Felly sgrifennodd
Norman Lynagh :
I'm a bit surprised at the minimum temperatures forecast on BBC1 at 6.30
p.m. this evening. Single figure mins were predicted for Scotland but
elsewhere double figure mins were predicted. I haven't looked into it in
much detail but for this part of the country the McKenzie method
predicts a min of around 7-8C, assuming light winds and only small
amounts of cloud. This would be quite a bit colder than the TV forecast
suggested.

Can someone please explain the McKenzie method? I can't find it in the FAQ.

Adrian

--
Adrian Shaw ais@
Adran Cyfrifiadureg, Prifysgol Cymru, aber.
Aberystwyth, Ceredigion, Cymru ac.
http://users.aber.ac.uk/ais uk

"Adrian D. Shaw" wrote in message
...

Can someone please explain the McKenzie method? I can't find it in the
FAQ.


.... it's on my 'to do' list, which is growing!

In essence, you take the day maximum (Tx) and the air-mass dew point
(Td), the latter is assumed not to change appreciably overnight.

Then T(min) = 1/2 (Tx + Td) - K

where K varies according to the expected surface wind speed and average
cloud overnight.

McKenzie originally did his work for Dyce (Aberdeen) and it was
presented (though not published) in 1944 in an internal memorandum
within the Meteorological Office. Since that time, constants for many
Met Office site have been calculated (Claude Kensett), but as a rough
guide:-

Dead calm/clear throughout night .. K =~ 8 or 9 (an extreme case)

Light wind/small amounts cloud .. K =~ 6 to 8
Light wind/cloud coming and going .. K =~ 4 or 5
Light wind/generally cloudy .. K =~ 2 or 3

Moderate wind/small amounts cloud .. K =~ 3 to 5
Moderate wind/cloud coming and going .. K =~ 3 or 4
Moderate wind/generally cloudy .. K =~1 or 2

Brisk winds (most cloud classes) .. K =~ 1 or 2

there are other corrections you can apply given the time of year etc.

You can see the beauty of the method, as it is easy to apply .. add
together a couple of commonly known variables, divide by 2, and then
fudge the 'correction' K.

Martin.

On Wednesday evening (24th) the BBC forecasted a minimum of 8C for
Aberdeen. However given that late afternoon dew points had been around
2-3C I was expecting it to be much colder.

I was proved right. I recorded a minimum of 5.5C while Dyce, a couple
of miles to the North recorded 3.3C. Further inland it got down to 1C
at Aboyne.

Last nights forecast was not as bad: 8C compared with an actual minimum
of 7.2C.

Richard Slessor,
Aberdeen.

"Norman Lynagh" wrote in message
...
I'm a bit surprised at the minimum temperatures forecast on BBC1 at 6.30
p.m. this evening. Single figure mins were predicted for Scotland but
elsewhere double figure mins were predicted. I haven't looked into it in
much detail but for this part of the country the McKenzie method
predicts a min of around 7-8C, assuming light winds and only small
amounts of cloud. This would be quite a bit colder than the TV forecast
suggested.

I have noticed for some time now that the forecast minimum temperatures
seem to err on the high side on radiation nights. I'm not certain but I
think that this apparent systematic error started at the time of the
introduction of the new graphics. It may be a presentation problem
rather than a forecasting problem.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England

You're all very lucky you can use McKenzie!

Try forecasting the min. at Haytor! McKenzie doesn't work as we rarely get
radiation nights. There are always patches of cloud to surprise you, always that
subtle increase in wind speed either due to the low-level jet or the effect of
the hills, always that subtle change in wind direction going from a land
direction to a sea direction causing the temperature to rise and vice-versa, and
of course in winter that surprise snowfall as the wet-bulb freezing level falls
in precipitation and the temperature plummets rapidly.

I ignore all TV and radio forecast mins as they will never apply to my locality
(nor should they, as hardly anyone lives here). Instead I use my judgement as
best I can using available data and also climatology. Wind direction seems to be
an important variable. With a wind speed 5 knots or more from a close sea
direction in November and December you can forget a frost at Haytor, mins will
normally then be in the range 0-2 degrees C on a clear night.

So in my area forecast mins are sometimes too high and sometimes too low and
occasionally right,
Oh the joys of weather forecasting :-)

Will.
--

" Visit Dartmoor National Park "
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You're all very lucky you can use McKenzie!

Try forecasting the min. at Haytor! McKenzie doesn't work as we rarely get
radiation nights. There are always patches of cloud to surprise you,
always that
subtle increase in wind speed either due to the low-level jet or the
effect of
the hills, always that subtle change in wind direction going from a land
direction to a sea direction causing the temperature to rise and
vice-versa, and
of course in winter that surprise snowfall as the wet-bulb freezing level
falls
in precipitation and the temperature plummets rapidly.

I ignore all TV and radio forecast mins as they will never apply to my
locality
(nor should they, as hardly anyone lives here). Instead I use my judgement
as
best I can using available data and also climatology. Wind direction seems
to be
an important variable. With a wind speed 5 knots or more from a close sea
direction in November and December you can forget a frost at Haytor, mins
will
normally then be in the range 0-2 degrees C on a clear night.

So in my area forecast mins are sometimes too high and sometimes too low
and
occasionally right,
Oh the joys of weather forecasting :-)

Will.

Hi, Will,

This post made me smile a little also. To forecast a min within 3C on
radiation nights is almost inpossible in hill country. In Copley alone at my
sites barely 400 metres apart horizontally and 35 metres vertically the
difference can be up to 4C. The Met O site rarely gets calm nights, as in
Haytor, but they are quite common at the Lead Mill, especially in winter.

Best wishes,

--
Ken Cook
Copley (253m, 830ft) & Copley Lead Mill (218m, 715ft)
5 miles north of Barnard Castle, County Durham.
http://mysite.wanadoo-members.co.uk/copley