Weather Tidbits: What Does a “Chance of Rain” Mean?

This edition of Weather Tidbits will be discussing the meaning behind a chance of rain. For example, you may have heard a meteorologist mention a day perhaps having a “40% chance of rain”. Where does that percentage come from? The percentage is derived from the probability of precipitation (POP) equation. The equation is equal to the percent confidence that an area will receive rain (C) multiplied by the percent of a viewing area that will receive rain (A). This is written as POP=C x A. The equation does not consider the duration of rain and time of day. For example, if a meteorologist is 80% confident that 60% of a viewing area will receive rain, the day will feature around a 50% chance of rain [{POP=C x A} → {(0.8 x 0.6)100=48%, round to 50%}].

In the short term, like within the span of a couple of days, confidence (C) in rainfall is generally higher. So the POP is more representative of just aerial coverage (A), based on the equation. In the long term, like within the span of 5–7 days, confidence in rainfall is generally lower. So the POP is less representative of just aerial coverage. Lastly, aerial coverage is lower for localized thunderstorms and higher for widespread rainfall.