Humidity
Humidity describes the water-vapour content of the atmosphere. Crucial for cloud formation, precipitation, visibility, and icing.
Source: WMO Guide to Meteorological Instruments; ICAO Annex 3; AMS Glossary.
Absolute vs relative humidity
| Term | Definition |
|---|---|
| Absolute humidity | Water-vapour mass per air volume (g/m³) |
| Relative humidity (RH) | Ratio current humidity / max possible at current temperature (%) |
| Specific humidity | Water-vapour mass per air mass (g/kg) |
| Mixing ratio | Water-vapour mass per dry-air mass (g/kg) |
RH alone does not show water amount
Relative humidity cannot be used to determine the actual amount of water which is contained in the air:
- 100 % RH at 0 °C = ~4.8 g/m³ water.
- 100 % RH at 30 °C = ~30 g/m³ water — 6× as much as at 0 °C!
- → RH alone says nothing about actual water content.
Maximum water uptake — temperature-dependent
The maximum possible amount of water vapour air can hold increases with increasing temperature:
- Warmer air can hold considerably more vapour (roughly doubles per 10 °C).
- Consequence: tropical hot air can carry a lot of water; polar cold air very little.
Dew point
The dew point is defined as the temperature to which air would have to cool in order for condensation to occur:
- For given vapour content: constant temperature at which RH = 100 % would be reached.
- Higher dew point = more vapour.
Spread — T and Td
Spread = temperature − dew point.
- Small spread = high RH = clouds/fog near.
- Large spread = low RH = dry air.
When the temperature increases at constant atmospheric pressure and dew point, relative humidity decreases.
The spread (difference between temperature and dew point) usually decreases in ascending air — rising air cools faster (adiabatic -1.0 °C/100 m) than the dew point falls (-0.2 °C/100 m).
Fog formation at spread = 0
Fog forms when the air temperature equals the dew point, resulting in a zero temperature-dew point spread and relative humidity approaching or reaching 100 %, causing water vapour to condense into visible droplets.
Spread and visibility
The reduction of spread causes a reduction of visibility in otherwise unchanged conditions:
- Spread falls → RH rises → vapour condenses → haze/fog → visibility loss.
Conditions for condensation
Supersaturation and condensation cores are preconditions that have to be met for condensation to occur:
- Supersaturation: RH > 100 % (briefly, at very fast cooling).
- Condensation nuclei: small particles (dust, salt, pollen) onto which water molecules attach.
- Without nuclei, RH can rise to ~400 % before spontaneous condensation — practically never reached in the atmosphere.
Hoar frost via sublimation in cloud-free air
Sublimation of water vapour in cloud-free air can cause formation of hoar frost on an aircraft:
- On a strongly supercooled aircraft (e.g. after long high-altitude flight) entering more humid, warmer air.
- Water vapour goes directly to ice (sublimation gas → solid).
- Result: wings and windshield ice up.
Supercooled aircraft + high-humidity air
When a significantly supercooled aircraft flies into a layer of air with high humidity, airframe icing can also occur in cloud-free air at temperatures above 0 °C.