Clouds, fog, or dew will always form when:

• A. the relative humidity falls below 50%
• B. the air pressure increases rapidly
• C. the air temperature drops below the dew point
• D. the sun's energy heats the ground

Answer: (C)

Verified step by step guidance

1. Understand the concept of relative humidity, which is the ratio of the current amount of water vapor in the air to the maximum amount the air can hold at that temperature, expressed as a percentage.
2. Recognize that clouds, fog, or dew form when water vapor condenses into liquid water, which happens when the air becomes saturated with moisture.
3. Know that the dew point is the temperature at which air becomes saturated (100% relative humidity) and water vapor begins to condense.
4. Identify that when the air temperature drops below the dew point, the air can no longer hold all the water vapor, causing condensation and the formation of clouds, fog, or dew.
5. Conclude that the correct condition for the formation of clouds, fog, or dew is when the air temperature drops below the dew point, not when relative humidity falls below 50%, air pressure increases rapidly, or the sun heats the ground.