Why do clear winter nights tend to be colder than cloudy ones; and why is the daily temperature variation in the desert greater than that found in a moist environment?
A. For this question, it is helpful to differentiate between absolute humidity and relative humidity. Absolute humidity refers to the amount of water vapor actually in the air, which can be higher than 2% of the atmosphere’s mass if it’s very warm, but is limited to around 0.1% or less of the atmosphere’s mass if it’s colder than about zero degrees Fahrenheit. Relative humidity is the amount of water vapor actually in the air expressed as a percentage of the amount that could be there (at a given temperature) given the amount of heat the atmosphere has available to maintain water molecules in the vapor state (i.e., without condensing to liquid water). At low temperatures, the air can’t maintain much water in the vapor state, but at high temperatures it can. (In hot, dry, weather, sweat evaporates faster because the surrounding air contains a lot less water vapor than it can maintain.) Over a desert, the absolute humidity can be pretty high (e.g., in the Middle East, near the Mediterran
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