Latent Heat and Humidity

Latent Heat

When a substance changes its state, its latent heat will be the hat absorbed or released without change of temperature.

Latent heat is absorbed when Ice changes into Water, or Water into Water Vapour
Latent heat is released when Water Vapour changes into Water, or Water into Ice

1. Evaporation

- Liquid to Vapour
- Latent heat is absorbed

There is a particular amount of water per unit volume that the air can hold. When this maximum is reached, evaporation will stop.

2. Saturation

- Air becomes saturated by adding more water vapour to it
- Saturation can be achieved by cooling the air
- If saturated air is called, condensation will occur

3. Condensation

- Vapour to Liquid
- Latent heat is released
- Requires condensation nucleii

4. Freezing

- Water droplet is cooled below zero
- Supercooling occurs when cooling it considerably below zero
- May also require a freezing nucleii to be present

5. Melting

- Solid to Liquid

6. Sublimation

- Directly from water vapour to ice (gas to solid)
- Latent heat is released

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Humidity Measurement

Absolute Humidity = The weight of water vapour in unit volume of air (gms/m3)

Humidity Mixing Ratio = The weight of water vapour contained in unit mass of dry air (gms/kg)

Saturation Mixing Ratio = Is the HMR when the parcel of air is saturated

Relative Humidity = The ratio between HMR and SMR (HMR/SMR x 100%)

In words: "Amount of water vapour in a volume of air divided by the maximum amount of water vapour which that volume could hold at that temperature, expressed as a percentage."

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Dew point = The temperature to which a mass of air must be cooled in order to reach saturation

Relationship with temperature;

When temperature increases, relative humidity decreases but dew point remains constant.