Questions: Saturation, Relative Humidity, and Dew Point

Question 1

On a summer afternoon, the temperature is 35°C and relative humidity is 40%. By nightfall, the temperature drops to 20°C with no rain, wind shift, or other change in the air mass. What happens to the dew point and relative humidity?

• A. Both dew point and relative humidity decrease as the air cools
• B. Dew point stays the same; relative humidity increases as the air cools because the air's capacity shrinks while moisture content stays constant
• C. Dew point rises as the air cools; relative humidity stays constant
• D. Both remain constant because no moisture has been added or removed

Answer: (B)

Dew point is a property of the air mass's actual moisture content — it only changes when moisture is added or removed. Cooling the air changes nothing about how much vapor is present, so dew point is unchanged. Relative humidity, however, is the ratio of actual vapor pressure to saturation vapor pressure at the current temperature. As temperature drops, the saturation vapor pressure drops (warmer air can hold more vapor), so the ratio increases — humidity rises even though no moisture was added. This is why dew forms on cool surfaces overnight in conditions that felt relatively dry during the day.

Question 2

A weather forecast shows a surface temperature of 28°C and a dew point of 26°C. What can you correctly infer about current conditions?

• A. The air is quite dry — a 2°C spread between temperature and dew point indicates low moisture content
• B. Relative humidity is approximately 50% and any clouds will have high bases
• C. The air is very moist; with only a 2°C spread, relative humidity is near 100% and low-level clouds, fog, or condensation are likely
• D. The dew point is close to the temperature, meaning the air is about to freeze

Answer: (C)

A narrow temperature-dew point spread means the air is close to saturation. When temperature equals dew point, relative humidity hits 100% and condensation begins. A 2°C spread means the air needs to cool only slightly before clouds, fog, or dew form. Option A reflects the common error of thinking 'spread = dryness' — actually, a wide spread (e.g., temperature 35°C, dew point 5°C) means very dry air, while a narrow spread means moist air near saturation. Forecasters use this spread routinely to predict cloud base heights and fog risk.

Question 3

Dew point is a more reliable indicator of actual moisture content than relative humidity because dew point changes only when moisture is physically added to or removed from the air mass, while relative humidity also changes when temperature changes.

• T. True
• F. False

Answer: True

Exactly right. A dew point of 20°C tells you the same thing about moisture content regardless of whether the air temperature is 22°C or 40°C — the amount of water vapor present is unchanged. Relative humidity at those temperatures would be very different: near 100% in the first case, much lower in the second. Because relative humidity depends on both moisture and temperature, it can give a misleading picture of atmospheric moisture. Dew point strips out the temperature dependence, making it a cleaner measure for forecasting moisture-related phenomena.

Question 4

At a relative humidity of 100%, the air contains the maximum possible number of water molecules that can exist in vapor form at any temperature.

• T. True
• F. False

Answer: False

100% relative humidity means the air is saturated at its current temperature — not at the absolute maximum possible for any temperature. Warmer air can hold much more vapor before reaching saturation. On a hot day at 35°C, saturation vapor pressure is about 56 hPa; on a cool day at 10°C, it is only about 12 hPa. Air at 100% RH on a cool day actually contains far less water vapor than air at 50% RH on a hot day. The 'maximum' is always temperature-relative. This is why meteorologists use dew point rather than relative humidity to quantify true moisture content.

Question 5

Explain why a rising air parcel cools and eventually forms clouds without any moisture being added to it, using the concepts of dew point and saturation.

Model answer: As an air parcel rises, it expands because atmospheric pressure decreases with altitude. This expansion is adiabatic — the parcel does work on its surroundings without exchanging heat, so it cools. The parcel's moisture content (and therefore its dew point) remains constant as it rises, because no moisture is added or removed. But its temperature drops steadily. At some altitude, the temperature reaches the dew point — the air becomes saturated (100% relative humidity). At this point, further cooling causes water vapor to condense into tiny droplets, forming cloud. This altitude is the lifted condensation level, and the surface temperature-dew point spread determines how high it is: a wide spread means the parcel must rise farther before cooling to saturation.

The key insight is that temperature and dew point converge as the parcel rises: temperature falls, dew point stays (nearly) constant, and cloud forms where they meet. This is why a large temperature-dew point spread on the surface predicts high cloud bases or no clouds, while a narrow spread predicts low bases. The mechanism requires no external moisture — the cooling itself drives the air to saturation by reducing the air's capacity to hold the vapor it already contains.