Some NOAA websites are beginning to display ASOS (i.e., airport weather) observations every 5-minutes. An example is http://www.wrh.noaa.gov/mtr/sfcobs2.php. While at first blush this seems like an awesome way to see more timely data, it can cause confusion when comparing it to daily maximum and minimum readings.
Example of 5-minute readings:
The 5-minute readings are actually an "instantaneous" 1-minute observation at the particular time noted. The problem arises in that the maxima and minima are a "5-minute average" of five adjacent individual 1-minute readings. Consequently, that average could be less than one of the 5-minute readings.
For example, at the hottest time of the day you have an instantaneous 5-minute reading of 85 degrees at 14:30. However, if the readings at 14:27, 14:28, 14:29 and 14:31 were all 84 degrees, then the maximum reading would be 84.2, which would be rounded to 84.
Correction: In rereading the ASOS criteria I think I misinterpreted how’s it’s calculated (i.e., the 5-min values are the average of
the previous five 1-min values) and that rounding due to C to F conversions may
be more of a factor. But overall I am right for the wrong reasons and for
the real world the confusion remains with seeing different 5-min values than the climatological max/min temps.
Jan Null, CCM
Golden Gate Weather Services