If air temperature is to be measured accurately, or at least in such a way that values measured at different places may be directly comparable, uniformity of method is essential.
Thus, meteorologists always measure temperature in the shade: their thermometers are housed at a standard height in louvred screens, so that the air can freely circulate around the instruments that are protected from direct sun radiation.
The white wooden box with louvred sides used for this purpose was designed in 1864 by Thomas Stevenson, father of the writer Robert Louis Stevenson. The device was cheap and effective, and was quickly adopted as the standard housing for thermometers globally. In this way, temperatures from place to place can be compared with confidence, knowing we are comparing exactly like with like.
But direct sunlight has a palpable effect on all of us; at the very least, we know that it is hotter in the sun than out of it. It would be nice, therefore, if meteorologists could provide us with a measure of how hot it might feel if we were in the sun in much the same way as wind-chill is used to indicate discomfort in the cold. One such measure, popular many years ago, was the reading from a "black-bulb in vacuo" thermometer.
A "black body", in scientific terms, is an object which is capable of absorbing all the electromagnetic radiation which may fall on it. It will appear black because visible light is such radiation in one particular guise, and since it is all absorbed by the object in question, no light of any colour is reflected to an observer's eye.
Now if the bulb of a thermometer is painted black, it becomes a reasonable approximation to a "black body", and since it will then absorb all the solar energy available at the time, its readings should give a measure of the "hotness" of the day in sunshine. The "in vacuo" simply means the blackened bulb is enclosed in an outer glass "jacket" from which the air has been removed, so that complications of convection or conduction are eliminated.
The readings from a blackbulb in vacuo thermometer were once regarded as a useful "comfort" indicator. But it came to be realised that the relationship between such a temperature and the perceived comfort or otherwise of a human being in the sunshine is very complex, so such readings nowadays are not thought of as being very useful.