Notes about ASOS Precipitation Reports

The Automated Surface Observation System ASOS is the primary automated weather observation system in the country. The network is maintained by the Federal Aviation Administration and National Weather Service. The IEM processes, archives, and makes available data from this network. The purpose of this page is to document some interesting aspects of the precipitation data from this network.

Please feel free to report any doubts or concerns you have with this documentation. This information has been pieced together based on conversations with the NWS and reviewing the ASOS Users Manual.

Regarding Time Zones...

Depending on the report type, observations from this network are reported in either local standard time or UTC (sometimes called GMT) time. The internal clocks on the ASOS sites are not updated for daylight time. This is an important consideration when using its local daily precipitation report found in the Daily Summary Message (DSM). This 24 hour period is between local midnight in standard time. So when the local calendar is in daylight time, the period will represent the time between 1 AM and 1 AM. This 24 hour is not exactly as stated and leads to the next section.

Regarding Exact Timing...

The ASOS reports time in hours and minutes without seconds. This creates some ambiguity when attempting to ascertain exactly when a reported precipitation rate occurred. When the time hits 00 seconds, the ASOS starts processing its memory of recently saved data. This processing ends promptly at 23 seconds after and various displays are updated, products disseminated, and one minute interval data archived.

The one minute precipitation data is thus some nearly 60 second period between when the previous minute's processing got to totaling the precipitation data to this minute's processing. So the one minute total is strictly not from a period between 00 seconds of the previous minute and 00 seconds of the current period. Instead this is approximately the 60 second period between 24 seconds after the previous minute to 23 seconds of the current minute. For example:

For one minute data, the 12:45 UTC observation approximately represents a period between 12:44:24 and 12:45:23 UTC.

For the hourly METAR precipitation, the 12:54 UTC observation represents a period between 11:54:24 and 12:54:23 UTC.

For the daily summary message, the observation represents a period between 11:59:24 PM LST of the previous day to 11:59:23 PM LST of the current day.

For the hourly data in the daily summary message, the observation is a period from (HH-1):59:24 to HH:59:23 LST.

Heavy Precipitation Rates...

The ASOS uses a tipping bucket method to measure precipitation. This means that a single tip of the bucket records 0.01 inch of precipitation with the data logger. Under intense rainfall rates, various physical things happen causing the tipping mechanism to not be able to keep up with the flowing water rate. The ASOS Users Manual notes in section 3.4.2 that a correction is applied to the measured accumulation on a minute by minute basis. The equation shown in the manual is as follows:

C = A(1 + 0.60A)

where C is the reported accumulation and A is the measured accumulation from the tipping bucket.

This means that when the ASOS reports a one minute accumulation of 0.25 inches, the actual measured value was around 0.22 inches. Please note that it is not our intention to claim this correction is wrong, but just to document that it is there. Extremely intense rainfall rates such as these are rare, so the typical adjustment is practically zero.

In Summary

The timing and precipitation totals are not exact, but they are conservative over time when summed. Please let us know of any corrections or clarifications you would like to see made.