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30 Oct 2019

Fog and Mist

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What is the difference between mist and fog?

Fog and mist differ only by how well you can see through them. What you see is fog if you have less than 1 KM visibility and mist if you can see more than 1 KM.

Surface Layer Type Visibility
Fog < 1KM
Mist > 1KM

How fog and mist form

Fog forms when the difference between air temperature and dew point is less than 2.5 °C and water vapor condenses into tiny liquid water droplets that are suspended in the air.

Sources of water vapour in the air can include

  • precipitation
  • virga
  • plant transpiration
  • cool dry air moving over warm water
  • heating evaporating water from wet land
  • heating evaporating water from water bodies
  • orographic air movement when air moves over mountains

To condense into these visible droplets, the water normally attaches to small particles of something else. These small particles act as what we call “condensation nuclei” and are usually particles such as

  • dust
  • smoke
  • smog
  • ice
  • salt

Fog normally occurs near 100% humidity and when a cool stable air mass is trapped underneath a warm air mass.

Types of fog

Radiation Fog

Formed by the infrared radiation after sunset when land cools and the sky is clear. Radiation fog forms at night and normally doesn’t last very long except in winter months.

Ground Fog

Ground for is low and does not reach as high as the base of the clouds overhead. Ground fog is often a thin, low layer of radiation fog.

Ground fog
Advection Fog

Advection, as you know from your weather studies, is the horizontal movement of air. Advection fog is formed wen warm air passes horizontally over a cool surface and cools.

Advection fog rolling into San Francisco

Advection fog is common when a warm front passes over cold ground or snow. It is most common over large bodies of water when warm moist air flows over cold water.

Evaportation / Steam Fog

Steam fog is formed when cold air moves over warm water. Steam for can cause steam devils over water bodies. Steam fog is typically convective with an upward movement.

Steam fog with a steam devil
Frontal fog

Frontal fog forms in much the same way as stratus cloud near a front when raindrops, falling from relatively warm air above a frontal surface, evaporate into cooler air close to the Earth’s surface and cause it to become saturated. This type of fog can be the result of a very low frontal stratus cloud subsiding to surface level in the absence of any lifting agent after the front passes.

Fog and Drones

The CARs state that you must not fly in conditions that impede your visual line of sight. This means that you cannot fly your drone into the fog if it means that you cannot provide the sense and avoid function to avoid collision.

The rules also state that you must follow your manufacturers manual and specifications. If, for example, you look at the manual for any DJI aircraft you will see that you are specifically told not to fly in fog

A snip of the DJI Mavic Pro manual.

That’s it! If you have any more questions about drones and drone training, meteorology, best practices, or anything else you can contact us any time.