By definition, fog has a visibility of less than 1000m. As such, flying in fog is restricted to IFR pilots with appropriate training and equipment. VFR pilots should stay well away!
Other Factors That Reduce Visibility
Rain Water droplets obscure light, so visibility is reduced in rain. Widespread drizzle will reduce visibility over a large area and heavy showers will significantly reduce visibility in a localised area.
Snow Snow similarly reduces visibility, sometimes to the point of ‘white out’ where visibility is virtually zero.
Haze Solid particles such as dust, smoke, and sand all create haze – reducing visibility. This is common when temperature inversions trap particles in the lower levels. It may be possible to climb into clear air above the haze, but slant visibility through the haze will still be poor.
Mist Mist and fog are both water droplets suspended in the atmosphere, reducing visibility. If visibility is below 1000m it is called fog. If visibility is between 1000m and 5000m it is called mist.
Radiation fog occurs when the surface temperature drops to the dew point in a light wind. Pilots should be cautious of a METAR that shows a temperature and dew point that are close together and light winds, particularly in the afternoon. Temperature can be expected to continue decreasing through the afternoon and night, leading to radiation fog. The below METAR is a perfect example:
121750Z 13005KT NCD 02/01 Q1002
Don’t worry if you’re not familiar with METARs, these are covered later in this course. We’ve included this one here as we find most student pilots are starting to become familiar with METARs from their flight training/instructor before they begin formal meteorology studies.