High humidity and dew-point

[Tudor Hughes]

Using a whirling psychrometer I measured a dew-point of over 20°C at about 1500Z today.  I thought there must be some mistake so I changed the wick and repeated the measurement - same answer.  It is very sweaty.

FWIW I reckon the whirling psychrometer, properly used, is just about the best humidity measure available unless you want to pay an arm and a leg.

Large amounts of Cu con and even Cb (just about) with a high base (about 4000 ft).  At 1415Z there was a very brief shower of large well-spaced drops, all the time with sunshine.

Tudor Hughes, Warlingham, NE Surrey, 557 ft, 170 m

[Brian Wakem]

FWIW my Davis VP2 was reporting a dew point of 19.5C at 1500Z.  In fact it peaked earlier in the day at 22.7C @ 0934Z.

-- 

Brian Wakem

Lower Bourne, Farnham, Surrey

http://www.brianwakem.co.uk/weather

Live obs @ 17:55:24 : 27.2C, DP 18.8C, RH 60%, 0.0 mm

[Graham Easterling]

It's an exceptionally warm & sticky evening down here in west Cornwall, even right next to the sea, where anything resembling a 'cooling sea breeze' seems to be facing extinction.

At 19:00

Camborne 26C with a dewpoint of 21C!  Even Lands End 25C & 19C respectively.

SST currently 20.6C at the Penzance buoy.

Graham

Penzance

On Monday, 19 July 2021 at 17:41:24 UTC+1 Tudor Hughes wrote:

[Len]

I agree Tudor. The whirling psychrometer was the go to instrument for a spot measurement of humidity when I was at Imperial College Met Dept. in the 1970s.

You need an aspirated instrument to get reliable measurements. Something AWSs do not usually have.

Temp here now a pleasant 22C after a max of 27.6C today.

Amber warning out here from Met Office for extreme temperatures.

But not for SE. Obviously you are used to 28 or 29C with little impact. LOL.

Len

Wembury

On Monday, 19 July 2021 at 17:41:24 UTC+1 Tudor Hughes wrote:

[Keith (Southend)]

I also peaked at 21.2°C DP this afternoon in SOuthend-on-Sea.

Keith (Southend)

[Tudor Hughes]

In a meteorological utopia all screen thermometers would be aspirated.  I have often noticed, on taking the psychrometer  out of the screen for a spin in the shade of a large bush, how much the screen temperature exceeds the dry-bulb of the psychrometer.  On a calm sunny day this effect is apparent even in winter.  I think the effect is well known but the authorities prefer to keep quiet about it because the cost and difficulty of equipping all screens with fans would be prohibitive..  It doesn't really matter - consistency of measurement over long periods is what counts even if the figures are "wrong".

       As for Tuesday's temperatures I think they will be just a little lower than Monday's here in the SE but higher further west.  Our case-hardening will not be put to the test.  The humidity will be more trying.  It's difficult at first sight to understand how the dewpoint can be higher than the surrounding sea surface temperature.  Possibly a very shallow evapotranspiration effect?

Tudor Hughes, Warlingham, NE Surrey, 557 ft, 170 m.

[Brian Wakem]

On Monday, 19 July 2021 at 20:53:02 UTC+1 Len wrote:

You need an aspirated instrument to get reliable measurements. Something AWSs do not usually have.

My VP2 is fan aspirated.  The fan has broken a couple of times over the 8yrs I've had it and it was very obvious something was wrong both times as the temperature gets silly very quickly in full sun.  I've long been of the opinion that Stevenson screens are not particularly accurate and a fan aspirated, quality AWS like a VP2 is probably quite a bit more accurate, especially on calm sunny days.  I'll never be able to compete with the likes of Heathrow, Northolt, Wisley etc for max temps despite being in the SE.

-- 

Brian Wakem

Lower Bourne, Farnham, Surrey

http://www.brianwakem.co.uk/weather

Live obs @ 08:14:10 : 19.8C, DP 18.3C, RH 91%, 0.0 mm

[Graham Easterling]

It doesn't really matter - consistency of measurement over long periods is what counts even if the figures are "wrong".

 I think that is the real issue when instrumentation is changed, even if it's just from a wooden to a plastic screen, other changes have major consequences. 

Then there are all these adjustments to compare new averages with old. Long parallel runs would be a solution. but is a very rare event.

I've managed to keep the seem instrumentation for 30 years, well I'm on the 2nd screen and at least the 3rd sheathed minimum thermometer. 

Graham

Penzance (too hot)

[Nick Gardner]

Recorded my highest ever temperature today with 31.4°C, narrowly beating last year's previous high of 31.3°C on the 11th August. Dew points at the time were around 21°C to 22°C, peaking at 22.9°C. With mostly calm conditions and a sea breeze that tried and failed several times, it made for a rather warm and humid afternoon. Quite amazing given the sea is just a tad under two miles away.

[Julian Mayes]

28 or 29C in the SE, Len?   What luxury that would be!   My max  31.4C.       As usual, most London sites only fractionally behind Heathrow, as Colin's data shows incl some with rural environments. This was despite storms breaking out in places late pm.  

Two thoughts though - not enough attention paid to wind speed, or lack of in forecasts.  No breeze to blow hot air out of houses, espec upstairs?  OK here tonight, outflow boundary breezes keep blowing the curtains everywhere - great.  Second thought, if I can manage it in this heat - yes, Met Office must assume greater tolerance to heat here. I think they underestimated our max temp today (I did see a 28C max on one of their twitter maps).  More interesting from tomorrow when the heat max moves more fully westwards.  It can stay there :)  

Julian  

On Monday, July 19, 2021 at 8:53:02 PM UTC+1 Len wrote:

[Len W]

28.3C max here today in my screen. This reached at 1200 UTC. Extremely weak sea breeze stopped it rising futher.

Strong subsidence over land and sea seems to be hindering the setting up of a decent pressure gradient.

Holiday makers are enjoying the beaches nonetheless. Back to normal on Friday though. Not quite like Med thankfully.

Len

[Graham Easterling]

I always find the wording in the warnings rather amusing.

" More people are likely to visit coastal areas" well, who'd have thought it! 

It is strange that the tolerance to heat varies in different parts of the Country. So if I moved from Cornwall to London (as I have in the past) my heat tolerance would suddenly increase.

Graham

Penzance

[Freddie]

On Wednesday, 21 July 2021 at 09:16:55 UTC+1 Graham Easterling wrote:

It is strange that the tolerance to heat varies in different parts of the Country. So if I moved from Cornwall to London (as I have in the past) my heat tolerance would suddenly increase.

It's more a case of difference from average for an area.  So 10 above average for Cornwall is a different number for 10 above average for London.  It's not going to work immediately if you move as you illustrated, but will do over time.

FWIW the heat warnings have been requested by PWS/PHE, so isn't some 'idea' that the Met Office has suddenly come up with :)

-- 

Freddie

Alcaston

Shropshire

148m AMSL

http://www.hosiene.co.uk/weather/

Stats for the month so far: https://www.hosiene.co.uk/weather/statistics/latest.xlsx