Fog is a cloud lying on the ground. Walk to school on a damp autumn morning and you walk straight through one. By mid-morning it has vanished. So why does fog come in the morning, and where does it go?
What is fog?
Fog is millions of tiny water droplets floating in the air. They are so light they hang in the air and scatter light, which is why fog looks white.
Many people think fog is a cloud that has sunk down from the sky. It is not.
Fog did not come down. It was made right where it is.
Air always holds some water as an invisible gas called water vapour. Warm air has plenty of room for it; cold air has less. When air cools far enough, the vapour gathers into tiny droplets. That is condensation — the same thing that fogs up a cold glass from the fridge or turns your breath white on a frosty day.
The droplets are astonishingly small. Line up about a hundred fog droplets and they only just span one ordinary raindrop. To build that raindrop, you would need about a million of them. For how much water a whole cloud holds, see how much does a cloud weigh.
Autumn is fog season because nights are long. On a clear night the ground gives off heat to space, and the air just above it grows colder until sunrise. Cold air is heavier than warm air, so it slides into valleys and settles over rivers and lakes, where there is plenty of moisture. When the sun warms the ground again, the air regains its room, and the droplets turn back into invisible vapour. More seasonal science is collected in autumn as a laboratory.
Try it at home: fog in a glass
Suitable for: 6–12 years
✅ A tall glass or jam jar ✅ Warm tap water ✅ A small plate ✅ Four or five ice cubes ✅ Dark paper ✅ A torch
- Fill the glass one third with warm water.
- Cover it with the plate and put the ice cubes on top.
- Hold the dark paper behind the glass.
- Shine the torch in from the side.
- Watch under the plate for a minute.
- Lift the plate gently and see what escapes.
⚠️ An adult helps with the warm water.
The glass is the valley, the water is the river, and the plate is the cold night. What happens if you use cold water instead?
Questions to wonder about
Where did the fog over the lake go this morning?
If fog and clouds are made of the same thing, why do some float high and others lie on the ground?
If every fog droplet needs a tiny grain of dust or salt to start, how many invisible grains did you walk through on the way to school?
Next time you walk through fog, run your hand through the air. You have just touched a cloud.
Every child is made of good atoms. At Good Atoms, we help them discover it.
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