A tropical storm is a rotating weather system with sustained winds between 39 and 73 mph that forms over warm ocean water. It sits in the middle of the tropical cyclone intensity scale, stronger than a tropical depression but not yet a hurricane. Once a storm’s winds reach 74 mph, it officially becomes a hurricane. Tropical storms bring heavy rain, flooding, and dangerous winds, and they’re the point at which a storm receives its official name.
How Tropical Storms Form
Tropical storms begin as clusters of thunderstorms over the ocean. For these storms to organize into a rotating system, a few conditions need to line up. The most important is warm ocean water, at least 80°F (27°C), extending to a depth of about 150 feet. That warm water is the storm’s fuel source, evaporating moisture into the atmosphere and feeding energy into the growing system.
Beyond warm water, the atmosphere needs to be unstable, meaning warm, moist air sits near the surface while cooler, drier air sits above it. This contrast encourages air to rise rapidly, building tall thunderstorm clouds. Low wind shear also matters: if winds at different altitudes blow in different directions or at very different speeds, they’ll tear the developing storm apart before it can organize. When all these ingredients come together near the equator, a tropical depression can form. If it strengthens to sustained winds of 39 mph, it’s reclassified as a tropical storm.
Wind Speed Classifications
The National Weather Service defines tropical cyclones by their maximum sustained surface winds, measured as a one-minute average:
- Tropical depression: 38 mph or less
- Tropical storm: 39 to 73 mph
- Hurricane: 74 mph or greater
A storm earns its name at the tropical storm stage. This naming convention exists because multiple storms can be active at the same time, and distinct names make tracking and public communication far easier than using numbers or coordinates.
Structure of a Tropical Storm
Tropical storms share the same basic anatomy as hurricanes, though their features tend to be less defined. Air spirals inward toward the center, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. At the very center, air sinks, creating a calmer area. In full-strength hurricanes, this becomes the clearly visible “eye,” but in tropical storms it’s often disorganized or partially obscured by clouds.
Surrounding that center is the eyewall, a ring of the tallest thunderstorms with the heaviest rain and strongest winds. Farther out, curved bands of clouds and thunderstorms called rainbands spiral away from the center. These bands can stretch hundreds of miles from the storm’s core and produce intense bursts of rain, gusty winds, and even tornadoes. A tropical storm doesn’t need a well-formed eye to cause serious damage; its rainbands alone can dump enormous amounts of water over a wide area.
Hazards Beyond Wind
Wind speed determines a storm’s classification, but wind is not always the biggest threat. Flooding from heavy rain is the second leading cause of death from landfalling tropical cyclones in the United States, and the rainfall from a tropical storm can cause flooding hundreds of miles inland, persisting for days after the storm dissipates. A tropical storm doesn’t need to be a hurricane to produce catastrophic flooding.
Storm surge, the abnormal rise of ocean water pushed ashore by a storm’s winds, is historically the leading cause of hurricane-related deaths in the U.S. Tropical storms generate smaller surges than hurricanes, but even a few feet of surge in low-lying coastal areas can be deadly, especially when combined with high tide. Other hazards include tornadoes that spin up within the rainbands, dangerous rip currents along the coast, and flying debris from winds strong enough to tear apart signs, roofing materials, and unsecured outdoor objects.
How Meteorologists Track Intensity
Measuring a tropical storm’s strength over open ocean, where there are no weather stations, requires specialized tools. Hurricane hunter aircraft fly directly into storms, releasing instruments called dropsondes from the plane’s belly. These small sensor packages deploy parachutes and fall toward the ocean surface, measuring temperature, pressure, humidity, wind speed, and wind direction as they descend. Researchers typically release 20 to 40 dropsondes during a single flight to build a picture of conditions throughout the storm’s core and outer bands.
The aircraft also carry instruments that measure surface wind speed and rain rate by reading the ocean’s thermal signature directly below the plane. Tail-mounted radar systems scan vertical slices of the storm as the aircraft passes through, allowing scientists to build a three-dimensional image showing where the strongest winds and heaviest rainfall are located. Satellite imagery fills in the gaps between flights, and analysts use pattern-recognition techniques to estimate a storm’s intensity based on its cloud structure when aircraft observations aren’t available.
Naming Conventions
Tropical storms receive names from pre-designated lists maintained by five regional bodies under the World Meteorological Organization. In the Atlantic and parts of the Southern Hemisphere, names alternate between men’s and women’s names in alphabetical order. The Atlantic uses six rotating lists, so the 2023 list will be reused in 2029.
Names are chosen to be short, easy to pronounce, and appropriate across multiple languages. They are not named after specific individuals. If a storm is exceptionally deadly or costly, its name is retired at the WMO’s annual meeting and replaced with a new one. In the rare event that more than 21 named storms form in a single Atlantic season, additional names come from a separate backup list approved by the WMO.
Watches and Warnings
When a tropical storm threatens land, the National Hurricane Center issues two levels of alert. A tropical storm watch means conditions with winds of 39 to 73 mph are possible in your area within the next 48 hours. A tropical storm warning means those same conditions are expected within 36 hours. The difference is essentially the timeline and the level of certainty. A watch means “start preparing.” A warning means “finish your preparations now.”
Different Names Around the World
The term “tropical storm” applies specifically to systems in the Atlantic and eastern Pacific basins. The same type of storm goes by different names in other parts of the world. In the western Pacific, these systems are called typhoons once they reach hurricane-equivalent intensity, though the tropical storm stage uses the same name globally. In the Indian Ocean, they’re called cyclones. The underlying weather phenomenon is identical regardless of the name: a warm-core, low-pressure system fueled by ocean heat, rotating around a central axis. The differences are purely geographic and naming conventions set by each region’s meteorological authority.