Relative humidity changes through the day mainly because temperature does. RH is the amount of water vapor in the air divided by the maximum the air could hold at its current temperature, and that maximum roughly doubles for every 10 °C rise. Air that holds the same water all day reads high in the cool early morning and low in the warm afternoon. On top of that, showers, cooking, laundry and people add moisture in pulses, and ventilation removes it. The steadier number is dew point, which tracks the water itself.
The ratio behind RH
Warm air can hold more water vapor than cold air. At 10 °C, saturation is about 9.4 g of water per cubic meter; at 20 °C about 17.3 g; at 30 °C about 30.4 g. Take air at 20 °C and 50 percent RH: it holds about 8.7 g/m³. Cool it to 10 °C without adding or removing water and it is at 92 percent RH. Warm it to 30 °C and it is at 29 percent. Nothing about the moisture changed; only the reference did. See what relative humidity is.
The outdoor daily cycle
- Dawn: coldest hour, highest RH, often near saturation with dew or fog.
- Mid-afternoon: warmest hour, lowest RH.
- Evening: temperature falls, RH climbs again.
The absolute moisture, expressed as dew point, typically moves only a couple of degrees over the same day unless a weather front passes. See what dew point is.
The indoor daily cycle
Indoor temperature is steadier, so the RH swing is smaller, but the moisture sources are closer:
| Event | Effect on room RH |
|---|---|
| Overnight, bedroom door closed, two sleepers | Rises 10 to 20 points from breath |
| Morning shower, bathroom door open | Spike in bathroom and hallway; 30 to 60 minutes to clear with extract fan |
| Cooking, especially boiling | Rise in kitchen and adjoining rooms |
| Heating switches on | RH falls as air warms |
| Drying laundry indoors | Sustained rise of several points for hours |
| Window opened on a cold day | RH falls as dry outdoor air warms indoors |
Why it matters
Comfort and mold risk depend on RH at the surface, which is why cold surfaces (window frames, external wall corners, behind wardrobes) can be above 80 percent RH while the room reads 55 percent. The daily peak in a bedroom is the number that decides whether condensation forms; see why condensation forms on windows. The comfort band of 30 to 50 percent RH applies to the room average.
Watching the right number
A hygrometer that also computes dew point, or the humidity checker, separates the two effects. If dew point is flat and RH is swinging, it is temperature. If dew point is rising, moisture is being added and ventilation is the fix. Hygrometers and data loggers that record both are covered in the environmental monitoring category.
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Frequently Asked Questions
Why is humidity highest in the morning?
Because the air is coldest. Overnight cooling raises relative humidity even though the water content barely changes; the same air warms during the day and RH falls.
Does humidity change indoors as much as outdoors?
Less, because indoor temperature is more stable, but showers, cooking, laundry and breathing add moisture in pulses that a bedroom or bathroom hygrometer sees clearly.
What number should I watch instead?
Dew point. It tracks the actual moisture in the air and does not swing with temperature, so it shows when moisture is genuinely being added or removed.
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