Severe weather refers to atmospheric events that pose a threat to life, property, and public safety. These phenomena include a wide range of powerful and destructive natural occurrences, such as strong thunderstorms, flooding, blizzards, hurricanes, and tornadoes. While there is no single, globally unified “5-level” scale that applies to every type of severe weather, multiple distinct categorization systems exist to rate the intensity of specific, highly destructive events. Storm severity is often classified using scales that measure either potential damage (like wind speed) or observed impact (like destruction left by a tornado).
Communicating Severe Weather Risk
Meteorological agencies use a tiered system to communicate the potential for severe weather, focusing on the likelihood and timing of an event, rather than its ultimate intensity. This communication structure helps the public understand the level of immediate threat and what actions should be taken. The system generally involves three key alert levels: Advisory, Watch, and Warning.
An Advisory is issued for weather conditions that may cause significant inconvenience or threaten life or property if caution is not exercised. These conditions are less severe than those that prompt a Watch or Warning. Examples include dense fog, strong winds below severe thresholds, or minor winter weather accumulations.
The alert level increases to a Watch when the risk of a hazardous weather event has significantly increased, but its exact occurrence, location, or timing remains uncertain. A Watch indicates that conditions are favorable for severe weather to develop, such as a tornado or severe thunderstorm, and serves as a planning notification. People should remain alert, monitor the forecast closely, and be prepared to act quickly if the threat materializes.
A Warning represents the most serious alert and is issued when a hazardous weather event is occurring or is imminent in the designated area. This means the threat to life and property is immediate, and protective action must be taken without delay. Warnings are based on confirmed reports from trained spotters or clear indications from weather radar, signifying that the severe weather is about to strike or is already underway.
Rating the Intensity of Hurricanes
The Saffir-Simpson Hurricane Wind Scale (SSHWS) is the primary 5-level system used in the Western Hemisphere to classify the intensity of tropical cyclones. This scale is based solely on a hurricane’s maximum sustained wind speed, which is the average wind speed measured over a one-minute period. The scale includes five categories, ranging from Category 1 (least intense) to Category 5 (most extreme).
A Category 1 hurricane has sustained winds between 74 and 95 miles per hour (mph), causing damage to well-constructed frame homes, including roofs and gutters. Moving up the scale, a Category 3 hurricane is considered a “major” storm, featuring winds from 111 to 129 mph. At this level, well-built homes sustain major damage, and widespread power outages are expected to last for days or weeks.
Category 5 is assigned to any hurricane with sustained winds of 157 mph or higher. Catastrophic damage is expected at this level, where a high percentage of framed homes will be completely destroyed with total roof failure and wall collapse. Most of the affected area is likely to be uninhabitable for weeks or months. The scale does not account for other destructive elements, such as storm surge, rainfall flooding, or tornadoes, which are communicated separately.
Measuring the Force of Tornadoes
For tornadoes, the Enhanced Fujita (EF) Scale is the standard system used to rate intensity, providing six possible ratings from EF0 to EF5. It is a damage-based system, meaning a tornado’s rating is determined by the destruction it leaves behind, rather than a direct measurement of wind speed. Survey teams assess damage indicators, such as the type of structure and the degree of damage sustained, to estimate the range of wind speeds.
The EF0 rating signifies light damage, with estimated wind speeds between 65 and 85 mph, involving minor roof damage or broken tree branches. An EF3 tornado indicates severe damage, with estimated winds of 136 to 165 mph, enough to completely destroy an entire story of a well-built house. The scale provides a better correlation between damage and wind speed than its predecessor by considering construction quality.
At the highest end of the scale is the EF5 rating, which represents incredible damage caused by estimated wind speeds exceeding 200 mph. This level of force can sweep clean a well-built frame house off its foundation and carry automobile-sized objects long distances. Although the scale technically starts at EF0, the categories EF1 through EF5 represent the five progressively more severe levels of destructive force.