What’s the difference in a Tropical Storm and a Hurricane?

Potato – Potata, right? It’s a massive amount of wind, rain and storm surge, but what are the actual levels that qualify the storms as one or the other? The difference is only 1 mile per hour more of sustained wind at the level from 74mph to 75mph. That’s the quick answer, but there is a lot more to the story than the 1mph marker. Keep reading and you’ll know enough about the measurements to be a grand wizard on Jeopardy ūüôā

History of Measuring Wind Speed

The Beaufort Wind Scale

Way back in 1805, Sir Frances Beaufort came up with Wind Scale that measured things from zero all the way up to Hurricane. Known today as the Beaufort Wind Scale, it measured a range of wind speed in knots, stopping at 64+ for Hurricanes.

The Beaufort scale broke up winds into 12 forces or grades from calm winds up through gale force winds to hurricane strength winds. The 12th grade on the Beaufort scale is “hurricane,” a grade defined by winds of 64 knots or 74 mph at its minimum bound and no maximum bound. 

Image credit @ Wikipedia

The Saffir-Simpson Hurricane Wind Scale

Saffir Simpson gave the Hurricanes a measure above 64 knots, which we refer to as categories 1 through 5 today. The scale is a rating based on a hurricane’s sustained wind speed. This scale estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage. Category 1 and 2 storms are still dangerous, however, and require preventative measures. In the western North Pacific, the term “super typhoon” is used for tropical cyclones with sustained winds exceeding 150 mph.¬†Note that all winds are using the U.S. 1-minute average.

hurricane categories explained
Graphic Kyung-Don, Korea Herald
Category One Hurricane

Winds 74-95 mph (64-82 kt or 119-153 km/hr). Very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days. Irene of 1999, Katrina of 2005, and several others were Category One hurricanes at landfall in South Florida.
 Category Two Hurricane

Winds 96-110 mph (83-95 kt or 154-177 km/hr). Extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks. Frances of 2004 was a Category Two when it hit just north of Palm Beach County, along with at least 10 other hurricanes which have struck South Florida since 1894.
 Category Three Hurricane

Winds 111-129 mph (96-112 kt or 178-208 km/hr). Devastating damage will occur: Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes. Unnamed hurricanes of 1909, 1910, 1929, 1933, 1945, and 1949 were all Category 3 storms when they struck South Florida, as were King of 1950, Betsy of 1965, Jeanne of 2004, and Irma of 2017.
 Category Four Hurricane

Winds 130-156 mph (113-136 kt or 209-251 km/hr). Catastrophic damage will occur: Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months. The 1888, 1900, 1919, 1926 Great Miami, 1928 Lake Okeechobee/Palm Beach, 1947, Donna of 1960 made landfall in South Florida as Category Four hurricanes.
 Category Five Hurricane

Winds 157 mph or higher (137 kt or higher or 252 km/hr or higher). Catastrophic damage will occur: A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months. The Keys Hurricane of 1935 and Andrew of 1992 made landfall in South Florida as Category Five hurricanes.

Is Wind the Only Factor?

Good Question! Short answer, No. Wind speed is something we can objectively measure but the amount of rainfall and water consumed while over the ocean, is a different story. Rainfall totals are a function of forward speed rather than tropical storm or hurricane intensity. 

Water, in the form of rain, flooding, storm surge, dictate how devastating a storm can be. In terms of the potential flooding it can bring tidal waves up to the height of rooftops in low lying coastal zones. The constant flow of rain can also saturate the ground and flood inland areas.

Read more about Flood risks and preparation here.