Humans operate inside a very narrow range of temperatures: from about minus forty to plus forty Celsius. That assumes we have clothes and some source of artificial heat or cooling. Without clothes or artificial heat/cooling, we could not survive temperatures lower than about the mid-teens at the low end and higher than the early forties at the high. We are fragile creatures. This is why we must modify our climate and create portable shelters (commonly referred to as clothing) that lower the rate of heat transfer to the outside, and permanent shelters (houses, offices, etc.) that contain artificial heating and cooling sources.
I never cease to marvel at our fragility. I spent the weekend at a dragonboat festival here in Ottawa (my team finished fourth overall). On Saturday the temperature was in the early twenties and the sky was so overcast you couldn’t feel the sun’s radiation. It rained sporadically, and I was caught outside a couple of times without a waterproof jacket; when that happened I shivered. This is because my natural body temperature is 37 °C, and when water from the sky lands on me its temperature is significantly lower than 37: it is more like five to fifteen degrees. Water is a significant absorber of heat, and the Second Law of Thermodynamics is an immutable fact. Rudolph Claussius stated the Second Law this way:
A transformation whose only final result is to transfer heat from a body at a given temperature to a body at a higher temperature is impossible.
If that transformation were not impossible, I would have no problem when ten-degree water landed on me. Unfortunately, Claussius was right. Which means that the heat transfer is from the body at the higher temperature (me, at 37 °C) to the 10 °C water. This threatens to lower the temperature of the overall “system” (me and the rainwater that has landed on me) to something less than 37. My body must therefore expend more energy to maintain those 37 degrees. Each gram of liquid water requires one calorie of energy to raise its temperature by one degree Celsius. So if say 500 grams of ten-degree water lands on me, my body must provide 13,500 calories of heat just to maintain its core temperature. This, together with the heat required to evaporate that water (which is much higher than that required to raise the temperature of liquid water, and which also comes from the higher-temperature source—which again is yours truly) is why it is quite possible, and common, to shiver uncomfortably on an otherwise comfortable 22 degree day.
On Sunday the temperature was in the high twenties, and the air was quite humid. The humidity lowered the rate of evaporation, which caused a quite-opposite problem when the sun came out and bathed the area with radiation. In this case, the rate of heat transfer from 37-degree human body cores was much lower than it was on the previous day, and human bodies enacted the opposite temperature-control measure, sweating. Sweating uses the evaporative property of water to ward off the threat of an increase in the body’s core temperature.
Think about this. We are talking about a temperature change of maybe six or seven degrees. At the low end of this range, I was shivering. At the high end I was sweating.
Right now I am inside, working at my computer. Outside, it is 24 °C and the relative humidity is 84 percent. Any physical exertion on my part would put my internal cooling system into operation, and soak my nice office clothes with sweat. The only reason I am able to comfortably work at my computer is because my basic need for a good convenient operating body temperature has been taken care of by an artificial cooling source.
That source is an electric-powered air conditioning and dehumidification system. And the electricity comes from the Ontario power grid, which right at this time (nine-fifty-five on Monday June 24) is being fed mostly with power from the three provincial nuclear plants; see Table 1.
I am one of thirteen million Ontarians, and most of us today will enjoy the comfort of artificial air conditioning. Air conditioning uses the latent heat of evaporation to create enough of a temperature differential so that the low end of that differential is lower than the ambient outside temperature. And to take advantage of the latent heat of evaporation the air conditioner first has to force a temperature differential in its working fluid; it does this by using heat. An electric air conditioner creates heat in the working fluid by forcing it int a liquid state using a compressor.
It takes a lot of energy to keep the air inside a building cooler than the outside air. This is why on days like today air conditioners run pretty much non stop. This would not be possible without a major source of reliable electricity.
So all Ontarians should thank the nuclear plants at Darlington, Pickering, and Bruce for providing most of the electricity that keeps air conditioners, and all other electric powered devices, big and small, running. Ontario would not be what it is today without the nuclear plants.