Because traditional temperature taking methods are overally simplified. A single dry bulb in the shade measures only air temperature and not humidity or solar radiation.
A ~~proposed new~~ better method of taking temperature for people working outside is called Wet Bulb Globe Temperature.
It is actually three thermometers: a wet bulb (which mimics sweat and accounts for humidity), a black bulb (which measures solar radiation), and a normal bulb in the shade (which measures air temperature.)
These three measurements can determine if it is safe for a person to work outside in the sun.
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