Stratus is a cloud without any distinct form and at low altitudes. They form in front systems or at various different occasions depending on temperature, pressure, humidity and the sun's activity. If stratus reaches ground level it's called fog. (see special stratus fog page)

At this occasion stratus clouds were formed over cold sea water and then drifted in over land. Observers on this hillside would experience this stratus cloud as fog.

Another fine example where the stratus clouds cover man-made objects, like here, the skyscrapers in downtown Chicago.

In the mountains you often encounter clouds at ground level. Here stratus clouds have formed at an altitude of around 2500 metres.

Just like clouds can form over land by rising air, clouds can form over water. Here some stratus clouds have formed by cooled down air over cold water. Note the icebergs in the water. Over land no or few clouds are found.