The meteorological definition of fog is a cloud (stratus) which has its cloud base on or close to ground, and reduces visibility to less than 1000 metres. 100% humidity at ground level can be reached in different ways. (They basically all look the same, though.)

'Radiation fog' is common in the mornings after heat has radiated into the atmosphere during night. It normally dissipates after sunrise. 'Advection fog' is formed by humid air moving horizontally, being cooled down from below. 'Frontal fog' is formed between warm and cold air in a front. There is also 'sea fog', 'hill fog', 'steam fog' and other forms of fog.

This picture of a tree is taken in the early morning, when radiation fog is common. Just an hour or two after sunrise the sky would be clear.

So what if the visibility is more than 1000 metres? This is very often the case, of course, and then we simply call it haze or mist. You can define it as 'an atmospheric aerosol of sufficient concentration to be visible', like here where you still can see distant objects.

A milky atmosphere, but still, you can see quite far in the mist.

When warm and moist air drifts over a colder surface being cooled from below, advection fog is formed. When over water it's called sea fog.

This cool picture shows radiation fog, which is nothing else than a stratus cloud, and its beginning (or end). To the left the approach lights from Guarulhos International Airport in Sao Paulo are visible and to the right, the sun is rising. The heat created by the sun will soon 'burn off' the fog, and maybe one hour later the sky will be clear. Touch down, shortly after this photo was taken, was, however, in quite thick fog with poor visibility.

Again, this is radiation fog at early morning (6 AM). Being not too thick the visibility at close range was quite OK but further away deteriorating. The rest of the day was beautiful with blue skies.

When watching fog stratus from above, it can look like this. The higher the terrain the less fog is encountered. A grey day at ground level, but beautiful grey colour shifting from above.

Early morning at 7 o'clock. The valley where this village is situated is covered by a low stratus cloud. A clear case of radiation fog where shortly after the layer would have dissipated. Standing on the hill overlooking the valley was a cool experience.

Similar to the one above, again, the village people would experience fog in this very clear-cut formation.

Again, early morning where the sun is still low over the horizon. And again, radiation fog is covering the ground just as a jet is taxying to the gate after landing at Zurich airport.

The further away the trees, the milkier they get.