Q. 9

Standing in the sun is more pleasant on a cold winter day than standing in shade. Is the temperature of air in the sun considerably higher than that of the air in shade?

Answer :

Yes. The temperature in an open place will be higher than that of a shady place due to the presence of more number of heated molecule in the former case.

Human body needs thermal comfort despite the surrounding condition. On a cold environment, human body needs to transfer less amount of heat compared to that in a normal environment.

The temperature of the surrounding is lower than the usual temperature on a winter day, and human body will transfer more heat to surrounding by radiation, convection and conduction*, causing a drop in the temperature of the outer skin. In an open space, where there is more radiation from sun, the temperature of the surrounding will be comparatively higher than a shady place. So, the heat transfer from human body will be lower in the open space since all the above modes of heat transfers are directly proportional to the Temperature difference between human body and the surroundings.

In the case of shady place, the temperature of the surroundings will be comparatively lower due to 2 reasons;

1. There is lesser availability of direct radiation from the sun

2. The phenomenon of transpiration** from the leaves of the trees will give a cooling effect.

So, there will be a higher amount of heat transfer from human skin to the colder surroundings.

To conclude, on a winter day, a hot environment will be convenient in order to reduce the heat transfer from the body.

*There are other types of heat transfer such as the heat transfer caused by Breathing, etc.

**Transpiration is the process of removal of water from leaves through the pores called Stomata. The evaporation of this will cause a cooling effect.

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