Scrutable

At 3.30pm today it was 37C on my balcony in the shade (NW London, West facing) so I thought I'd put it in the sun for a bit to see if that made a difference. It went up to 46C at which point I quickly put it back in the shade as it only goes up to 50C and I didn't want it to explode (it's just a cheap one).

My question is: are temperatures normally measured in the shade or in the sun? If it's in the shade, isn't that a false reading? What about places where there isn't any shade like in the desert or at the Poles?

Tessa K wrote: Fri Aug 07, 2020 7:16 pm At 3.30pm today it was 37C on my balcony in the shade (NW London, West facing) so I thought I'd put it in the sun for a bit to see if that made a difference. It went up to 46C at which point I quickly put it back in the shade as it only goes up to 50C and I didn't want it to explode (it's just a cheap one).

My question is: are temperatures normally measured in the shade or in the sun? If it's in the shade, isn't that a false reading? What about places where there isn't any shade like in the desert or at the Poles?

The air temperature is the one that counts, and 'proper' measurement are taken by devices housed in a Stevenson screen, which has louvres to keep direct sun off - and at least allows a measure of standardisation.

Always in the shade, as I learnt in a geography class a long time ago. Here's an explanation: https://www.chicagotribune.com/news/ct- ... olumn.html

Tessa K wrote: Fri Aug 07, 2020 7:16 pm At 3.30pm today it was 37C on my balcony in the shade (NW London, West facing) so I thought I'd put it in the sun for a bit to see if that made a difference. It went up to 46C at which point I quickly put it back in the shade as it only goes up to 50C and I didn't want it to explode (it's just a cheap one).

My question is: are temperatures normally measured in the shade or in the sun? If it's in the shade, isn't that a false reading? What about places where there isn't any shade like in the desert or at the Poles?

On -10C day, you can leave a thermometer in the sun here, and it will register >30C. While the ground is covered in ice and snow, and any water you pour on the ground (at least, not on black tarmac) will freeze.

So if I was standing in the sun, the heat on my skin was 37C or 46C? Sorry if I'm being dim.

The air is 37C which will warm your skin a bit. The energy from the sun is also hitting your skin and causing direct heating. I don't know what this energy is measured in, but it isn't C, and there's bucketloads of it.

What temperature an object will rise to from absorbing the sun energy will depend on the properties of the object. A thermometer bulb will be different to a blade of grass which will be different to black tarmac. The human body is complicated by its own energy mechanisms. I'd recommend running the experiment while dead - lie dead in the sun and see what temperature your corpse reaches.

lpm wrote: Sat Aug 08, 2020 8:09 am The air is 37C which will warm your skin a bit. The energy from the sun is also hitting your skin and causing direct heating. I don't know what this energy is measured in, but it isn't C, and there's bucketloads of it.

What temperature an object will rise to from absorbing the sun energy will depend on the properties of the object. A thermometer bulb will be different to a blade of grass which will be different to black tarmac. The human body is complicated by its own energy mechanisms. I'd recommend running the experiment while dead - lie dead in the sun and see what temperature your corpse reaches.

If the neighbours play music again as loud and late as last night there will be no shortage of corpses on which to experiment.

Also, it's handy that I have a lot of surface area and not much density on hot nights as I cool down comparatively easily.

Tessa K wrote: Sat Aug 08, 2020 8:23 am

lpm wrote: Sat Aug 08, 2020 8:09 am The air is 37C which will warm your skin a bit. The energy from the sun is also hitting your skin and causing direct heating. I don't know what this energy is measured in, but it isn't C, and there's bucketloads of it.

What temperature an object will rise to from absorbing the sun energy will depend on the properties of the object. A thermometer bulb will be different to a blade of grass which will be different to black tarmac. The human body is complicated by its own energy mechanisms. I'd recommend running the experiment while dead - lie dead in the sun and see what temperature your corpse reaches.

If the neighbours play music again as loud and late as last night there will be no shortage of corpses on which to experiment.

Skin tone will make a difference as well, so don't forget to keep some of them aside to let the bruises develop nicely before finishing the job.

Gfamily wrote: Sat Aug 08, 2020 8:44 am

Tessa K wrote: Sat Aug 08, 2020 8:23 am

lpm wrote: Sat Aug 08, 2020 8:09 am The air is 37C which will warm your skin a bit. The energy from the sun is also hitting your skin and causing direct heating. I don't know what this energy is measured in, but it isn't C, and there's bucketloads of it.

What temperature an object will rise to from absorbing the sun energy will depend on the properties of the object. A thermometer bulb will be different to a blade of grass which will be different to black tarmac. The human body is complicated by its own energy mechanisms. I'd recommend running the experiment while dead - lie dead in the sun and see what temperature your corpse reaches.

If the neighbours play music again as loud and late as last night there will be no shortage of corpses on which to experiment.

Skin tone will make a difference as well, so don't forget to keep some of them aside to let the bruises develop nicely before finishing the job.

I'll make sure there's a range of height, weight, gender, age and skin tone so it's proper science.

lpm wrote: Sat Aug 08, 2020 8:09 am The air is 37C which will warm your skin a bit. The energy from the sun is also hitting your skin and causing direct heating. I don't know what this energy is measured in, but it isn't C, and there's bucketloads of it.

What temperature an object will rise to from absorbing the sun energy will depend on the properties of the object. A thermometer bulb will be different to a blade of grass which will be different to black tarmac. The human body is complicated by its own energy mechanisms. I'd recommend running the experiment while dead - lie dead in the sun and see what temperature your corpse reaches.

It's power, so Watts per unit area... remembering that you are also radiating to your surroundings as well as losing heat through convection and not much conduction.

The heat capacity is what is important, after all a small wooden spark might hurt but not as much as a kettle of boiling water

lpm wrote: Sat Aug 08, 2020 8:09 am The air is 37C which will warm your skin a bit. The energy from the sun is also hitting your skin and causing direct heating. I don't know what this energy is measured in, but it isn't C, and there's bucketloads of it.

Calories. This is why people left in the sun get fatter.

Tessa K wrote: Sat Aug 08, 2020 8:00 am So if I was standing in the sun, the heat on my skin was 37C or 46C? Sorry if I'm being dim.

The temperature of sunlight is around 6000 K.

The temperature of light from an incandescent bulb is around 3000 K.

The color temperature* of daylight on a cloudy day is ~8500 K.

Temperature is not a measure of how much heat you feel or receive from an object, but of the statistical distribution of energy among the constituents of the object/fluid/radiation field. When you change the thing you are measurement the temperature of, you can't use temperature to compare how much sensible heat you receive from it, or how much energy flows from one body to another via that medium.

It's not the temperature of sunlight/air/water that warms you, but the flow of energy. But what temperature can tell you is how much heat flows compared to what would flow from a similar body at a different temperature.

*This doesn't apply to the spectrum outside the visible range here, so temperature is caveated as being the color temperature in the visible wavelength range. Clouds block IR radiation, so the heat flux is less than from direct sunlight.

dyqik wrote:6000K

Or 5726.75 oC if you are watching in old money.

It may be tedious to some of you having to explain this stuff but I do appreciate it.

I'm looking forward to getting on the Overground later. It's only a few stops but it's always nice and cool.

I believe the issue of human hotness in the sun was extensively investigated by the army of Alexander the Great marching home from India through deserts. They surely found that this was a non-equilibrium situation so temperature may not be well defined.

Different aspects were researched by Napoleons multinational flocks trying to get home from Moscow.

During the US Civil War with Shermans troops trampling the South I'm told that Confederate authorities promised the Bluecoats would soon have a worse slog than Napoleon's retreating through Russia. General Grant watching from the North is supposed to have asked: 'But who'll supply the snow?'