Why is white breath hard to see in Antarctica?

It is easy to call the white part of your breath water vapor, but vapor is a gas and cannot be seen. What you see is a mass of tiny droplets that formed when the vapor cooled.

Air from your lungs is nearly as warm as your body and carries plenty of water vapor. How much vapor air can hold depends strongly on temperature: a cubic meter of air can hold about 50 g at 40°C but only a little over 10 g at 10°C. When warm breath mixes with cold outside air and cools suddenly, the vapor it can no longer hold turns into droplets. Each droplet is tiny, but countless droplets together scatter light in every direction and look white. Clouds and fog look white for the same reason.

Extra vapor does not become droplets on its own. It needs very small specks floating in the air, and vapor gathers around them to form droplets. These specks include dust from the ground, salt particles blown in from the sea, and fine particles from car exhaust.

The air in our cities and neighborhoods always has plenty of these specks. That is why your breath turns white as soon as the temperature drops a little. Even at the same temperature, if the air is very dry, there is less leftover vapor, so your breath looks faint and vanishes quickly.

The Antarctic and Arctic Science Museum of Japan's National Institute of Polar Research notes that at Syowa Station in Antarctica, breath does not turn white even on days below freezing. Antarctic air is so clean that specks to serve as the centers of droplets are rare. The ground is covered with ice, so there is little dust, and there are almost no people or cars.

Interestingly, when a snow vehicle drives by, the specks in its exhaust suddenly increase, and only then does breath turn white. So if your breath shows clearly in your neighborhood, it means the weather has turned cold, and also that the air carries that many tiny specks. When you breathe on a cold window and it fogs up, that is also vapor from your cooled breath turning into droplets.