Question: The thermometer used to measure temperature of a particular place is always installed inside a ventilated wooden box at the meteorological laboratory. Ideally one feels the heat of the sun when directly exposed to sunlight, then why is it that the thermometers are not installed in places where they can come in direct contact with the sun?
Answer: The reason is that the wavelength of each of the seven colours refracted from the sun rays is not the same. Each colour has its own particular wavelength. Depending upon their compositions, substances like plastic, glass, iron, etc when exposed to sunlight absorb the sunrays of a particular wavelength and accordingly absorb heat in varying degrees without any uniformity.
This does not happen in shade where the sunrays do not reach directly. The thermom meeer computes the exact temperature of the atmosphere, taking into account the cumulative effect ofall seven wavelengths of the sunrays.