Humidity and its Measures

4.1 What is Humidity?

Humidity is the water vapour content of the atmosphere. Water vapour is colourless and odourless but crucial for weather — it carries latent heat, forms clouds and precipitation, and is itself a potent greenhouse gas.

Maximum water vapour capacity of air increases with temperature — warm air can hold much more water vapour than cold air (Clausius-Clapeyron relation: ~7% more water vapour per 1°C warming).

4.2 Types of Humidity

1. Absolute Humidity

• Mass of water vapour per unit volume of air: g/m³
• Typically: 0–30 g/m³ (nearly zero in polar air; 30 g/m³ in hot tropical air)
• Limitation: Changes with temperature and pressure, making comparison difficult

2. Relative Humidity

• RH (%) = (actual vapour pressure / saturated vapour pressure) × 100
• The most practically important measure — determines comfort level (RH 40–70% comfortable; RH>80% oppressive)
• Measured by: Wet-bulb and dry-bulb thermometer (psychrometer) or hygrometer
• Changes with temperature (without changing actual moisture) — warmer air has lower RH; cooler air has higher RH
• Why it's hotter in Delhi (dry, RH 20–30% in summer) than Mumbai (humid, RH 80–90%) at same temperature: RH affects evaporative cooling of perspiration

3. Specific Humidity

• Mass of water vapour per mass of moist air: g/kg
• Does NOT change with temperature or pressure (unlike relative humidity) — most scientifically useful for atmospheric analysis

4. Dew Point

• The temperature at which air must be cooled (at constant pressure) to become saturated (100% RH)
• Below dew point → condensation occurs → dew, frost, fog, or precipitation
• High dew point = humid air; low dew point = dry air
• Frost: When dew point below 0°C — water vapour deposits directly as ice crystals (sublimation)
• Fog: When air near ground is cooled to dew point by radiation cooling on clear nights

4.3 Condensation Types

When air is cooled below its dew point, water vapour condenses:

Type	Formation Condition	Example
Dew	Radiation cooling of surface below dew point (calm, clear nights)	Morning dew on grass
Frost	As dew but below 0°C — direct sublimation to ice	Winter frost on windows
Fog	Bulk cooling of air near surface to dew point	Valley/radiation fog; sea fog (cold current)
Mist	Thin fog; visibility 1–2 km	Morning mist
Clouds	Air rises → cools adiabatically → reaches dew point at altitude	Cumulus, cirrus, stratus
Precipitation	Cloud droplets coalesce into drops large enough to fall	Rain, drizzle, hail, snow

---