Humidity is determined by the relationship between the amount of water vapor in the air and temperature. When rising air is cooled, clouds form.
Basic Knowledge
There is a limit to the amount of water vapor that can be contained in 1m³ of air. This is called the saturated water vapor amount. The higher the air temperature, the greater the saturated water vapor amount (for example, about 17.3g/m³ at 20℃ and about 30.4g/m³ at 30℃).
Humidity (%) = amount of water vapor in the air ÷ saturated water vapor amount × 100.
When air is cooled, it reaches saturation, and if it cools further, water droplets begin to form. This temperature is called the dew point.
When warm, moist air near the surface rises, the pressure is lower aloft and the temperature falls through adiabatic expansion. At the dew point, water vapor becomes water droplets or ice particles, forming clouds. An air mass falls by about 1℃ for every 100m of ascent in dry conditions and by about 0.5℃ in moist conditions after clouds form.
Saturated water vapor amount（The maximum amount of water vapor [g] that 1m³ of air can contain at a certain temperature）
Humidity（The ratio [%] of the amount of water vapor in the air to the saturated amount）
Dew point（The temperature at which water vapor begins to condense when air is cooled）
Adiabatic expansion・adiabatic compression（A phenomenon in which volume changes without exchanging heat with the surroundings）
Condensation（The change of water vapor into water droplets. Forms clouds, dew, and fog）
Cloud（A collection of tiny water droplets or ice crystals. Forms through rising air）
Further Study (Background and Significance)
Three conditions are needed for clouds to form: ① enough water vapor, ② rising air, and ③ condensation nuclei (such as aerosols).
Clouds are classified into 10 types according to shape and altitude. High clouds (cirrus, cirrocumulus, cirrostratus: 5–13km), middle clouds (altocumulus, altostratus, nimbostratus: 2–7km), low clouds (stratocumulus, stratus: up to 2km), and convective clouds (cumulus, cumulonimbus: vertical development).
Cumulonimbus clouds can bring lightning, tornadoes, and sudden heavy rain, and their tops may reach heights above 10km. This is why aircraft avoid thunderclouds.
Humidity is also included in the heat stress index (WBGT), an indicator of heatstroke risk, and is directly related to health.
- The higher the temperature, the greater the saturated water vapor amount
- Humidity = water vapor amount ÷ saturated amount × 100
- Dew point = temperature at which water droplets begin to form
- Cloud formation = rising → adiabatic expansion → cooling → condensation
- Dry adiabatic lapse rate = about 1℃ per 100m
- Moist adiabatic lapse rate = about 0.5℃ per 100m
- There are 10 cloud types (high, middle, low, and convective clouds)
Points to Note (Easily Confused)
① Humidity (relative humidity) and water vapor amount (absolute amount) are different. Even at 100% humidity, the water vapor amount differs if the temperature differs. ② The dew point is a temperature, while humidity is a ratio. ③ Clouds do not form through descending air; they form through rising air. ④ The rates of temperature decrease differ for dry and moist adiabatic processes because condensation releases latent heat after clouds form.
Practice
- What is the temperature at which water vapor begins to condense called?
- State all three conditions required for clouds to form.
- By approximately how many ℃ does dry air cool when it rises 100m?