Air with the same amount of water vapor feels dry on a hot day and damp on a chilly one, because the most vapor air can hold, its saturation vapor amount, depends on temperature.
Relative humidity tells what percentage of that maximum the air actually holds, and at 100 % the air cannot take in any more vapor.
In the diagram, keeping the vapor amount fixed and lowering only the temperature moves the saturation line to the left, so relative humidity rises, and whatever lies past the line condenses into droplets.
The same thing happens when air rises and cools, and the tiny droplets that form gather into clouds.
Water Vapor in the Air
💧 How Much Vapor Air Can Hold
①The saturation vapor amount is the most water vapor (in g) that 1 m³ of air can hold
②It grows with temperature: about 17.3 g/m³ at 20 °C and about 9.4 g/m³ at 10 °C
③Relative humidity is the current vapor amount as a percentage of the saturation amount
④With the same vapor amount, warming lowers relative humidity and cooling raises it
Relative Humidity and Dew Point
20°C
10 g/m³
seegongsik.com
Bars comparing the current vapor amount with the saturation amount, with relative humidity and dew point
☁️ How Clouds Form
①When a parcel of air rises, the surrounding pressure drops, the air expands and its temperature falls
②As the temperature falls, the saturation vapor amount shrinks
③Once the air cools to its dew point, relative humidity reaches 100 % and vapor begins to condense
④Droplets form on tiny particles such as dust (condensation nuclei), and these droplets and ice crystals gather into clouds
Summary
Relative humidity
RH(%) = current vaporsaturation vapor × 100
What percentage of the saturation amount the air currently holds
Dew point
Dew point = the temperature at which vapor begins to condense as air cools
When the temperature equals the dew point, relative humidity is 100 %. The more vapor the air holds, the higher its dew point
②Warming raises the saturation amount, so relative humidity falls
③Cooling to the dew point brings relative humidity to 100 % and condensation begins
④Clouds form when rising air expands and cools
⑤The dew point is set by how much vapor the air contains
Examples and Unit Review
Example 1
On a certain day the saturation vapor amount is 20 g/m³ and the air actually holds 15 g/m³. What is the relative humidity?
1
Relative humidity is the current vapor amount divided by the saturation amount, times 100.
2
(15 ÷ 20) × 100 = 75.
▸ 75 %
Cooling this air until its saturation amount drops to 15 g/m³ brings it to 100 %; that temperature is its dew point.
Example 2
A parcel of air with a fixed amount of vapor rises. How does its relative humidity change, and when does a cloud form?
1
Rising air expands and cools, and colder air has a smaller saturation amount.
2
The vapor amount stays the same while the saturation amount shrinks, so relative humidity rises; when the air cools to its dew point and reaches 100 %, vapor condenses and a cloud forms.
▸ Relative humidity rises, and a cloud forms when the air cools to its dew point and reaches 100 %
In the diagram above, fixing the vapor amount and lowering the temperature walks through exactly this process.
Unit Test
Which statement about humidity and clouds is correct?
①When temperature rises, the saturation vapor amount becomes smaller
②At the dew point the relative humidity is 50%
③Relative humidity is the current vapor amount divided by the saturation amount, times 100
④When air rises its temperature goes up and clouds scatter
⑤With the same vapor amount, relative humidity stays fixed even if temperature changes
▸ ③ Relative humidity is the current vapor amount divided by the saturation amount, times 100
1
When temperature rises the saturation amount grows (① wrong), and at the dew point relative humidity is 100% (② wrong).
2
When air rises its temperature falls and condensation occurs (④ wrong), and with the same vapor amount a change in temperature changes the relative humidity (⑤ wrong); relative humidity = (current ÷ saturation) × 100 (③ correct).