Humidity and dew point
1The lesson in plain words
Air contains invisible water vapour. At its dew point it reaches saturation. Further cooling normally forms droplets: dew, fog or cloud.
Water vapour
Air always contains some water vapour, water as an invisible gas. Warm air can hold much more than cold air: the amount needed for saturation rises steeply with temperature. When air holds the most it can at its temperature, it is saturated.
Absolute and relative humidity
Absolute humidity is the mass of water vapour in a volume of air, in grams per cubic metre. Relative humidity (RH) compares the vapour present with the most the air could hold at that temperature, in per cent. At 100 % RH the air is saturated. Cooling unsaturated air without changing its vapour raises its RH until saturation.
Dew point
The dew point is the temperature to which air must cool, with no change in its vapour, to become saturated. Cool it below the dew point and the extra vapour condenses into droplets. This is how dew on the deck, fog and clouds form. The gap between air temperature and dew point is the dew point spread. The smaller the spread, the less cooling is needed to reach saturation.
Measuring it on board
A psychrometer has two thermometers: a dry bulb and a wet bulb wrapped in clean, wet cloth. Ventilate them as instructed. Evaporation cools the wet bulb; drier air gives a larger temperature gap. With a properly wetted, ventilated instrument, equal readings indicate saturation. Its chart gives RH and dew point. Electronic humidity sensors calculate these automatically.
2Key data
- Saturated air
- Relative humidity 100 %; air temperature = dew point
- Warm air holds more
- Much more vapour is needed for saturation at higher temperatures
- Dew point spread
- Air temperature − dew point; small spread = near saturation
- Psychrometer
- Dry and wet bulb; a bigger difference means drier air
3Key points
- 1
Warm air can hold much more water vapour than cold air.
- 2
Relative humidity rises as unsaturated air cools towards its dew point.
- 3
At the dew point air is saturated; cooler still, vapour condenses.
- 4
A small dew point spread means the air is close to saturation.
- 5
A psychrometer gives RH and dew point from dry- and wet-bulb readings.
4Frequent mistakes
Wrong: Relative humidity tells you how much water the air contains.
Right: It is a percentage of what the air could hold at its temperature. Unsaturated air shows a higher RH as it cools towards its dew point.
Wrong: The dew point is the temperature at which water boils off the deck.
Right: It is the temperature at which air becomes saturated when cooled, so droplets start to form.
Wrong: If the wet bulb reads lower than the dry bulb, the air is saturated.
Right: With a properly wetted, ventilated instrument, equal readings mean saturation. A larger difference means drier air.
5Practice case & exam keys
Your air temperature is 18 °C and the dew point is 16 °C. What is the dew point spread, and what happens if the air cools to 16 °C without changing its water-vapour content?
Show the answer
The spread is 2 °C. At 16 °C the air reaches saturation: its relative humidity is 100 %. Further cooling normally causes some vapour to condense.
Exam keys
- Relative humidity is vapour present as a percentage of the maximum at that temperature.
- Dew point: the temperature at which air becomes saturated when cooled.
- Psychrometer: with wet cloth and correct ventilation, equal bulb readings mean saturated air.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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