Precipitation in weather refers to water that falls from the sky, such as rain, drizzle, snow, sleet, hail, or something rarer. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and falls due to gravitational pull. The amount of precipitation is usually measured in millimeters or inches, and it is important to know which type of precipitation the weather is going to bring as it can tell you about other things going on in the atmosphere, as well as what you might need to do, like taking an umbrella or getting ready to build a snowman.

The probability of precipitation is a statistical probability of 0.01 inches or more of precipitation at a given area in the given forecast area in the time period specified. It is usually expressed as a percentage, and it describes the likelihood of measurable precipitation occurring at a location during a specific timeframe. For example, a "40% chance of rain" means that there is a 40% probability for at least 0.01 inches of rain at the specific forecast point of interest. It does not mean that 40% of the area will be covered by precipitation at a given time in the given forecast area or that you will be seeing precipitation 40% of the time in the given forecast area for the given forecast time period.