Annual rainfall in 2015, Source: NASA Earth Observatory


In situ measures of precipitation are typically collected using a rain gauge. The frequency with which gauges are checked can determine what information may be available. At some stations, gauges are checked only once a month, providing information on total rainfall but no information on the intensity, length, or frequency of individual rainfall episodes. Stations that check rain gauges daily or hourly may be able to provide additional information important to end-users, such as the intensity of individual rainfall episodes and the length of time between rainfall episodes. Some precipitation indices of interest may include:

  • Greatest total precipitation over a 1-day period
  • Greatest total precipitation over a 5-day period
  • Average rainfall amount on rainy days (also called Simple Precipitation Daily Index)
  • Number of days when Precipitation exceeds 10mm
  • Number of days when precipitation exceeds 20mm
  • Maximum length of a dry spell
  • Maximum length of a wet spell
  • The precipitation amount when the year is in the 95th percentile
  • The precipitation amount when the year is in the 99th percentile

While station data on precipitation can provide very accurate information for the location from which it was collected, it may not provide much information about even very close locations without weather stations. While temperature fluctuations are relatively homogeneous across space, high precipitation in one location may not indicate an increase in rainfall in neighboring locations. For this reason, interpolating precipitation measures across space can prove challenging. Where there is considerable topographic variation, interpolation may be less reliable over even smaller distances. 

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