Definition of Humidity

• Humidity refers to the amount of water vapour present in the air.
• It plays a crucial role in weather conditions, precipitation formation, and human comfort.
• Humidity is a key factor in heat index, cloud formation, and energy balance in the atmosphere.

Measurement of Humidity

Humidity can be measured using:

• Hygrometers (e.g., dry and wet bulb hygrometer)
• Psychrometers
• Electronic humidity sensors

These instruments calculate:

• Relative humidity
• Dew point
• Vapour pressure depending on application.

Types of Humidity

a) Absolute Humidity

• The actual mass of water vapour present in a unit volume of air.
• Expressed in grams per cubic metre (g/m³).
• Varies with temperature and altitude.

Example: If 1 m³ of air contains 10 g of water vapour, its absolute humidity is 10 g/m³.

b) Relative Humidity

• The ratio of the actual amount of water vapour present in the air to the maximum amount the air can hold at that temperature.
• Expressed as a percentage:
• High relative humidity indicates air is near saturation → possibility of dew, fog, or precipitation.

Example: If air at 30°C can hold 30 g/m³ of water vapour but contains only 15 g/m³, then RH = 50%.

Factors Affecting Humidity

Humidity in the atmosphere is influenced by multiple factors:

a) Temperature

• Higher temperature → air can hold more moisture.
• Hence, humidity increases with warmth and decreases with cold.

b) Water Body Proximity

• Areas near oceans, lakes, and rivers have higher atmospheric moisture due to evaporation.

c) Vegetation and Soil Moisture

• Dense forests and moist soils release more water vapour via transpiration.

d) Air Movement

• Winds help in mixing dry and moist air.
• Strong wind may reduce local humidity by dispersing water vapour.

e) Topography and Altitude

• Higher altitudes generally have lower humidity due to lower temperatures and pressure.

Temperature and Humidity Relationship

• The capacity of air to hold moisture increases with temperature.
• Therefore, for the same amount of water vapour, relative humidity decreases as temperature rises, and vice versa.

Example: Warm air with 10 g/m³ vapour has low RH; the same amount in cool air may give high RH.

This relationship is crucial in weather forecasting and climate studies, especially in understanding dew point, fog formation, and heat stress.

Geographical Variation in Humidity

Humidity distribution varies across the globe: