marine meterology

Relative humidity (RH) is the ratio of the partial pressure of water vapour in an air-water mixture to the saturated vapour pressure of water at a prescribed temperature.

The ratio is in percentage.

When moisture content remains constant, temperature increases, relative humidity decreases.

The RH value of saturated air is 100%.

The RH value of unsaturated air is less than 100%

Dew point:

The dew point is the temperature below that the water vapour in air condenses into liquid water.

The condensed water is called dew when it forms on a solid surface.

Relative humidity 100% means that the dew point is equal to current air temperature. The air is saturated with water.

A higher relative humidity means that dew point is closer to current air temperature. A lower relative humidity means that the dew point is lower.

Condensation:

Condensation of water vapour occurs when air temperature decreases below the dew point temperature.

Advection fog:

Advection fog develops when a mass of warm air, with a high relative humidity value, moving over a cooler surface, whose temperature is below the dew point temperature of the air.

With conduction aided by turbulence, the warm air is cooled below its dew point temperature, water vapour condenses, the water droplets produce fog.

The degree of turbulence dictates the height of fog. Higher wind speed, higher height of fog.

Low wind speed and steep temperature gradient are good for developing advection fog.

With a steep temperature gradient, advection fog can form in gale force winds.

Sea fog = advection fog at sea

Predict sea fog:

When sea temperature is below the dew point temperature.

Sea smoke:

It forms by the movement of cold air over a warmer surface.

Wind of very cold air mixes with a shallow layer of saturated warm air immediately above warmer water.

The warmer air is cooled below the dew point and condenses. Then it ascends and mixes with colder unsaturated air above.

Great temperature difference is needed.

With small temperature difference, higher wind speed is needed for providing cold air above surface.