Precipitation occurs when tiny droplets of water, ice or frozen water vapor join together into masses too big to be held above the earth. They then fall to ground as precipitation.

The term precipitation denotes all forms of water that reach the earth from the atmosphere. Usual forms are rainfall, snowfall, hail, frost and dew. Of all these, only the first two contribute significant amounts of water. Magnitude of precipitation varies with time and space.

What is dew ?

• When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew.
• The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights.
• For the formation of dew, it is necessary that the dew point is above the freezing point.

How is White Frost formed?

• Frost forms on cold surfaces when condensation takes place below freezing point (0° C), i.e. the dew point is at or below the freezing point.
• The excess moisture is deposited in the form of minute ice crystals instead of water droplets.
• The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.

What is Fog ?

• When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles.
• So, the fog is a cloud with its base at or very near to the ground. Because of the fog and mist, the visibility becomes poor to zero.
• In urban and industrial centers smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke, is described as Smog.

Mist

• The difference between the mist and fog is that mist contains more moisture than fog.
• In mist each nuclei contains a thicker layer of moisture.
• Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.
• Mist is also formed by water droplets, but with less merging or coalescing. This means mist is less dense and quicker to dissipate.

What is Haze?

• Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky (No condensation. Smog is similar to haze but there is condensation in smog).
• Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires.

Also read : Weather & Climate