cyclones do not form on their own and need this kind of disturbance to get started. Finally, vertical wind shear below about 10 meters per second is required, because stronger shear tears a developing storm apart before it can organize.

NOAA's Ocean Service describes the same process in simpler terms: a tropical wave moves across warm tropical water, warm air rises and creates a zone of low pressure, more air rushes in and rises, and condensation in the growing clouds releases heat that intensifies the system further. AOML cautions that even when all six conditions are present, a storm does not always form—they are necessary but not sufficient for development.

From disturbance to hurricane: the four stages

Meteorologists classify the growth of a tropical system into four stages. It begins as a tropical disturbance, a cluster of thunderstorms with little or no rotation. If winds start circulating around a center, it becomes a tropical depression. Once sustained winds reach 39 mph, the system is named and classified as a tropical storm. When sustained winds reach 74 mph, NOAA classifies it as a hurricane, typhoon, or tropical cyclone, depending on where in the world it occurs.

NOAA's Ocean Service explains that these are the same type of storm with different regional names: hurricane in the North Atlantic, central North Pacific and eastern North Pacific; typhoon in the Northwest Pacific; and tropical cyclone as the generic term used in the South Pacific and Indian Ocean. The classification is based purely on geography, not on any physical difference between the storms.

How strong hurricanes are measured

Once a storm reaches hurricane strength, the U.S. National Hurricane Center rates it on the Saffir-Simpson Hurricane Wind Scale, which is based solely on maximum sustained wind speed and estimates potential property damage. The agency stresses that the scale does not take into account other potentially deadly hazards such as storm surge, rainfall flooding and tornadoes.

Which hazards actually kill people

The U.S. National Weather Service (NWS) identifies storm surge—the abnormal rise of water pushed by a storm's winds—as historically the deadliest hurricane hazard, able to push inland for miles along bays and estuaries. Rainfall flooding is described as the second-leading cause of fatalities, and the NWS notes it can occur hundreds of miles from the coast and continue for days after the storm passes. The agency also lists destructive winds, tornadoes that can spin up in a hurricane's outer rain bands, and dangerous waves and rip currents that can threaten swimmers even when a storm is over 1,000 miles away.

How to prepare: step by step

The U.S. Centers for Disease Control and Prevention (CDC) and NWS outline preparation across three phases: before hurricane season, as a storm approaches, and during the event itself.

1. Know your evacuation zone and flood risk, and plan more than one travel route in advance, following guidance from NOAA's hurricane preparedness resources.
2. Make a family communication plan in case cell service is knocked out, including a meeting point if people get separated.
3. Build a supply kit with at least three days of water and food, medications, flashlights, batteries, important documents, cash, and a phone charger, per CDC guidance.
4. As a storm approaches, fill your vehicle's gas tank, fill water containers, and secure or bring in outdoor items that wind could turn into projectiles, per CDC recommendations.
5. Follow official evacuation orders without delay. According to the CDC, protecting property is not a reason to stay, since even strongly built houses may not withstand a hurricane.
6. If sheltering in place, stay in an interior room away from windows and doors, and remain there even during a lull, since the calm eye of the storm can be followed by a return of violent winds.
7. Never drive or walk through flood water. The CDC warns that vehicles can be swept away or can stall in as little as 6 inches of moving water.
8. After the storm, continue monitoring official alerts before returning home, since hazards such as downed power lines and floodwater can persist.

When hurricane season runs

The CDC states that the Atlantic and Caribbean hurricane season runs from June 1 through November 30 each year, the period when NOAA and the National Hurricane Center maintain routine tracking of tropical systems in that basin.