--- title: Return Periods description: Learn more about return periods and some common misconceptions to keep in mind --- [Skip to content](https://knowledgebase.earth-scan.com/knowledge/return-periods#main-content) English Show submenu for translations ![EarthScan\_ID\_White\_RGB-01.png\]](https://knowledgebase.earth-scan.com/hs-fs/hubfs/EarthScan_ID_White_RGB-01.png?height=40&name=EarthScan_ID_White_RGB-01.png) - [EarthScan](https://www.mitigasolutions.com/earthscan) - [About](https://www.mitigasolutions.com/about-us) - [Contact](https://www.mitigasolutions.com/contact-us) Open main navigation Close main navigation - [EarthScan](https://www.mitigasolutions.com/earthscan) - [About](https://www.mitigasolutions.com/about-us) - [Contact](https://www.mitigasolutions.com/contact-us) - English Show submenu for translations - Go to EarthScan Go to EarthScan Welcome to the EarthScan Knowledge Base. How can we help you? - There are no suggestions because the search field is empty. 1. [Knowledge Base](https://knowledgebase.earth-scan.com/knowledge?hsLang=en) 2. [Using EarthScan](https://knowledgebase.earth-scan.com/knowledge/using-earthscan?hsLang=en) 3. [Insights](https://knowledgebase.earth-scan.com/knowledge/using-earthscan?hsLang=en#insights) # Return Periods ## Learn more about return periods and some common misconceptions to keep in mind > After reading this article, you will learn: > > - [What return periods are](https://knowledgebase.earth-scan.com/knowledge/return-periods#what) > - [Some common misconceptions about return periods](https://knowledgebase.earth-scan.com/knowledge/return-periods#misconception) ### **What are return periods?** Return periods are a statistical measurement often used in risk analysis. They’re used to convey information about the probability of an event, such as flooding or extreme rainfall, happening in a particular location. You’ll find return periods within your most detailed [EarthScan Insights](https://knowledgebase.earth-scan.com/knowledge/get-started-with-earthscan-insights?hsLang=en) for each individual [Risk Category](https://knowledgebase.earth-scan.com/knowledge/the-earthscan-navigation-panel-risk-categories?hsLang=en), as well as in the [CVaR](https://knowledgebase.earth-scan.com/knowledge/climate-value-at-risk-cvar?hsLang=en) Insight for Wind and Flooding Risk. Return periods, also called recurrence intervals or repeat intervals, estimate the average time between events of a similar intensity occurring. This probability is often given in years e.g. 10-year flood, 50-year flood, and describes the average time between events of a similar magnitude. The *higher* the return period, the *greater* the intensity of the event.   The scientific community describes return periods as a “1 in 10-year event”, or a “1 in 50-year event”. Sometimes, they’re expressed as a percentage. For example, a 50-year flood event can also be described as having a 2% chance of happening in any one year. Turning this around, if there is a 10% chance of a heatwave occurring, then we say it has a 10-year return period. ### **What are some common misconceptions about return periods?** Although they’re widely-used metrics, return periods are easy to misinterpret.  The key thing to remember is thatif an event has a **100-year return period,** it could happen **multiple times,** or **not happen at all** within a given 100 years. A 100-year return period, or a 1 in 100-year event, means there is a 1/100 or 1% chance of an event of that magnitude happening in any given year. For example, a flooding event with a 100-year return period could have occurred 4-5 times between 1920 and 2020. It’s also possible it might not have occurred at all.  Another common misconception is that if a 100-year event has just happened, another similar event won’t happen for another 99 years. However, a 100-year event could conceivably happen two years in a row. For example, if a 10 year extreme rainfall event happened in 2021, it is equally as likely that it could happen in 2022 or 2023. Return periods remain **unchanged** even if an event at a similar level of intensity has occurred **recently** within the timeframe. It’s also important to note that return periods do not account for any interventions or mitigating factors that may influence how much impact a climate hazard could have at a particular location. Examples of interventions and mitigations include: adding flood defences, improving ventilation or upgrading water storage facilities. 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