MIST – Mist is said to exist when visibility is reduced by water particles that have condensed on the dust, minute salt particles of salt.
When due to mist the visibility reduces below 1 km, it is called FOG. Mist occur when relative humidity is as low as 80%. Radius of water droplet is less than 1 micron.
Fog occurs when relative humidity is 90% or more and the radios of the droplet is between 1 to 10 microns.
What are the types of fog?
- Radiation fog is also known as land fog.
- It forms over land only.
- During the night, land gives off its heat very quickly (On clear night it cools more rapidly) .
- The air in contact with the ground gets cooled and if cooled below its dew point, so large quantity of dew is deposited.
- Now when light breeze is blowing, turbulence causes the cold from the land surface to be communicated to the air a couple of meters above the ground. So it is called ground fog.
- If wind is bit stronger, radiation fog may extend upto a height of about 150m.
- Strong wind causes too much turbulence which leads to formation of cloud – Stratus Type.
- It is also called sea fog because it is mostly found over sea. It can however form over land also.
- It is formed when moist wind blows over a relatively cold surface of sea or land.
- When most air is cooled below its dew point, the excess water vapour condenses into small droplet of water on the dust or minute particles of salt, resulting in Advection fog.
- Wind cause advection fog to form and also to spread.
- However very strong wind can lead to formation of clouds stratus type.
- Example of advection fog are –
- Off the east coast of japan where the warm, moist Westerlies, blowing over the warm Kuro Shio, cross over the cold Oyo Shio.
- On the Grand bank of New Foundland where the warm, moist westerlies, blowing over the warmGulf stream, crosses over the cold Labrador current.
- Smog is radiation fog mixed with Industrial smoke
- Smoke + Fog = Smog
- Example of places – London, Kolkata, Tokyo, Newcastle, Glasgow.
Steam fog or Arctic sea smoke
- When very cold dry air passes over relatively warm sea surface, The water surface evaporating from sea is quickly condensed into water droplet.
- It appears as if vertical streaks if smoke are rising from the sea surface. This is called steam fog or Arctic sea smoke.
Hill or Orographic fog
- When wind comes against a mountain or hill and begins to climb over it, It cools adiabatically.
- After it reaches dew point, any further cooling causes excess moisture to condense into water droplets forming hill fog or Orographic fog.