Humidity

Dew is formed when water vapour in the air condenses into liquid water upon cooling. This occurs when the temperature of the surrounding air falls below the dew point, resulting in the formation of tiny water droplets on surfaces.

Dew Point (D.P): This is the temperature to which air must be cooled for it to become saturated with water vapour.

Example: If water vapour in an air container has a pressure of 8 mm of mercury and is cooled, dew will form at 7.9°C.

Absolute Humidity: This is the mass of water vapour present in a unit volume of air, usually expressed in grams per cubic metre (g/m³). It is not widely used, since most practical applications require knowing how "wet" the air feels.

Relative Humidity (R.H): This is the ratio of the amount of water vapour actually present in the air to the amount required to saturate the air at the same temperature.

$\text{R.H} = \frac{\text{Mass of water vapour actually present}}{\text{Mass required to saturate the air at the same temperature}} \times 100\%$

Alternatively, using saturation vapour pressure:

$\text{R.H} = \frac{\text{S.V.P at dew point}}{\text{S.V.P at air temperature}} \times 100\%$

• A low value of relative humidity means that evaporation occurs easily.
• A high value of relative humidity means that evaporation is slow.
• A humid atmosphere is nearly or fully saturated with water vapour.

Perspiration (sweat evaporation) is less effective at cooling the body in a humid atmosphere.

1. Cotton industries prefer areas with high relative humidity. If cotton becomes too dry, it becomes brittle and hard to spin.
2. Dry atmospheres are required in warehouses storing food, tobacco, and for assembling certain electronic components.
3. Air-conditioning systems are used in ships and buildings to regulate humidity.

Hygrometers are used to measure the relative humidity of a place.

Mason's Hygrometer (Wet and Dry Bulb Thermometer):

1. Has two thermometers: one dry bulb and one wrapped with muslin and dipped into water.
2. Capillary action keeps the muslin wet.
3. The wet bulb reads a lower temperature due to evaporation.
4. The wet bulb depression (difference between dry and wet readings) is used to calculate relative humidity from standard tables.

Example: If the dry bulb temperature is 30°C and the wet bulb temperature is 20°C, the depression is 10°C. A greater depression indicates drier air.

This instrument is used to determine dew point and relative humidity.

Above show Regnault (dew point ) hygrometer.

Description:

• Contains two test tubes A and B with silvered ends C and D respectively.
• Test tube A contains ether, with a thermometer and narrow tubes E and F.
• Test tube B is empty and used for comparison.

How it Works:

1. Air is pumped through tube E, causing ether in A to evaporate.
2. Evaporation cools the surroundings, and dew forms on the silvered surface C.
3. The temperature at which dew appears is recorded.
4. Airflow is stopped, and dew disappears at a slightly higher temperature.
5. The dew point is taken as the average of the two temperatures.
6. Relative humidity is calculated as:

$\text{R.H} = \frac{\text{S.V.P at dew point}}{\text{S.V.P at air temperature}} \times 100\%$

Example: Dew point temperature = 12.55°C Air temperature = 10°C S.V.P at dew point = 10.9 mm Hg S.V.P at air temperature = 15.5 mm Hg

$\text{R.H} = \frac{10.9}{15.5} \times 100\% = 71\%$

Water exists in the atmosphere in many forms including clouds, rain, snow, hailstones, mist, fog, and smog.

Clouds:

1. Made of tiny droplets of water or ice.
2. Formed when air rises and cools below the dew point.
3. Dust or salt particles act as nuclei for condensation.
4. Clouds may also form when warm moist air meets cold air.
5. When droplets grow large enough, they fall as rain.

Rain: Formed when cooling occurs in clouds and water droplets combine and become heavy enough to fall.

Snow: Formed when the dew point is below 0°C, causing water vapour to directly condense into ice crystals.

Hailstones: Formed from supercooled water droplets (below 0°C but still liquid). When carried upward by air currents, they freeze upon contact with ice crystals.

Mist: Tiny water droplets formed near the ground by condensation of water vapour.

Fog: A thick mist with visibility less than 1 km.

Smog: A dense fog often mixed with pollutants, reducing visibility to a few meters.