Humidity and Dew Point: Temperature Traps for Prelims

Core Concept

Humidity describes atmospheric water vapour. Relative humidity (RH) compares actual vapour pressure with saturation vapour pressure at the same temperature. Dew point is the temperature to which air must cool to reach saturation, with pressure and water-vapour content held constant.

Formula: RH = (actual vapour pressure ÷ saturation vapour pressure at the air temperature) × 100. RH is a percentage; dew point is a temperature.

Key Comparisons

Absolute humidity: mass of water vapour per unit volume of air, commonly expressed in g/m³. It can change when air expands or contracts even without moisture being added.
Relative humidity: proximity to saturation at the current temperature. It changes with temperature as well as moisture.
Dew point: an indicator of moisture content at a given pressure. A higher dew point generally indicates more water vapour.
Air temperature minus dew point: a smaller difference generally means higher RH. At saturation, the two temperatures are equal.

The Temperature Trap

Heat the air without changing its water-vapour content, at constant pressure: saturation vapour pressure rises, so RH falls. Dew point remains approximately unchanged. The RH decrease does not prove that water vapour was removed.

Cool the same air towards its dew point: RH rises towards 100%. Further cooling can cause condensation. Once condensation begins, do not continue assuming unchanged vapour content.

Illustrative calculation: suppose actual vapour pressure is 10 units and saturation vapour pressure is 20 units: RH = 50%. If warming raises the saturation value to 25 units while the actual value stays 10, RH becomes 40%. The numbers are hypothetical; they illustrate the ratio rather than a particular weather observation.

Memory Hook

“Relative compares; dew point marks saturation.” Before deciding how RH changes, ask two questions: Did the temperature change? Was water vapour added or removed?

Condensation, Fog and Rain

Cooling towards saturation helps explain dew on a cold surface and fog formation. However, 100% RH does not guarantee rainfall. Cloud particles must grow sufficiently to fall as precipitation. Surface saturation and the processes occurring throughout a cloud are different observations.

Warm and cool places with the same RH can contain different amounts of water vapour. Compare their dew points, at comparable pressure, rather than interpreting equal percentages as equal moisture content.

Common Mistakes

Calling dew point a percentage.
Assuming a fall in RH always means moisture loss.
Assuming identical RH implies identical water-vapour content.
Treating air as a sponge with a fixed water capacity: saturation depends strongly on temperature.
Assuming saturated air must immediately produce rain.

Quick Checks

Q1. Consider the following statements:

At constant pressure and unchanged water-vapour content, warming generally reduces relative humidity.
Such warming necessarily reduces the dew-point temperature.

Which of the statements given above is/are correct?

A. 1 only
B. 2 only
C. Both 1 and 2
D. Neither 1 nor 2

Answer: A

Explanation: The saturation reference rises with air temperature, lowering RH. Dew point depends on the moisture condition and remains approximately unchanged under the stated assumptions.

Q2. Consider the following statements:

At saturation, air temperature and dew point are equal.
A relative humidity of 100% necessarily means that rain is falling.