Interesting arithmetic in the climate summary for Seattle from May 8...

How do they arrive at a +1 degree departure from normal? What is the
rounding method used that makes (61+48)/2 = 55 while (63+46)/2 = 54?
...or (-2) + 2 = 1?

Or is it just 'Intel math'...?

CDUS46 KSEW 090844 CLIMATE REPORT
NATIONAL WEATHER SERVICE SEATTLE WASHINGTON
131 AM PDT MON MAY 09 2005
...........................................
....THE SEA-TAC AIRPORT CLIMATE SUMMARY FOR 08 MAY 2005...
CLIMATE NORMAL PERIOD 1971 TO 2000 CLIMATE RECORD PERIOD 1945 TO 2004
WEATHER ITEM OBSERVED TIME RECORD YEAR NORMAL DEPARTURE LAST
VALUE (LST) VALUE VALUE FROM YEAR
NORMAL
.................................................. .................
TEMPERATURE (F)
YESTERDAY
MAXIMUM 61 339 PM 84 1987 63 -2 63
MINIMUM 48 358 AM 36 1999 46 2 47
AVERAGE 55 54 1 55

There are 24 hourly readings averaged, not just a daily maximum and
minimum.

There are 24 hourly readings averaged, not just a daily maximum and
minimum.

I'd be surprised if this were the case, unless the US has changed its
method of calculating daily mean temperature - they've always used
(max+min)/2 as far as I know. (Many other parts of the world use
different methods - won't go into the details now but can post
separately if anyone is interested).

My guess is that the normals are calculated by doing some sort of
smoothing/curve-fitting on either the raw daily means or the monthly
means, and that this process is done separately for the maximum,
minimum and mean - which will result in the occasional small
inconsistency between them.

Blair Trewin
National Climate Centre
Australian Bureau of Meteorology