Unlike the ASOS & AWOS networks, the RWIS data is not reported at regular intervals. This creates a problem when comparisons are to be made between the networks. Traditionally, the ASOS stations report at about :53 past the hour and this observation is thus reported as being valid for the top of the hour. For example, the Ames ASOS station makes an observation at 4:53 PM. This observation is then considered as the conditions valid at 5:00 PM. Since this time conversion is similar for all ASOS stations, comparisons between ASOS stations is not a problem.

The AWOS network in Iowa reports 3 times per hour. Most of the stations report at :05, :25 and :45 minutes after the hour. Traditionally, these observations are then grouped into three time bins per hour. These bins are :00, :20 and :40 after. On the IEM site, you may notice plots that are valid for :20 or :40 after. The obs in the :00 bin are then compared with the ASOS obs at :00, eventhough the true observational times are seperated by ~12 minutes.

Sample Time	Ob Valid	Time Bin
:05 after	:01 after	:00
:20 after	:08 after	:00
:35 after	:31 after	:40
:50 after	:38 after	:40

The RWIS network is sampled every 7 minutes, by the DOT. The results of this sampling are saved on the IEM every 15 minutes. The hopes are to catch enough observations to fill the 3 time periods per hour. Sometimes this does not work. Consider this situation represented by the table to the right. The network was sampled 4 times that hour and the station generated 4 unique reports, but none of the reports would have been classified valid for :20 after. This creates a missing value for comparison with AWOS stations.

The original RWIS observations are saved on the IEM server with their none regular timestamps. It is our hope to generate better algorithms to handle these time difficulties.

Return to RWIS page.