How Typhoons and Hurricanes Form: A Guide to Tropical Cyclones

As Super Typhoon Champi tracked across the western Pacific toward Taiwan, eastern China and Japan in late October 2026, it illustrated a process that repeats dozens of times a year in tropical oceans worldwide. The storm reached sustained winds of about 290 km/h at its peak, equivalent to a Category 5 hurricane on the Saffir-Simpson scale, before Hong Kong's Observatory projected it would weaken to a severe tropical storm by landfall, according to a report by The Star based on regional weather agency forecasts.

Same storm, different names

According to NOAA, a hurricane, a typhoon and a tropical cyclone are the same weather phenomenon. The generic scientific term used everywhere is 'tropical cyclone'; depending on where the storm forms, it may also be called a hurricane, typhoon, cyclone or severe tropical cyclone, NOAA's satellite service NESDIS notes.

The four ingredients a storm needs

NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) lists the physical conditions required for a tropical cyclone to form. Ocean water must be at least 26.5°C (80°F) down to a depth of roughly 50 meters (150 feet). The mid-troposphere, around 5 km (3 miles) up, must be relatively moist, because dry air suppresses the thunderstorms that drive the system. Vertical wind shear — the change in wind speed and direction with height — must stay low, under about 10 m/s (20 knots, 23 mph), since strong shear tears a developing storm apart.

A storm also needs to form at least 500 km (300 miles) from the equator, AOML notes, because the Coriolis force — created by Earth's rotation — is too weak closer to the equator to sustain the spinning motion. Finally, a pre-existing disturbance with organized rotation and converging winds must already be in place; AOML emphasizes that tropical cyclones do not form spontaneously even when every other condition is favorable.

From disturbance to full storm: the four stages

NOAA's satellite service NESDIS describes tropical cyclone development as a sequence of four stages. In the tropical disturbance stage, clusters of thunderstorms form as water vapor from warm ocean water condenses and releases heat into the air. If winds organize into circular motion and reach 25 to 38 mph, the system becomes a tropical depression. At 39 mph, it is classified as a tropical storm and given an official name. Once sustained winds reach 74 mph, NOAA classifies it as a hurricane, typhoon or severe tropical cyclone, depending on the region.

• Tropical disturbance: thunderstorms cluster, no defined wind threshold
• Tropical depression: winds of 25–38 mph (40–61 km/h)
• Tropical storm: winds of 39–73 mph (63–118 km/h), storm receives a name
• Hurricane/typhoon/severe tropical cyclone: winds of 74 mph (119 km/h) or higher

Inside a mature storm

At full strength, NESDIS describes a hurricane as roughly 125 miles (about 200 km) across, with cloud tops reaching at least 50,000 feet. At its center sits the eye, a relatively calm zone 5 to 30 miles (8–48 km) wide, surrounded by the eyewall, where the strongest winds and heaviest rain occur. NESDIS notes that low pressure in the eye draws up a mound of ocean water that, on reaching the coast, can produce destructive storm surge.

Why a storm can still fail to form

NOAA's Ocean Service points out that even when warm water, moisture, a disturbance and low wind shear are all present, a hurricane does not always develop — the four ingredients are necessary but not sufficient. This is why forecasters at agencies such as the U.S. National Hurricane Center and regional centers in Asia issue probability outlooks rather than certainties for developing disturbances.

How to prepare before a storm arrives

The U.S. Centers for Disease Control and Prevention (CDC) recommends residents in storm-prone areas prepare in advance rather than waiting for a warning. Its guidance covers planning, supplies and what to do when a storm is approaching.

1. Write down emergency phone numbers and post them where every household member can find them.
2. Identify shelter locations and more than one route to reach them before a storm threatens.
3. Stock food, water and medicines, plus flashlights with extra batteries, in advance of hurricane season.
4. Keep important documents — identification, medical records, insurance papers — together and accessible.
5. Clear yards of items that could become airborne, and cover windows and doors with shutters or plywood.
6. Fill clean containers with drinking water in case supplies are interrupted, and check carbon monoxide detector batteries.
7. Fill vehicle fuel tanks ahead of the storm and move vehicles under cover where possible.
8. If authorities order an evacuation, take only essential items and follow designated evacuation routes.
9. Never drive through flooded roads: the CDC warns that cars can be swept away or stall in as little as 6 inches of moving water.
10. If sheltering in place, stay away from windows and move to an interior room or closet without windows, then wait for official all-clear information before going outside.