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Catastrophe Vocabulary: Understanding the language of severe weather

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What is a supercell? What is wind shear and why is it important in tropical storm development? Does a red flag warning mean that a wildfire is imminent? Sometimes weather terminology can be confusing, and it seems like there’s a new term to learn each year. 

To properly plan and prepare for extreme weather conditions it’s important to understand the language used in current weather forecasting and reporting. 


According to NOAA, “a hurricane is a type of storm called a tropical cyclone, which forms over the tropical or subtropical waters” and has sustained wind speeds of 74mph or greater.1 Hurricanes are rated based on windspeeds using the Saffir-Simpson Wind Scale, ranging from 1-5. This rating system measures the sustained windspeeds but does not consider other impacts that occur as a result of the storm. 


  • Invest: term used by the National Hurricane Center to refer to an area they are monitoring for potential cyclone development.
  • Tropical Cyclone: a rotating system of low pressure that does not have fronts but does have organized thunderstorms and generally form in the tropics driven by warm ocean water. 
  • Tropical Depression: a tropical cyclone with maximum sustained windspeeds under 39 mph. 
  • Tropical Storm: a cyclone with sustained windspeeds over 39 mph, but under 74mph. 
  • Weather Front: the zone where areas of different air temperature, humidity, and/or pressure meet.
  • Wind Shear: the change of direction and speed of wind, which can occur as vertical wind shear or horizontal wind shear. Vertical wind shear is most impactful in terms of tropical development, which helps to disrupt the organized thunderstorms found within a tropical cyclone. Vertical wind shear can break up a storm and prevent additional development.2

Did you know?

The Cape Verde Islands are located off the western coast of Africa and are often mentioned in tropical cyclone development because of the many the tropical storms that originate there that often make landfall in the U.S.3


The Federal Emergency Management Agency, or FEMA, defines a wildfire is an accidental and undesired blaze that occurs in a natural environment, such as a forest, grassland, or prairie.4


  • Containment: indicates how much of a fire perimeter has been controlled, preventing the fire from expanding further in that direction. Typically, the extent of a fire's containment is presented as a percentage, denoting the proportion of the fire perimeter that is effectively controlled. For example, a wildfire that is 50% contained means that 50% of the fire perimeter is controlled without threat of the fire growing.5
  • Controlled Burn: also known as “prescribed fires” are planned burns to help control tree growth and overcrowding that can be fuel for wildfires. Periodic burning is also good for the environment because it returns nutrients to the soil and creates additional space for indigenous wildlife.6
  • Defensible Space: is an area surrounding a property with low vegetation and limited combustible material. Creating a defensible space of 5 feet or greater around a residential property can help prevent fire ignition from flying embers, which is the number one cause of wildfire-related property damage. View our infographic for ideas and tips on how to create defensible space.
  • Fire Perimeter: is the outside boundary of a wildfire. 
  • Red Flag Warning: indicates an increased risk of fire spread due to a combination of factors, including low humidity, high temperature, and strong winds.
  • Relative Humidity: amount of moisture needed to saturate the air in relation to the temperature and pressure. Relative Humidity, or RH, plays a significant role in the wildfire conditions. If the RH is high, meaning there is a lot of moisture in the air, the risk of wildfire spread is low.

Did you know? 

Although lightning can be a factor, human carelessness is the primary cause of wildfires in the U.S. 4



NOAA defines a tornado as a “narrow, violently rotating column of air that extends from a thunderstorm to the ground9.” Tornadoes pose a significant threat and can result in widespread destruction. While tornadoes can happen anywhere in the world, the United States consistently experiences the greatest frequency of reported tornado occurrences each year.10 


  • Enhanced Fujita Scale: rating scale used to determine the wind strength of a tornado. The National Weather Service rates the strength using 28 different damage indicators to determine a rating of EF0 to EF5.10
  • Condensation Funnel: water droplets that rotate and extend from the base of a thunderstorm. Once the funnel touches the ground it is considered a tornado.
  • Supercells: storms, generally thunderstorms, that rotate on a vertical axis. This rotation is generated when there is a change in wind speed and direction. All tornadoes originate from a supercell, but not all supercells spawn tornadoes.11 
  • Tornado Ally: refers to an area in the United States that regularly see tornadoes. This includes Kansas, Missouri, Oklahoma and parts of Colorado, Iowa, Louisiana, Nebraska, and Texas.  These areas foster tornado development because of the regular fluctuation of wind speeds and direction.12
  • Watch vs Warning: a watch means that you should be prepared because conditions are favorable for tornado development, but a warning means that a tornado has been spotted. This same terminology is also used by the National Hurricane Center to communicate the threat of tropical cyclones. 
  • Waterspout: a tornado that occurs over water. 

Did you know?

According to the NOAA, on average there are 1,000 tornadoes reported in the U.S. each year.13



The United States Geological Survey, or USGS, says that earthquake is a “term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth”.14 Earthquakes cannot be predicted but it is possible to estimate the possibility of a seismic event taking place in a particular location. 


  • Aftershock: smaller earthquakes that follow the mainshock. These can occur anywhere from weeks to years after the first earthquake. The magnitude of the mainshock will determine the number and intensity of the subsequent earthquakes.14
  • Epicenter: the point on the earth’s surface that is directly above where the seismic activity originated.
  • Liquefaction: the term used to describe when over-saturated sediment loses its rigidity and acts like a fluid; a common side effect of an earthquake.14
  • Richter Scale: measures the magnitude of earthquakes. The magnitude is the intensity of the earthquake at the epicenter.14 
  • Seismic retrofitting: the process of marking property more resilient to ground movement caused by earthquakes and earth movement. There are different forms of retrofitting options available based on the construction and foundation of the property. Click here to learn more about how to stay safe during an earthquake. 
  • Tsunami: is a massive wave that is often caused by an earthquake or a volcanic eruption that occurs in the ocean and displaces large amounts of water. As the displaced water moves towards shore and reaches shallow water, the tsunami wave increases in height.15

Did you know? 

Approximately 100 earthquakes a year cause damage but there are an estimated 500,000 detectable earthquakes annually. Southern California experiences about 100,000 earthquakes a year, but most are small and never felt.16 


Miscellaneous Weather Terms    

  • Bomb Cyclone: also known as “bombogenesis” refers to a cyclone that is multilateral (between the tropic and polar latitude regions) that rapidly intensifies over a 24-hour period. These occurs when warm air collides with cooler air.17
  • Derecho: a prolonged, widespread, windstorm associated with rapidly moving storms or thunderstorms.18
  • El Niño / La Niña: the warm (Niño) and cool (Niña) phases of the most influential weather pattern on earth in the tropical Pacific, ENSO (El Niño-Southern Oscillation). El Niño has weaker surface winds that move across the Pacific and ocean temperatures are warmer than normal. La Niña is the opposite, strong winds and cooler ocean temperatures.19

This document is advisory in nature and is offered as a resource to be used together with your professional insurance advisors in maintaining a loss prevention program. It is an overview only and is not intended as a substitute for consultation with your insurance broker, or for legal, engineering or other professional advice.

Chubb is the marketing name used to refer to subsidiaries of Chubb Limited providing insurance and related services. For a list of these subsidiaries, please visit our website at Insurance provided by ACE American Insurance Company and its U.S. based Chubb underwriting company affiliates. All products may not be available in all states. This communication contains product summaries only. Coverage is subject to the language of the policies as actually issued. Surplus lines insurance sold only through licensed surplus lines producers. Chubb, 202 Hall's Mill Road, Whitehouse Station, NJ 08889-1600.

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