Flying VFR means that we need a minimum amount of visibility to fly safely to our destination. This depends on a number of factors, they are all weather related and indicate the state of the air. Day or night makes no difference to the visibility or transparency of air.
Aviation law requires a minimum flight visibility in all types of airspaces, ranging from 1 NM and up depending on the airspace you fly in.
For example: airspace class G is 1 NM (1,5 km) in some countries where others require 3 NM (5 km) as a minimum (check the AIP).
As pilots we must be aware of these visibility limiting factors and we need to know the local regulations regarding minimum visibility required so that we can ensure a safe outcome of our flight.
This page, and the rest in this section, will lay the groundwork of some basic meteorological effects important for pilots. Remember that these pages are not ment to be a replacement for a good meteorology course.
Visibility is the ability to see through air, its transparency if you like. For our purposes it is measured horizontally. Clear clean air has a better visibility than air polluted with dust or other particles. Thus, how far a meteorological observer can see an object of a given size, regardless day or night, against a background. And keep in mind that sun or moonlight does not alter the transparency of the air, only our ability to see objects.
A number of factors influence visibility, they are: precipitation, fog / mist, haze, smoke and in coastal areas sea spray under the right conditions.
Rain or snow will reduce visibility. Of course it depends a bit on how heavy the precipitation, drop or snow flake size and the intensity are. A light drizzle will not hinder VFR operations (although commercial operations usually will have higher limits, see part 91 vs 125/135) but heavy precipitation in Cb or TCu can reduce visibility to 100 meters or even less accompanied with effects like wind shear and turbulence.
Fog / mist
People confuse these two sometimes but fog is visibility less than 1000 meters and mist, by definition, is visibility between 1000 and 5000 meters. Both have their origins in light suspended cloud droplets with a nearly 100% relative humidity and an abundance of condensation nuclei for the condensation process to start.
When visibility is reduced to 5000 meters or less by the presence of dust particles it is called haze. It is not related with cloud forming factors as is the case with fog or mist. When dust or sand particles are blown off and visibility reduces to less than 1000 meters it is referred to as a dust or sand storm, with altitudes usually not higher than around 150 - 200 ft. In desert areas and with unstable air conditions (steep ELR) fine dust particles can go up to 8000 ft or higher and this condition can last for hours and have their effects on other continents too.
In Europe it is not uncommon to experience sand dust from the Sahara carried by high altitude winds from the south and eventually raining down well into the mid and northern parts of Europe.
Smoke / smog
Activity from industrial districts and fires in residential areas (think of wood fires, they are still being used in some parts of the world) add soot and carbon to the environment reducing visibility even more. This adds enough condensation nuclei to the air so that condensation will take place before reaching a RH of 100% and thus smog is formed. This was very common phenomena in certain cities in the last century (London, Los Angeles).
Commonly seen in coastal areas, sea spray adds salt particles to the air thus increasing the amount of condensation nuclei and condensation can take place with a RH lower than 100% and thereby reducing visibility greatly. With strong onshore winds, reduced visibility can be experienced many miles inland.
The contrast of an object to its background has a notable effect on its visibility, if contrast is low (for example a white building against a snowy white mountain) then the range at which objects can be seen is reduced. A clean and dry windscreen also helps increase visibility range (and reduces accidents), as do clean sun glasses and spectacles.
Illumination by the sun or the moon does not alter visibility, it does alter range. The best conditions occur when looking with the sun in your back (down sun) or towards the moon, one can see the silhouette of the objects better that way.
Slant range is visibility at an angle, thus not vertical or horizontal. In low light conditions (end of the day) and with a haze layer, a pilot overflying an airport can easily see the runway looking down, but when turning onto final the angle is now slant and this reduces visibility greatly. Especially when flying toward the setting sun.
At higher altitudes, reaching space, the sky gets darker. But the sun retains its brightness resulting in a dazzling effect rendering the pilot or astronaut unable to focus on far away objects.
While we do not fly that high, in situations with a hazy grey background our eyes will tend to focus about 3 meters out without the pilot knowing it, making it very difficult to detect other aircraft possibly on a collision course.