Fog and light

Early Saturday morning I looked out the bathroom window and saw... fog! I haven't seen fog for a long time because our summer has been so hot and dry. It cooled on Friday night and this lovely, murky glow was the predawn result facing east.


What is fog? Fog is a low-lying cloud caused by moisture condensing into droplets that can't rise up into the air. Here's a great Yahoo Answer:

During the day, the sunlight heats the earth and warm air right above it rises. During the night, the earth radiates back heat into space. At sunrise the earth is often colder than the air above. This is called an inversion because air doesn't rises anymore.

If the temperature near the ground has fallen under what is called the dew point temperature then the air can't sustain any more moisture as vapour and has to condense it into droplets. That's when fog appear.

Fog can also appear for several other reasons but it must involve very moist and relatively cold air that doesn't rise. As said above, fog is exactly the same as a cloud at ground level.

Saturday morning's fog is a sign of the changing seasons. Fall is coming.