Explainer #14
How does fog form?
Fog is simply a cloud that touches the ground. Air always carries some water, spread out as an invisible gas. On a clear, calm night the ground loses its warmth up into the sky and chills the air just above it. Cold air can't hold as much water as warm air, so cooling it is like squeezing a sponge, and the extra water gathers on tiny specks of dust, salt and smoke and turns into droplets far too small to fall. Billions of them bounce light in every direction, so the air looks white and you can't see far. In the morning the sun warms the ground and the air, the droplets turn back into invisible water, and the fog fades away.
The principle
Fog is a cloud on the ground. It is made of the same thing as the clouds in the sky: tiny drops of water floating in the air. The only difference is where it forms.
Step 1: Air holds invisible water. There is always some water in the air, spread out as a gas you can’t see. The air also carries countless tiny specks of dust, sea salt and smoke. In the video both are drawn hugely bigger.
Step 2: The air cools at night. On a clear, calm night there are no clouds to keep the warmth in, so the ground loses its heat up into the sky and gets colder. The ground then chills the layer of air just above it. That’s why fog likes autumn and winter nights, and low spots like valleys and the land around lakes and rivers.
Step 3: Water gathers on specks. Cold air can’t hold as much water as warm air. Once the air has cooled enough, it’s like squeezing a sponge: the water the air was holding gets pushed out, and it clings to the tiny specks and forms droplets. It is the same thing you see when your breath turns into a little cloud on a cold day, or a cold window mists up.
Step 4: Billions of droplets make fog. Each droplet is so small and light that it just floats instead of falling. Together, billions of them bounce light in every direction, so the air looks white and murky and you can’t see very far.
Step 5: The sun clears it away. After sunrise the sun warms the ground, and the ground warms the air. Warmer air can hold more water again, so the droplets turn back into invisible water and the fog thins out and disappears, often within a few hours.
Common myth
Fog is smoke or steam rising from the ground
Fog doesn’t come up out of the ground. It forms right there in the air, wherever the air gets cold enough. On a still night the fog often starts as a thin layer close to the ground, because that is where the air cools first, and it grows thicker as the night goes on. It is clean water droplets, not smoke, although dirty city air with lots of specks can make the thickest fogs of all.How this video was made: Rendered 3D animation, simplified and drawn flat. The water, specks and droplets are drawn many thousands of times bigger, and a whole night is sped up to under a minute.
