The Importance of Observation Time

This weekend there was a long discussion on the CoCoRaHS group page on Facebook (not the HQ page) regarding observation time. After reading through this discussion it was clear that many observers may not realize why things are done the way they are, so here is an attempt to try and clarify things a bit.

We ask observers to measure their precipitation in the morning, typically 7:00 a.m. There are a number of reasons for this, but the primary one is so that CoCoRaHS observations are consistent with other precipitation observations made in the U.S. Starting in the early 1960s the (then) U.S. Weather Bureau requested that U.S. Cooperative Observers start taking their measurements in the morning as that would minimize the amount of evaporation from rain gauges and result in more accurate precipitation measurements. Prior to this both temperature and precipitation measurements were taken in the late afternoon, typically 5-7 p.m.. The current instructions for U.S. Cooperative Observers states:

"Observations at precipitation stations should be taken at 7 a.m. local time, although you may usually choose any time between 6 a.m. and 8 a.m. Be sure, however, to take observations at the same time everyday throughout the year if at all possible. Continue observing at the same time whether standard or daylight saving time is in effect; i.e., convert from 7 a.m. standard time to 7 a.m. daylight saving time when the latter takes effect." 

CoCoRaHS follows this same guideline. However, because the morning is not the best for everyone who wants to participate, we allow observers to choose an observation time that is more convenient.
Many National Weather Service precipitation products are based on these observations. A map showing 24-hour precipitation amounts, such as this one from the Advanced Hydrologic Prediction Service (AHPS), uses a combination observed precipitation from Coop, CoCoRaHS, first-order stations (any meteorological station that is staffed in whole or in part by National Weather Service , FAA, or civil service personnel), and radar to map the precipitation. You can read a more thorough description here.

Some argue that a calendar day (midnight-to midnight) total is more representative of daily precipitation. That may be true, but precipitation measurement is still by and large a manual process performed by humans, many of whom don't want to or can't wait until midnight to make an observation. The 7:00 a.m. time also closely aligns with 12:00 UTC, one of the standard synoptic hours for weather measurement. These synoptic hours (in Universal Coordinated Time, or UTC), based on international agreement through the World Meteorological Organization (WMO), are hours which meteorological observations are made simultaneously throughout the world at three or six-hourly intervals. The primary synoptic hours are every six hours, commencing at 00:00 UTC.  12:00 UTC also marks the end of the "hydrologic day", a standard used by hydrologic modelers and the River Forecast Centers.

You may hear of other networks making weather measurements at one-minute or five-minute intervals. These are automated stations, such as those located at airports or the Climate Reference Network. However, for the purposes of climate, data from these stations can be aggregated and summarized on a daily basis or longer.

The CoCoRaHS Precipitation Map

The CoCoRaHS precipitation map displays all reports of precipitation with observation times within 2.5 hours of 7:00 a.m., i.e. 4:30 a.m. to 9:30 a.m. local time. Any observations made outside of this window will not appear on the daily map, but remain in the database for users. This data is still needed and valuable for time periods longer than a day, such as weekly, monthly and longer precipitation summaries. Our map represents 24-hour precipitation amounts, in line with the synoptic time, the end of the hydrologic day, and represents most of the CoCoRaHS observations made that day. For example, on Friday, June 5, out of nearly 13,000 CoCoRaHS observations only 340, about 2.5 percent, were made outside of the 4:30 a.m. - 9:30 a.m. window.

The take-way from all of this is pick an observation time that is convenient to your schedule. If 9:00 a.m. or 5:00 a.m. or 4:00 p.m. work better for you - no problem. The important thing is to be consistent. Don't switch observation times from day to day (for example,  from 7:00 a.m. on one day to 11:00 a.m. the next to 8:00 a.m. on the next). If on one day you take your observation time earlier or later than usual, that's OK - be sure to enter that time in the Observation Time Field.

Here is one more thing to remember. It's not about when you enter your data (the map itself updates all the time to reflect late entries), rather it's the time you actually look at your gauge - and what is entered on the form - that is important. Time of observation is the time you make your measurement, not the time you submit your observation to CoCoRaHS. If you make your observation at 7:00 a.m. but aren't able enter it until 11:00 a.m., it is still a 7:00 a.m. observation.

If you have been taking your observation at one time, say 6:00 a.m., and you want to change it to 8:00 a.m., contact your state coordinator or headquarters to make that change. You can't change your default ob time using the My Account menu.

If you have any questions about observation time, let us know!