Umbrella trapping heat My question is. If you have a large black backyard umbrella and after a long hot day with the sun on the umbrella . Then the sun goes down ,will the temperature stay warmer longer under the umbrella by trapping the heat or is there no change in temperature between the surroundings and under the umbrella?
 A: I'm not sure if the heat capacity and black body radiation are large enough to be experienced. They do contribute, however, I'm not sure about their dominance.
My guess would be that the obscured air flow causes the effect. To explain what I mean let's assume the air temperature is 30°C.

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*The air around our body will have a higher temperature than the air $1m$ away from us, because we are at 37°. Heat is always  transferred from warm to cold. Therefore, our body heats the air which surrounds us.

*If wind is present, the air around us is blown away constantly. Hence, the air, which we just heated, is replaced by colder air.

*If the air flow is obscured by the umbrella, than the reduced exchange of air will make us feel hot.

*Therefore, my conjecture is: You won't feel this effect on a hot, but windy day.

A: There are two things to consider:


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*Heat capacity. Air has low heat capacity, and umbrella material is usually quite thin. So the total amount of heat that can be stored is very low.

*Speed of heat loss. Your umbrella is surrounded by air. At the very least, it has a large upper surface area. This surface will both heat the surrounding air and radiate heat away as infrared light. Both have the effect of cooling the umbrella. Give it even a tiny bit of wind, and the hot air beneath the umbrella is quickly blown away, cooling the umbrella from the other side as well.
Low heat capacity and high speed of heat loss mean that your umbrella cannot hold a high temperature for long. I only give it five minutes to cool down after the sun vanishes suddenly, and you won't get any appreciable effect after the sun sets in the normal way.
A: "Trappining" heat, has to do with the heat capacity of the material you are dealing with. Air has low heat capacity, that means it will take a short amount of time in order to reach the temperature of the surroundings.
Now lets take a look to the black umbrella. Black means absorption of all wavelenghts of the visible light and no reflection, but visible light is not what heats up the umbrella or the air. Infrared light is what warms everything up. The ability of an object to absorb and emit IR light has nothing to do with its visible color, so that means that it doesn't matter if your umbrella is black or purple.
A: The "heat capacity" mentioned in the other answers is irrelevant.
The important thing is that the umbrella has the temperature of the surrounding air, so it radiates with that temperature in the near infrared. Without that, on a clear evening, one gets only radiation from the upper atmosphere which is much colder.
This is how carports work when the windshield does not get frosty in a cold night.
