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

Just saying...
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)

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!
Just saying...


You can say that again.

I really hate it when the temperature soars between 8 and 9 am. Quite
often in winter the maximum is 8.59 a.m. December 21st here was very
cold - a maximum of -2.4C during the day, dropping to -6.8C around 2
am, then climbing rapidly to -1.4C at 8.59, thus spoiling my record
lowest maximum. And there have been several occasions where an
otherwise "frost day" has been ruined by rising temperatures around
dawn (which is 8.45 a.m. here).

Trevor
NW Bonnie Dundee
http://www.personal.dundee.ac.uk/~taharley/

On Fri, 18 Jan 2008 19:39:51 +0000, Trevor Harley wrote in
news:2008011819395116807-taharley@dundeeacuk

I really hate it when the temperature soars between 8 and 9 am. Quite
often in winter the maximum is 8.59 a.m. December 21st here was very
cold - a maximum of -2.4C during the day, dropping to -6.8C around 2
am, then climbing rapidly to -1.4C at 8.59, thus spoiling my record
lowest maximum. And there have been several occasions where an
otherwise "frost day" has been ruined by rising temperatures around
dawn (which is 8.45 a.m. here).

The most notable occasion that happened here was in the early 80s. The
daytime max was about -11C but by 0900 next day the temp was up to +3C.

--
Mike Tullett - Coleraine 55.13°N 6.69°W posted 18/01/2008 20:36:23 GMT

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, 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.

And these figures are for 09-21/21-09h. Choosing 06-18/18-06 shows
even larger effects, mainly because the peak frequency of winter
minima is between 0600 and 0900 UTC. Arguably the method with least
possible objection is 00-00h, but this still leads to a high frequency
of max or min at 0000 or 2359 (and unless you have an AWS it's a mite
unpopular ... )

It reinforces how important it is, when undertaking serious
climatology, to (a) agree a standard for terminal hours (i.e. 09-09h
or close to it in UK and Ireland) and (b) stick to it across the
network and for long-period sites. Changing terminal hours will dwarf
many other genuine climatological variations.


Stephen Burt
Stratfield Mortimer, Berkshire

On 18 Jan, 20:36, Mike Tullett
wrote:
On Fri, 18 Jan 2008 19:39:51 +0000, Trevor Harley wrote in
news:2008011819395116807-taharley@dundeeacuk

I really hate it when the temperature soars between 8 and 9 am. Quite
often in winter the maximum is 8.59 a.m. December 21st here was very
cold - a maximum of -2.4C during the day, dropping to -6.8C around 2
am, then climbing rapidly to -1.4C at 8.59, thus spoiling my record
lowest maximum. And there have been several occasions where an
otherwise "frost day" has been ruined by rising temperatures around
dawn (which is 8.45 a.m. here).

The most notable occasion that happened here was in the early 80s. *The
daytime max was about -11C but by 0900 next day the temp was up to +3C.

--
Mike Tullett - Coleraine 55.13°N 6.69°W *posted 18/01/2008 20:36:23 *GMT

I can remember in the cold spell at the end of January 1972, the
morning of 31 Jan was bitterly cold (my min that morning, in
Warwickshire, was -12.2°C, around 8.30 am). At around a third of
climatological sites in the Midlands, the lowest temperature occurred
a little after the (nominal) 0900 UTC observation, so the minimum
thermometer reading was, following standard practice, credited to the
following morning - i.e. 1 February. However, milder air spread in
quickly during the day on 31 January, and February was a mostly mild
month, resulting in most places seeing their official lowest February
(and annual) minimum occurring ... erm, in January.

I'd say it's about once every 2-3 years here when the nominal 24 hour
09-09h max and min actually occur exactly 48 hours apart. How can this
happen?

Day 1 - temp at 0900 0°C
Day 2 - temp rising throughout, temp 5°C at 0900
Day 3 - temp rising throughout, now 10°C at 0900

For Day 2, the 09-09h min will be 0°C (temp at 0900 Day 1) while the
max will be 10°C (temp at 0900 Day 3). The 24 hour extremes are
actually exactly 48 hours apart.

Of course, if the temp is falling throughout then min = max = 0.0
degC. But that's another quirk of the system.


Stephen Burt
Stratfield Mortimer, Berkshire

wrote:

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, 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.

And these figures are for 09-21/21-09h. Choosing 06-18/18-06 shows
even larger effects, mainly because the peak frequency of winter
minima is between 0600 and 0900 UTC. Arguably the method with least
possible objection is 00-00h, but this still leads to a high frequency
of max or min at 0000 or 2359 (and unless you have an AWS it's a mite
unpopular ... )

It reinforces how important it is, when undertaking serious
climatology, to (a) agree a standard for terminal hours (i.e. 09-09h
or close to it in UK and Ireland) and (b) stick to it across the
network and for long-period sites. Changing terminal hours will dwarf
many other genuine climatological variations.


Stephen Burt
Stratfield Mortimer, Berkshire

There are also anomalies in rainfall statistics. What is recorded as
the wettest "day" in a month will not be the wettest 24-hour period in
the month if the wettest 24-hour period straddles 0900z, or whatever
cut-off time is used for rainfall measurements. I have no idea how
often this happens but I suspect it is not particularly unusual. Also,
when frontal rain is moving across the country the rainfall event at
some locations may occur during a single rainfall day while at other it
may be split between 2 rainfall days.

No matter what system for measuring is used there will always be
anomalies but I agree, Stephen, that for serious work it is essential
to have a standard and stick to it.

Norman
--
Norman Lynagh
Chalfont St Giles, Buckinghamshire
85m a.s.l.
(remove "thisbit" twice to e-mail)

All this interesting stuff of course arises from the fact that we are trying to
discretize a continuous variable.

Will
--

wrote in message
...
On 18 Jan, 20:36, Mike Tullett
wrote:
On Fri, 18 Jan 2008 19:39:51 +0000, Trevor Harley wrote in
news:2008011819395116807-taharley@dundeeacuk

I really hate it when the temperature soars between 8 and 9 am. Quite
often in winter the maximum is 8.59 a.m. December 21st here was very
cold - a maximum of -2.4C during the day, dropping to -6.8C around 2
am, then climbing rapidly to -1.4C at 8.59, thus spoiling my record
lowest maximum. And there have been several occasions where an
otherwise "frost day" has been ruined by rising temperatures around
dawn (which is 8.45 a.m. here).

The most notable occasion that happened here was in the early 80s. The
daytime max was about -11C but by 0900 next day the temp was up to +3C.

--
Mike Tullett - Coleraine 55.13°N 6.69°W posted 18/01/2008 20:36:23 GMT

I can remember in the cold spell at the end of January 1972, the
morning of 31 Jan was bitterly cold (my min that morning, in
Warwickshire, was -12.2°C, around 8.30 am). At around a third of
climatological sites in the Midlands, the lowest temperature occurred
a little after the (nominal) 0900 UTC observation, so the minimum
thermometer reading was, following standard practice, credited to the
following morning - i.e. 1 February. However, milder air spread in
quickly during the day on 31 January, and February was a mostly mild
month, resulting in most places seeing their official lowest February
(and annual) minimum occurring ... erm, in January.

I'd say it's about once every 2-3 years here when the nominal 24 hour
09-09h max and min actually occur exactly 48 hours apart. How can this
happen?

Day 1 - temp at 0900 0°C
Day 2 - temp rising throughout, temp 5°C at 0900
Day 3 - temp rising throughout, now 10°C at 0900

For Day 2, the 09-09h min will be 0°C (temp at 0900 Day 1) while the
max will be 10°C (temp at 0900 Day 3). The 24 hour extremes are
actually exactly 48 hours apart.

Of course, if the temp is falling throughout then min = max = 0.0
degC. But that's another quirk of the system.


Stephen Burt
Stratfield Mortimer, Berkshire

For many years figures from Met Office sites were based on 0900-2100 max and
2100-0900 min (these are still calculated from NCM reports) precisely to
avoid singleton cold nights counting twice. Average differences between
monthly averages worked out from 09-21/21-09 stations and 0909 stations were
as much as 0.7C for December minima, though depending on the character of
the month the figure could be even higher.

The standardisation on 0909 was presumably to make it easy for once a day
stations, though there is no ideal time of day for a reset that would be
valid all year.

I think it is a great limitation of most (if not all) AWS weather software
interfaces that you are stuck with a non-standardised 00-24 daily cycle,
though perhaps that means it is the new standard....

Stuart

Norman wrote:

There are also anomalies in rainfall statistics.

Does the same problem apply when counting, say, "number of air frosts"?
The temperature could be below freezing at 0859Z through 0901Z only but
would this technically count as two days with air frost?

Jonathan

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/
Reverse my e-mail address to reply by e-mail

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)