What Is a Storm? Recurrence Intervals and Why “100-Year” Doesn’t Mean What You Think
Plain-English definitions of storms, how engineers use recurrence intervals (1-, 10-, 100-year), and why probabilities matter for drainage design and flood risk.
Storms are typically categorized based on factors such as wind velocity, rain intensity, duration, and the extent of the affected area. Agencies like the National Oceanic and Atmospheric Administration (NOAA) employ additional parameters, including barometric pressure, to gauge the potential severity of storms. Utilizing these storm characteristics, federal, state, and local agencies assess the potential for property damage, urging residents in impacted areas to be prepared, prioritize safety measures, and anticipate challenges such as flooding and high winds. The NOAA National Hurricane Center employs the Saffir-Simpson Hurricane Wind Scale to classify the severity of winds and storm surge flooding generated by tropical storms and hurricanes.
In addition to these criteria, storms are classified in terms of recurrence intervals, representing the anticipated time between occurrences of a storm of a particular severity. The probability or likelihood of a storm of a specific magnitude happening in any given year is inversely related to the recurrence interval. A lower probability or chance of occurrence indicates a more severe storm. This comprehensive classification system aids in effectively communicating the potential impacts of storms and informs communities about the urgency of preparedness and safety measures.
Engineers employ the following probabilities, along with their corresponding recurrence intervals, for diverse applications. This includes the design of urban stormwater systems and the assessment of whether existing systems possess sufficient capacity to accommodate expected rainfall runoff volumes:
- 100-percent chance of occurring in a year (1-year storm)
- 20-percent chance of occurring in a year (5-year storm)
- 10-percent chance of occurring in a year (10-year storm)
- 1-percent chance of occurring in a year (100-year storm)
- 0.2-percent chance of occurring in a year (500-year storm)
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