When I want to know how hot it is, I want to know how hot it is in the sun not in the shade.
WAG: Because the aim is to get the ambient air temperature. Temperature in the sun is meaningless because it depends on how long you are out in the sun for.
If you put a thermometer in the sun the temperature reading it gives will depend on the size, shape color and composition of the thermometer. Dark colored thermometers, will absorb and convert more of the suns light into heat than lighter ones, while the size, shape and composition of an object affect the rate at which they give off the absorbed heat.
You could you a thermometer with a fast response time, or a small thermocouple to quickly measure the temperature in the sunshine, but I don’t see that that number would be useful very often, since anything you put out in the sun will reach an equilibrium temperature that depends on its size, shape color and composition. By measuring temperatures in the shade you eliminate the need to be concerned about all those variables.
Well, a thermometer doesn’t really display the temperature of the surrounding air - it displays the temperature of the active element (the mercury, or the bimetal coil, or whatever).
We make the assumption that these two temperatures are the same, and many times this is a perfectly reasonable thing to do. Set the thermometer in the middle of a room, leave it for long enough to come to thermal equilibrium with the ambient air, and you’re set. If it’s a mercury thermometer, sitting in a 72 [sup]O[/sup]F room, then the glass container, the mercury inside, everything will be at 72 [sup]O[/sup]F, and the thermometer’s reading will be spot on (assuming that it’s properly calibrated and all that jazz).
Now, take it outside. We have a pretty strong direct source of radiant heat in the sun, which kind of complicates matters. The air is at some temperature, but a thermometer stuck out in the sun will likely not find itself at that same temperature.
Why? The thermometer itself will absorb a goodly amount of the solar energy, and will thus rise in temperature. The sun’s also directly heating the air, but opaque objects tend to absorb a much higher fraction of the energy that hits them than do the various molecules in the air. The upshot of this is that the equilibrium temperature of the thermometer itself is probably higher than that of the surrounding air. You can stick the thermometer in the sun, but the reading will likely be high.
In the shade, you remove the sun’s direct input on the thermometer, and you’re coming closer to measuring actual air temperature, just like indoors. But, isn’t the air cooler in the shade than it is in the sun? My suspicion is that it’s not - especially not if there’s any appreciable breeze. Temperature differences between “sunny air” and “shady air” will diffuse themselves out, and will be helped by the wind, which moves a given volume of air through both sunshine and shade.
There are many factors besides simply air temperature that contribute to feeling “hot”. These variables can be pretty tough to account for (humidity, solar radiation, clothing, reflection/absorbtion properties of the person/clothing, etc.), and need more than a thermometer. If you really do want to try, you should use the thermometer to get the actual air temperature (done in the shade), and make corrections to that.
Off on a slight tangent/nitpick:
The air gets it’s heat from the ground which is heated by the sun. The air may absorb a very small amount of energy directly from the sun, but it is neglible. This is why if you get a layer of stratus cloud over a valley, it may stay there for the whole day regardless of how sunny it may be above the cloud. The direct sun is not enough to warm the air above dew point. First the ground must be heated and then the air is heated from the ground up so to speak. The cloud layer can prevent the ground from heating thus preventing the air from heating above dew point and so the cloud remains.
This also explains why the air temp is the same in the sun as in the shade, provided the shaded areas aren’t large enough to provide different ground temps.
Thanks for the correction, Skogcat - you’re quite right.