Dew Point and Humidity

About 71% of the Earth's surface is covered with water. Due to constant evaporation, water vapour is always present in the atmosphere. The amount of water vapour in the air affects weather conditions and determines whether the air feels humid or dry. The moisture present in the atmosphere is called humidity.

1. Humidity and Saturation

Air can hold a limited amount of water vapour depending on its temperature. When air reaches its maximum vapour-holding capacity, it becomes saturated. If the vapour exceeds this limit, condensation occurs, forming water droplets.

The amount of vapour required for saturation depends on air temperature:

• Higher temperature → Air can hold more water vapour.
• Lower temperature → Air can hold less water vapour.

Example:

• At 40°C, air can hold 49 g of water vapour per kg of dry air before condensation.
• At 20°C, air can hold only 14.7 g per kg of dry air before condensation.

This shows that warmer air can retain more moisture, while cooler air reaches saturation faster, leading to condensation.

2. Dew Point Temperature

Dew point is the temperature at which air becomes saturated with water vapour.

• If unsaturated air cools, a point is reached where the air holds maximum water vapour and cannot hold more.
• Any further cooling causes condensation, leading to the formation of dew, fog, or clouds.
• At the dew point, the relative humidity is 100%.

1. Absolute Humidity

Absolute humidity is the mass of water vapour per unit volume of air (measured in kg/m³). It does not depend on temperature, only on the actual amount of vapour present.

2. Relative Humidity

Relative humidity is the ratio of the actual amount of water vapour in the air to the amount required for saturation at a given temperature.

It is expressed as:

\[\%\text{Relative humidity}=\frac{\text{actual mass of water vapour content in the air in a given volume}}{\text{mass of vapour needed to make the air saturated in that volume}}\mathrm{x~100}\]

• If relative humidity > 60%, air feels humid.
• If relative humidity < 60%, air feels dry.
• When the air reaches 100% relative humidity, condensation begins, forming dew, mist, or rain.

1. White Trails Behind Aeroplanes: In cold, humid air, water vapour from aeroplane engines condenses and forms white trails (contrails).

• High humidity → trails last longer.
• Low humidity → trails disappear quickly.

2. Fog Formation in Winter: On cold mornings, air near the ground reaches the dew point, forming fog due to condensation.

3. Coastal vs. Desert Climate:

• Coastal areas (e.g., Mumbai) have high humidity, making the air feel hot and sticky.
• Desert areas (e.g., Rajasthan) have low humidity, making the air feel dry and cooler at night.

4. Dew Formation on Grass: On cool nights, air cools to the dew point, causing water droplets (dew) to form on grass and leaves.

5. Sweating and Humidity:

• On humid days, sweat evaporates slowly, making us feel hotter.
• On dry days, sweat evaporates quickly, making us feel cooler.

Take a cold water bottle from the refrigerator and place it in a normal room environment. Wait for a few minutes and observe the outer surface of the bottle. Tiny water droplets start forming on the surface. Similar droplets can also be seen on plant leaves, grass, or vehicle windows in the early morning.

• Water vapour present in the air is usually invisible, but when warm air comes in contact with a cold surface, it cools down.
• The excess moisture condenses into water droplets. This process is called condensation, and the temperature at which it occurs is known as the dew point.
• This activity demonstrates how humidity and temperature changes cause condensation in nature.

The presence of water vapour in the air makes it moist. The moisture in the atmosphere is called humidity.

Suppose unsaturated air at a certain temperature is taken and its temperature is decreased, a temperature is reached at which the air becomes saturated with vapour. This temperature is called the dew point temperature.

The vapour content in the air is measured using a physical quantity called absolute humidity. The mass of vapour present in a unit volume of air is called absolute humidity. Generally absolute humidity is measured in kg/m³.

The ratio of actual mass of vapour content in the air for a given volume and temperature to that required to make the air saturated with vapour at that temperature is called the relative humidity.

% Relative humidity = \[\frac{\text{actual mass of water vapour content in the air in a given volume}}{\text{mass of vapour needed to make the air saturated in that volume}}\] × 100