1. The previous value in a time series. Thus, if x(t) denotes the present value, the value of persistence would be x(t − 1), whence the latter value is regarded as “persisting. ” It is used as an objective standard in the verification of weather forecasts. Sometimes it is extended to mean x(t − h) where h is arbitrary. 2. (Also called constancy, steadiness. ) With respect to the long-term nature of the wind at a given location, the ratio of the magnitude of the main wind vector to the average speed of the wind without regard to direction. 3. (Also called inertial forecast. ) In general, the tendency for the occurrence of a specific event to be more probable, at a given time, if that same event has occurred in the immediately preceding time period.