Quick primer on the 5-min obs from NWS/MesoWest

[MN Transplant]

There has been a lot of confusion over the 5 minute observations from our main ASOS sites.  I have absolutely no inside information on any of this, but I'm fairly certain I've figured out what is happening.

Here is the link to DCA's obs:  https://www.wrh.noaa.gov/mesowest/getobext.php?wfo=lwx&sid=KDCA&num=72&raw=0#

That'll take you to the METARs, and below is how you get to the 5-min obs (you have to open a dropdown box on mobile).

 

However, you have to be very careful with the 5 minute obs.  What appears to happen is that the temperatures are initially communicated in rounded F degrees.  They are then converted to C, but (and this is huge), the C degrees are rounded.  Then they are converted back to F and rounded again!  So, with the rounding, the temperature in F can be wrong by up to 1 degree.  Here is a representation of the calculations and the reason why we don't see any 98s except for on the hourly obs.

So, moral of the story for today is that a 99 could be a 99 or a 98.  But 100 should be 100.