Water vapour in the air
Water from seas, rivers and wet clothes slowly changes into vapour. Vapour is water as an invisible gas. It mixes with the air. Fog and clouds are not vapour: they are tiny drops of liquid water.
Absolute humidity is the mass of vapour in one cubic metre of air. Its unit is g/m³. On a dry day it may be 5 g/m³. On a damp day it may be 20 g/m³.
Saturated vapour: when air is full
Air at a given temperature can hold only a limited amount of vapour. When it holds the most it can, the vapour is saturated. Then the number of molecules leaving the water equals the number returning. There is no net evaporation.
The limit grows with temperature. At 0 °C, 1 m³ holds about 4.8 g. At 20 °C it holds about 17.3 g. At 30 °C it holds about 30.4 g. Warm air is a bigger bus.
Relative humidity
Relative humidity (RH) tells how full the air is:
RH = (vapour in 1 m³ now) ÷ (most vapour 1 m³ can hold at that temperature) × 100%
RH 100% means saturated air. RH 50% means half full. Air feels sticky above about 70% because sweat cannot evaporate. Very low RH (below 30%) dries skin and lips.
If you warm air without adding vapour, absolute humidity stays the same, but RH goes down. Cooling does the opposite.
Dew point
The dew point is the temperature to which air must be cooled for its vapour to become saturated. Below it, extra vapour condenses into drops: dew on grass at dawn, mist, fog, drops on a cold bottle.
Example: air with 9.4 g/m³ of vapour has a dew point of 10 °C, because 10 °C air can hold exactly 9.4 g/m³. A high dew point means moist air.
Measuring humidity: hygrometer and psychrometer
A condensation hygrometer cools a shiny surface until dew appears. That temperature is the dew point, and a table gives RH.
A psychrometer has two thermometers. One is dry. The other has its bulb wrapped in a wet cloth. Water evaporating from the cloth cools it. In dry air evaporation is fast, so the wet thermometer reads much lower. In humid air the two readings are close. A table converts the difference to RH. A hair hygrometer uses a hair that gets longer when the air is more humid.
Try it: dew on a glass
Fill a glass with water and add ice. Wait two minutes. Drops appear outside the glass. Where did the water come from? From the air. The cold glass cooled the nearby air below its dew point. Now try it on a dry winter day: the drops come later, because the dew point is lower. Predict first, then check.
Key formulas and definitions
- Absolute humidity = mass of vapour ÷ volume of air (g/m³)
- Relative humidity RH = vapour density ÷ saturated vapour density × 100%
- Dew point: the temperature where saturated vapour density equals the vapour density now
- Mass of vapour in a room = absolute humidity × volume
Worked examples
1. Air at 20 °C holds 8 g/m³ of vapour. Saturated air at 20 °C holds 17.3 g/m³. Find the RH.
RH = 8 ÷ 17.3 × 100 ≈ 46%.
2. At 25 °C saturated air holds 23 g/m³. The RH is 60%. Find the absolute humidity.
Vapour = 0.60 × 23 = 13.8 g/m³.
3. Air has 9.4 g/m³ of vapour. Saturated air holds 9.4 g/m³ at 10 °C. What is the dew point?
The air is saturated at 10 °C, so the dew point is 10 °C.
4. A room of 50 m³ is at 20 °C with RH 50%. How much vapour is in it? (Saturated: 17.3 g/m³)
Absolute humidity = 0.5 × 17.3 = 8.65 g/m³. Mass = 8.65 × 50 ≈ 432 g.
5. Air at 30 °C holds 12.8 g/m³. Find the RH and the dew point. (Saturated at 30 °C: 30.4 g/m³; at 15 °C: 12.8 g/m³)
RH = 12.8 ÷ 30.4 × 100 ≈ 42%. 15 °C air holds exactly 12.8 g/m³, so the dew point is 15 °C.
6. 1 m³ of air at 30 °C is saturated (30.4 g). It is cooled to 20 °C (17.3 g/m³ capacity). How much water condenses?
30.4 − 17.3 = 13.1 g of water condenses as drops.
Common mistakes
- Thinking relative humidity means the mass of vapour. It is a percent of what the air could hold.
- Calling fog or steam from a kettle vapour. Those are tiny liquid drops. Vapour is invisible.
- Saying warming air adds water. Warming only raises the capacity, so RH falls.
- Mixing up the units: absolute humidity is in g/m³, relative humidity is in %.