The temperature of air in a greenhouse is usually much higher than outdoor. Which of the following is the main reason? *
A The glass top of greenhouse does not allow hot air to lose heat by convection.
B The glass walls of greenhouse absorbs large amount of heat from the Sun.
C The glass walls of greenhouse conducts heat quickly from outdoor to indoor.
The correct answer and explanation is :
Correct Answer: A
“The glass top of greenhouse does not allow hot air to lose heat by convection.”
Explanation (300 words):
A greenhouse stays warmer than the outdoor air mainly due to the greenhouse effect, a process that traps heat inside the structure. The correct answer, option A, highlights the key mechanism: the glass roof and walls prevent the warm air from escaping by convection.
Here’s how it works:
Sunlight, which includes shortwave radiation, easily passes through the transparent glass or plastic walls and roof of the greenhouse. This radiation heats up the soil, plants, and surfaces inside. These warmed surfaces then emit infrared (longwave) radiation. However, glass is not transparent to infrared radiation—it traps this heat energy inside. As the interior warms, the air inside becomes hotter.
Normally, in open air, hot air would rise and escape through convection, being replaced by cooler air from the surroundings. But in a greenhouse, the enclosed environment blocks this air movement. The warm air is trapped inside, and without a way to escape, the temperature continues to rise.
Let’s break down why the other options are incorrect:
- Option B is misleading: while the glass may absorb some heat, its main role is transparency, not absorption. It lets in light rather than storing much heat itself.
- Option C is incorrect: glass is actually a poor conductor of heat. It doesn’t conduct heat quickly from outside to inside. Heat enters mainly through radiation, not conduction.
In conclusion, the higher temperature inside a greenhouse is primarily due to the inability of the warm air to escape by convection, combined with the trapping of infrared radiation by the glass. This makes the greenhouse an effective method for growing plants in cooler climates or seasons.