Reading of thermometer

A thermometer is a temperature measuring instrument. We know that almost all substances expand by heating and thermometer is making use of this property of thermal expansion of matter.

When we measure the temperature of a body, we bring the thermometer (and its thermometric substance, like mercury) in thermal contact with that body. For example, finding the temperature of the patient, the doctor keeps thermometer in his armpit.

When thermometer is placed in direct sunlight, it becomes in thermal contact with the surrounding air. The process of heat exchange is established and after some time the thermometer and the surrounding air become equal.

Thus the thermometer placed in direct sunlight, will record the temperature of the surrounding air.