We are going to take a look to radiation if we would like to know the temperature in our room or house. We just put in a thermometer and measure the temperature if the thermomemeter is in equilibrium with the surroundings.

We assume there is no air flow, so Nusselt is 2. What about radiation if the walls are 6 degreees Celsius colder than the rest of the room, which has a temperature ? What will now the temperature measured by the thermomemeter be?

The thermal conductivity of air is and the diameter of the thermometer is .