The next time you watch a snowfall, just think that among the falling flakes are some that house bacteria at their core.
It’s a well known fact that water freezes at 0°C, but it only does so without assistance at -40°C or colder. At higher temperatures, it needs help and relies on microscopic particles to provide a core around which water molecules can clump and crystallise. These particles act as seeds for condensation and they are rather dramatically known as “ice nucleators”.
Dust and soot are reasonable ice nucleators but they are completely surpassed by bacteria, which can kick-start the freezing process at higher temperatures of around -2°C.