While reading this story about the extreme heat currently felt in Death Valley, I noticed this quote:
So that got me thinking:
–Why are extreme temperatures measured in the shade, rather than in the sunlight? Wouldn’t ground or air temperatures in the sun give a much better idea than shade temperatures?
–Does anyone keep well authenticated ground or sunlight temperatures? And if so, what is the alltime record?
Well there’s this…
“For weather purposes, we measure temperatures in the shade, because we want to measure air temperature. Measuring in the sun causes the liquid or the other materials in a thermometer to raise in temperature, causing all kinds of anomalies in the final reading.”
Temperatures are measured in a Stevenson Screen.
The reason is, obviously, is that you need standardization if measurements are to be comparable between locations and through time. Taking the temperature in the shade, protected from windchill, and a few feet off the ground is a good way to eliminate some confounding factors. The temperature on (and close to) the ground is sensitive the composition and color of the ground. If temperature were taken in open sunlight, the reading would be sensitive to the material and design of the thermometer, and to which way the thermometer were positioned. Windchill would also affect an exposed thermometer in unpredictable ways, depending how the thermometer were positioned.
So ground temperatures are not used for meterological purposes, but they are sometimes recorded:
You can get ground surface temperatures much higher than that. The heat source would be mostly geothermal, not from the Sun, but then, every spot on Earth has at least some contribution from geothermal sources.
And a thermometer held in open air won’t necessarily record the temperature of the air, because the radiative coupling might be stronger than the convective or conductive. On a clear summer night, you can actually freeze water in a bowl that’s appropriately isolated from the ground and the air.
Well, I guess you’re right…
We measure temperature in the shade because that’s a measure of air temperature. The temperature of a thermometer in the shade is going to equilibrate at the same as air temperature, since (in theory) it’s absorbing heat from the air by conduction.
A thermometer in the sun is going to absorb heat by radiation as well as by conduction, so it could potentially get quite a bit hotter than the air temperature, and so it isn’t a good index of air temperature anymore.
Windchill is irrelevant.
Windchill is a measure of how fast an object (a human body) sheds heat in a cool/cold breeze vs. still air. If the temperature is 4F but the windchill is -17F, it just means your body loses heat in the wind as fast as it would at -17F in still air. Your fingers will still NOT go below 4F, they’ll just get there much faster. And of course, once your fingers or nose reach 31F, it really does not matter to them if they get colder. (Technically, depending on what’s in you blood, maybe it won’t just freeze until a bit further below 32F…)
A thermometer in the sun will be heated by the sun. What you want to measure is ambient air temperature - as others mentioned, insulated from effects of strong sunshine, say, hitting the thermometer innards or the asphalt below it… we know some parts of some settings get hotter than the general air around. Just walk barefoot on a sandy beach or nice black asphalt during the summer.
Is that correct?* Latent heat of evaporation makes it possible fora windchilled object to be cooled below the temperature of the air blowing across it.
*Earnest question - because I don’t know if the meteorological term ‘windchill’ accounts for this or if it’s irrelevant.
In still conditions, the heat from your body warms up the air directly around your body by a a little bit. If you look at a thermometer and it says 60 degrees F, you think "so this is what 60 degrees F feels like, but what you are really feeling is the 65 degree F air that has been warmed by your body. If the wind is blowing, the air around you is blown away and replaced before it can warm up, and you see what 60 degrees F actually feels like.
I would think windchill wouldn’t be irrelevant if the bulb were not raised and in the shade, since it could rise above air temperature.
The critical point about windchill is that it’s based on evaporative cooling and it’s based on a device that’s trying actively to maintain 98.6 in the presence of wind.
A thermometer doesn’t have any sweat on it to evaporate. It also isn’t trying to generate heat to maintain a temperature.
So windchill is measuring something that’s real but irrelevant to thermometers.
ETA: @ludovic: Yes, but. The windchill charts are calibrated for thermodynamic measuring devices called “humans”. With the active heating & evaporative components I mentioned.
Yes, a thermometer in the sun in the wind will transfer more heat by conduction to the moving wind than it would to stationary air. But since the rate will be different than for a human, the number won’t be comparable to the number we call windchill.
It’s worth pointing out that the US & Canadian weather departments have different and non-compatible definitions of wind chill. Because of the imprecise nature of the thermodynamics of a typical human being.
We’d have the same problem with needing different windchill scales for each model of official thermometer and its mounting details.
A news report from the Phoenix airport said that the temperature on the runway where the baggage handlers worked was 168, fifty degrees higher than the official temperature.
But that temperature would vary a few feet away in the shade, and vary a lot more on the other side of terminal if there were grass and vary over asphalt and every other type of ground. Measurements would quickly become meaningless without one standard procedure that minimizes the variables as much as possible. And taking the standard air temperatures also minimize the gaming that some would indulge in to claim “records” for whatever local glory that gave.
NM, answering the wrong question
Evaporation is relatively irrelevant by the time you are talking about wind chill. You are not really sweating significantly -we hope. What is really important is the heat of conduction to surrounding air and speed at which heat travels through air.
As Darren G. points out - in still air, your body heats the air around you in a thin layer, which then heats the air around it, and so on… so you lose heat gradually. Remember the old school science experiment where you put a series of globs of wax on a metal rod, heat one end, and see how long before each glob melts? Speed of conduction of heat. Of course in cool air there will be a convection effect too, that heated air around you will rise upward.
A breeze disrupts this, and speeds the conduction of heat to the outside air because the air you heated is pushed away faster, so you are in contact with (and transfer heat to) cooler air. net result you cool faster. Yes, if you sweat you could cool even faster, plus even get a bit colder than ambient temperature. But then, if you took the challenge and dumped a bucket of water over your head, you would cool even faster again. (Partly because wet clothing transmits heat faster than dry “air gap insulation” clothing.) Wind chill assumes a body normal for the situation, AFAIK.
In fact Canada has tried to use some bizarre “Kilopascals per cm^2” measure to better reflect wind chill, but it is meaningless to most people.
A thermometer exposed to the sun would see solar heating effects. Perhaps less if also exposed to a stiff breeze, since the (cooler) airflow would carry away heat above ambient air temperature.
but then, the whole point of measuring temperatures would be to get the average general overall temperature - not to measure sun heating effects or what the local patch of asphalt is doing. local effects and construction can mean significant deviation from ambient air temperature, but generally shade won’t make things cooler than ambient unless the structure providing the shade also absorbs some of the heat (I.e. maybe concrete is cool from overnight or something).
Yes, some conditions can get really bad. When I researched a trip to Egypt, I read someone’s comments about going to Aswan in the summertime. The tarmac felt sticky walking from the plane, until they got to the terminal and realized it was their luggage wheels and sneakers’ soles that were actually melting from the heat of the asphalt. The ambient air temperature may have been a mere 46C but the tropical sun meant that you could fry an egg on the black asphalt.
Wait, kilopascals per square centimeter? I’m having a hard time seeing how anything with those units could ever be relevant-- Pascals are already a pressure, a force per area. Pressure per area, then, would dimensionally be a force per hypervolume.
The air temperature at the 1966 All-Star (baseball) Game in St. Louis was reported at 103-105F, while the temperature on the playing field registered 130F. And at that time, the field was natural grass, not artificial turf.
In the older days, we had a thing called a “wet bulb” temperature … what we’ve discussed so far here in this thread would have been known as “dry bulb” temperature … when the bulb of the thermometer is dry …
For wet bulb, we wrap cloth around the bulb and get it soaking wet, then move as much air past this wet bulb as possible, like swinging it on a string around and around, trying to get the water to evaporate as fast as possible off the wet bulb … this draws off the latent heat of evaporation until the temperature reading is at the dew point, the temperature the air must be lowered to in order to reach 100% humidity …
Here’s a picture of a sling psychrometer which is used to measure both wet and dry bulb temperatures at the same time (from the Wikipedia article “Hygrometer”) …
Windchill is a calculated value based on air temperature (dry bulb) and wind speed … and it’s a measure of human danger to the elements … here’s a handy calculator from NOAA …
md2000 might just be misremembering or wooshing us a bit, but we (Canada) used watts per square metre for a while. I don’t remember the thresh-holds very well (it’s been years), but something like 700 was starting to get dangerous and over 2000 was downright brutal.
(My job at the time involved monitoring the weather at some of Canada’s more northerly airports. Some strips on the northwest shore of Hudson Bay could have outrageous windchills in winter.)
