📖 PART 1: Meaning of Humidity

• Water vapour is added to the air mainly through Evaporation from oceans, lakes, and rivers, and Transpiration from plants.
• The capacity of air to hold water vapour depends entirely on its Temperature. Warm air can hold more moisture than cold air.
• When a given mass of air holds maximum water vapour it can at a specific temperature, it is called Saturated Air. The temperature at which this occurs is the Dew Point.

Important Terms

Term	Definition
Absolute Humidity	The actual weight of water vapour in a given volume of air (usually expressed in g/m³). Changes with air volume/temperature.
Relative Humidity (RH)	The ratio (percentage) of the actual amount of water vapour in the air vs the maximum amount the air could hold at that same temperature. Critical for weather forecasting. 100% RH = Saturated air = Rain/Dew likely.
Condensation	The process by which water vapour changes into liquid water. Requires cooling of the air below its Dew Point, and the presence of condensation nuclei (dust, smoke particles).
Precipitation	The falling of condensed water vapour from clouds to the Earth's surface (rain, snow, hail, sleet).

🌫️ PART 2: Forms of Condensation

• Dew: Moisture condensing on cool surfaces (grass, metal) at night when ground temperature drops below dew point.
• Frost: Forms when the dew point is below 0°C (freezing point) and water vapour directly turns into solid ice crystals on surfaces.
• Fog: A cloud formed at or near ground level due to the cooling of moist air below its dew point. Very common over oceans where warm and cold currents meet.
• Mist: Similar to fog but less dense (visibility is greater than 1 km). Common in mountainous areas.
• Clouds: Masses of condensed water droplets or ice crystals floating in the upper atmosphere. Main types: Cirrus (high, feathery, ice), Cumulus (cotton-like, fair weather), Stratus (low, flat layers), Cumulonimbus (tall, dark storm clouds bringing heavy rain & lightning).

🌧️ PART 3: Types of Rainfall (Precipitation)

Rainfall occurs when warm, moist air is forced to rise, cool, condense, and form heavy water droplets that fall due to gravity. There are 3 main types depending on why the air rises:

Type	Process	Key Features & Location
1. Convectional Rainfall	Intense heating of land (mostly in tropical/equatorial regions) → Air expands, becomes lighter, and rises rapidly in strong convection currents → Cools at high altitudes → Forms massive Cumulonimbus clouds → Heavy thunder showers.	Occurs mainly in the Equatorial region (daily at 4 PM — "4 o'clock showers"). Heavy, short-lived rain accompanied by thunderstorms and lightning.
2. Orographic (Relief) Rainfall	A mountain acts as a barrier. Moist winds from the sea hit the mountain → are forced to rise upwards along the slope → expand, cool, condense → rain falls on the Windward side.	Windward side: Heavy rain (e.g., Mumbai, Mawsynram). Leeward side (Rain shadow area): Descending air warms up, holds moisture → very little rain (e.g., Pune, Deccan Plateau).
3. Cyclonic (Frontal) Rainfall	Warm, light moist air meets cold, heavy dry air. The warm air is forced to rise over the cold air at the boundary (called a Front) → cools and condenses → prolonged rainfall.	Common in mid-latitudes and coastal regions. Associated with Cyclones. Light to moderate rain lasting for hours or days. Typical of the British Isles and western Europe in winter.

📝 Quick Facts on Measurement

• Hygrometer or Dry & Wet Bulb Thermometer is used to measure Humidity.
• Rain Gauge is used to measure Rainfall (usually in mm or inches).
• Isohyets are lines on a map joining places with equal rainfall.