Weather Banter - View Single Post - Measuring max/min temperatures in winter...

Paul Hyett · January 19th 08, 09:09 AM posted to uk.sci.weather

On Fri, 18 Jan 2008 at 12:51:11, wrote in
uk.sci.weather :

On 18 Jan, 19:39, Trevor Harley wrote:
On 2008-01-18 18:12:59 +0000, Paul Hyett said:

... can be a total PITA, as you can never tell what time of day or
night they will occur!

It happens much more frequently than you might think

Yes, it's been more obvious since I got a datalogger.

What is the official way of recording max & mins anyway? Should they
both be read at 9am GMT, and the max ascribed to the previous day, with
the min to the current day?

, and has a
significant effect on your mean max and min. Looking at my records
just now over 20 years (1988-2007) -

- a higher night max (i.e. max 09-09h 09-21h) occurred on average 35
days per year (and an average of 8 days in December, a little over 5
in both November and January, so over the Nov-Dec-Jan period an
average somewhat more than once per week) - but only 6 days in 20
years in July

- a higher night min (i.e. min 09-09h 21-09h) occurred on average 58
days per year (and an average of 11 days in December, 10 in January
and 9 in November, so over the Nov-Dec-Jan period on average every 3
days) - but only 5 days in 20 years in July

- Difference between 09-21h/09-09h average max is 0.10 degC over the
year as a whole, 0.37 degC in December;

- Difference between 21-09h/09-09h average min is 0.28 degC over the
year as a whole, 0.81 degC in December.

The difference in the means is significant (0.20 degC or more, i.e.
the difference you'd expect between normal calibrated instruments) in
mean maximum for three months of the year but for mean min *six*
months of the year. It dwarfs decadal variations from e.g. climate
change and for minimum temperatures is comparable with urban heat
island effects in a large conurbation.

Interesting - I'd never the effect quantified before.
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)