Most of the atmospheric water vapor interacts and forms clouds in the: stratosphere mesosphere thermosphere troposphere

Troposphere

Explanation:

Clouds form in the troposphere, which is the layer of the atmosphere that is closest to Earth.

The process of cloud formation starts with warm, moist air that rises because warm air rises. This air cools as it rises. The temperature of the rising air drops until the air can no longer rise further and it expands into the troposphere. The water vapor in the air then condenses to form water droplets which form clouds.

81 is a composite number.