The short answer: Fog usually contains only about 0.05 to 0.5 grams of liquid water per cubic meter of air (a tiny fraction of an ounce). So a cubic meter of fog weighs just a sliver more than the same volume of clear air.
Fog weight by type
Fog looks dense but holds surprisingly little water. Its extra weight over clear air comes from suspended droplets, which are sparse even in thick fog.
| Fog type (example) | Approximate liquid water per cubic meter |
|---|---|
| Light mist | around 0.05 g |
| Moderate fog | about 0.1-0.2 g |
| Dense fog (low visibility) | about 0.3-0.5 g |
| Heavy maritime fog | up to about 0.5 g or more |
What affects fog weight
- Droplet density. More suspended droplets per volume means slightly more weight.
- Visibility. Thicker, lower-visibility fog generally holds more liquid water.
- Temperature. Cooler conditions help more water condense into droplets.
- Humidity and air mass. The clear air itself still makes up most of the weight.
- Fog type. Maritime and radiation fogs differ in droplet content.
- Volume considered. Across a whole valley, the tiny per-meter water adds up to tons.
How fog weight compares
A cubic meter of even thick fog holds less liquid water than a teaspoon, yet a fog bank filling a large valley can still contain water weighing more than several swimming pools combined.
Frequently asked questions
Why does fog look so dense if it weighs so little?
Tiny droplets scatter light very effectively, so even a small amount of water sharply reduces visibility while adding almost no weight.
How much does a whole fog bank weigh?
Per cubic meter the water is tiny, but spread over a large area the total liquid water can amount to many tons across a valley or coastline.
Is fog heavier than clear air?
Only very slightly. The suspended droplets add a fraction of a gram per cubic meter on top of the air's roughly 1.2 kilograms.
