40 degrees: reflections

Friends, Wow! So that was it. I have lived through temperatures probably never experienced in these islands since human beings lived on them.

I am finding it hard to grasp the significance of what has just passed, so these notes are mainly for myself, but I hope you find them relevant.


I wrote about this weather event last Friday because it had been predicted with high confidence by many computer models.

And as it turned out, the models did pretty well.

Click image for larger version. The left-hand panel shows the difference from ‘normal’ temperatures’ (aka ‘temperature anomaly) predicted one week in advance. The right-hand panel shows the same quantity as it actually happened. See text for details.

The images above show the forecast temperatures and the actual temperatures across Europe. My reference for this is a slightly obscure resource which Karsten Haustein  kindly tweeted about.

Considering the complexities of this forecast – think about the different landscapes across the continent for example – this is really impressive.

Note that the ‘actual’ temperatures are a ‘re-analysis product’ – this means they are based on weather station data but the gaps between the weather station locations are interpolated.

From this we can conclude something which is not obvious – this was not a ‘freak’ occurrence. It can be understood using the same basic physics that underpins all weather forecasts and climate models.

The experience

Click image for larger version. The temperature recorded by the weather station in my back garden over 18th and 19th July. The peak temperature was 39.9 °C.

I deliberately went out in the hot weather – 35 °C by 10 a.m. – just to experience it. It felt like being in another country.

And I was reminded that once air temperature exceeds body temperature (~37 °C) a breeze no longer cools the skin by aiding evaporation of sweat – it simply heats the body more effectively. And in the afternoon, with the air temperature around 39 °C, a breeze blew up and it felt like I was walking into a giant hairdryer. It felt shocking.

‘Popping down’ to the local supermarket for salad ingredients I found that the central refrigeration system had failed and all the food was being thrown out.

Click image for larger version. Total refrigeration failure at Teddington Tesco.

I felt a big relief in the evening when temperatures cooled down to (still hot!) 24 °C at 9 p.m.


Aside from the experience of walking in what felt like the output of a giant hairdryer, three things shocked me.

The first was the image of wildfires burning homes on the outskirts of London. The pictures below (stolen from a DW News Report entitled “Hot Weather Brings UK to a Halt“) are truly appalling, and reports indicate that the Fire Service was massively overstretched with similar wildfires across the UK.

The major parks near me – Bushy park and Richmond Park – are both extremely dry and these wildfires made me reflect that this danger was not restricted to the outskirts of London.

The second was an appreciation of the pan-continental scale of the heatwave.

Click image for larger version. The heatwave was pan-continental – see text for details . Image courtesy of NASA.

This image is from the NASA Earth Observatory and shows the surface air temperatures across most of the Eastern Hemisphere on July 13, 2022. It was produced by combining observations with a version of the Goddard Earth Observing System (GEOS) global model, which uses mathematical equations to represent physical processes in the atmosphere.

Of course it’s summer in the Northern Hemisphere, so one expects the map to be red. But as the article explains, temperature records are being broken right around the world.

And the heatwave has not gone away. The hot air has returned to the mainland of Europe, but it could of course return: August is usually just as hot as July in the UK.

And the finally the most shocking thing of all is that this event is still not enough to persuade politicians that this needs to be the focus of all their actions.

It seems they are simply unable to grasp that ‘business as usual’ is the problem, not the solution.

8 Responses to “40 degrees: reflections”

  1. Bruce MacNeil Says:

    This ought convince a few people to install heat pumps.

  2. protonsforbreakfast Says:

    Indeed. The UK is still coming to terms with the concept of Air-to-Air heat pumps. Give us time: it’s not called ‘the old country’ for nothing. M

  3. Ross Mason Says:

    Your comment about a hot breeze heating the body more efficiently. A few years ago I was in Victoria, Austroylya visiting a friend. We took the car to Philip Island. It was hot. A dunger with no AC but wind down windows. Lets put my arm out the window to cool down. Ummm…nope. At 80kph and the temperature ridiculously tropical it was hotter out than in.

    • protonsforbreakfast Says:

      Woah! at that air speed the heat transfer is incredibly effective – like putting your hand in a fan oven!

  4. Geoff Milward Says:

    I thought that that the latent heat of vaporisation of sweat on the skin was what cooled us down even in ambient temperatures above body heat. The process being limited by the saturation of the air next to the skin. So a breeze, even one at 40°C, which moves unsaturated air to the skin increases cooling. Dry skin would be heated by a 40°C breeze but wet skin would be cooled. Am I missing something?

    After all you were my first thermodynamics lecturer!


    • protonsforbreakfast Says:

      Geoff, Good Morning. I trust you are well.

      Yes and no. This is how I think it works. I think there are two competing effects: (a) heating of the skin by direct heat transfer from the air and (b) cooling of the skin due to evaporation.

      Imagine conducting an experiment where one sits in an air flow of constant speed at a slowly increasing temperature.

      At 25 °C, the air movement feels ‘cool’ – the air is at just about the natural temperature if the skin and sweat evaporation cools the skin.

      At 30 °C, the air is now above the natural temperature of the skin and there is direct heating of skin from the air. My guess is that sweating is now close to it’s maximum 2 to 4 litres/hour (wikipedia)

      At 35 °C the direct heat transfer from the air gets stronger still but the cooling by evaporation is maxed out. My guess is that this is about the balance point – there is roughly net zero cooling from the breeze.

      At 40 °C and above direct heat transfer from the air gets stronger still but the cooling by evaporation is maxed out. The hotter the air and the faster it blows – the more heat transfer there is.

      I think that’s how it works. I suggest some experimental work on this. Expose yourself to hot temperatures in a variety of different breezes of differing relative humidities. I recommend multiple holiday destinations (with low carbon travel!). You should be able to find the break-even point at which a breeze no longer feels like a relief but instead feels ‘stifling’.

      Happy Travels

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