Because you’re using the thermometer to measure the temperature of the air. You’re making an assumption that the thermometer bulb/sensor is at the same temperature as the air.
If the sunlight shines on the thermometer, it will heat it up. It’s temperature will then depend on its aborbtivity, emissivity, mass, etc., etc.. The temperature indicated will just be the thermometer temperature.
It’s usual in building control systems to mount air temperature sensors on the north face of a building (northern hemisphere, always in the shade). The Contractors on one project put it on the west face. At some random time in the afternoon, the sun would hit it, the indicated temperature would go from 10 or 20 degC to 40 or 45 degC and the HVAC control would go loopy.
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