Not really good for sleeping at all as temps at that range during the night are more often than not the result of high humidity, which is the big snooze killer. Good for plants though. 

It's very easy actually to sleep.. 🙂 The actual average low for here in July & August is 15C but that's the 1976-2005 average, and with the warming trend since i think its probably around half a degree or so warmer now. Humidity can be a bitch after a very warm day, especially if there is no breeze on the night and temperatures don't drop much below 18C.

Humidity value ranged between 97 and 100% here for a few nights (with similar value during the day) last week with high Dps. Sickly beyond belief and starts to grate after a while. 

ECM has 20c 850s in the south on Friday , if sunny could be pushing mid 30s.

I think Louise Lear is a hot weather fan. She was positively salivating at lunchtime over the prospects of 30c being reached on Friday. Personally I can't stand the thought of it. Not unless I was lucky enough to have AC unit indoors, which I don't have. I reckon Louise spends her days off cooking herself to a frazzle on a sun lounger in her back garden. 

Same here because of the rain but humidity is relative..  85-95% humidity at 14C will feel rather cold at this time of year. I'm guessing from your lack of tolerability to mild nights that your idea of high Dps aren't especially high 🙄 

For example outside here now it's a rather cool overcast evening 18C with 83% R/H & DP of 15C. 

This weekend coming looks very good with some very humid day's, but I'm unsure what the dp's will be with this upcoming plume. 

The 'feel' of temperature is also relative to the water content of the air mass. EG: 16/17c will feel quite cool & fresh with good breeze and 30-50% Rh, but insufferably uncomfortable with a RH value of 98-100% humidity, esp when trying to sleep, and there is a reason for that as sweat cannot evaporate (to help cool the body) under such conditions. 

The water content is the dew point.  It's the point at which the water vapour will begin to condense. Like i said humidity is only relative, and doesn't tell you anything on its own.  

100% humidity at 15C will feel different to 100% humidity 18C. 

By the way a heat index of 18C @ 100% humidity is only 18C but if it was 50% humidity it would be 17C. You need to be into the twenties before the heat index at 100% humidity begins to increase. Obviously at any point the lower the humidity the cooler it will feel. For example 25C @ 50% humidity would only feel like 25C 

Yes, and there is a greater water content in the air at 18c with 100% than at 15c with the same. Also, standing in direct sunshine in summer with a temp of 13c (for example) at midday will feel no less cool than standing in a 25c under cloudy conditions. Sorta all relative, + the I think we Celtic types are better suited and more adapted to colder, harsher weather than you south coast England people.  It's all we know. 

13C in summer at anytime of day regardless of sunshine/humidity/wind will feel rather cool at the very least 🤣 To be fair i don't think it's a case of southerners not being able to handle cool, but more of a case of yourself and perhaps others those from your climatic area not handling a little warmth. 

Not true. Regardless of the air temperature, the sun will warm your skin, or any surface, much quicker than it will the air. In fact, the sun cannot warm the air at all. Any heat in the air tends to be warmed from the ground beneath or buildings etc. 

Outer reaches of ECM looks lovely tonight:

I never mind those odd sweltering nights. Yeah, your sleep is a bit disrupted, but then you get up and have a tepid shower that always feels so refreshing. 

GFS - current LP moves away then by end of this week LP to W of Ireland and HP over E England bring some very warm air up from the S but by Tue 4th the plume slides off E-wards and is replaced by W-ly dominated weather with shallow LPs crossing the UK (not so shallow on Fri 7th and Tue 11th) and brief spells of HP in between. GFS has again acceded to the ECM view.

GEFS - Big temp peak Fri 31st, dropping rapidly to below norm only a couple of days later and slowly rising to a little above norm from ca Thu 6th. Not much rain for the S, best chances around Mon 3rd as yesterday.Same sort of temp profile for Scotland though with peak flattened, and rain on and off continually from about Sat 1st.

ECM - similar to GFS but during the W-ly spell referred to the LPs are further N and although the associated troughs do dip S-wards, they do so without much conviction

 Yes i remember a few years  ago whilst staying with family near Bonar Bridge in Scotland during a short spell of high 20,s temps. The talk in the queue at the Post Office of was how unbearable the heat was. Two days of heat and they were wilting 

LOL at run 10 of the 00Z GEFS, 850s hit 26C on the south coast!

26.1C around Eastbourne according to the Meteociel table view.

The 06z op run nearly gets the 20°C t850hPa isotherm into the far SE on Friday:

What sort of max temps would that deliver.

Only low-res charts that I could see, only show about 34C. The tabulated output on Meteociel didn't go above 30C!

I'm halfway through switching to the high-res GEFS data sets. Hopefully in place later today or tomorrow.

Am I right in thinking that a rough guide is to add 10C onto the 850s for the surface temp assuming essentially sunny conditions & no sea breeze effect? If so, exceptionally hot!

The BBC 5 day red button forecast for London now has a max of 32c for Friday, which is the peak of the upcoming heatwave. It's amazing how quickly the temps can rise in the S.E. given a little bit of a continental waft. Pretty impressive for what is only a 2 day hot spell!

If it were that simple. A temperature at 850 of 5°C and sunshine can deliver >20°C in summer, but perhaps only 10°C in winter.

There is no doubt that high 850 temps play a part in the occurrence of extremes; but don't forget the dreadful set up in early summer last year, when Europe was roasting (before our record breaking moment), when surface temperatures were dictated by the upstream surface temperatures (N Sea) despite high 850s (fuzzy memory- help out here) that might have been around or even above 20.Conversely, I don't think the 3 c35C days, peaking at 35.6 at Southampton in June 1976, had exceptionally high 850s.

Then there is the altitude factor- 850s are I believe judged at being about 1500m ASL; some surfaces are above that height.

I prefer to think of the 850 temps as a significant, but not necessarily dominant factor.