We humans are keen both on controlling our environment (building houses, central heating etc), and on moving about (planes, trains and automobiles). We are not the only species to do some of this, beaver build dams, bees build hives, many birds migrate, etc. Friends at Surrey, Joe, and Bath, Douglas, independently drew my attention to work on a type of bacteria that manipulates the freezing of water to build itself temporary (and heavy) homes and so drop out of the atmosphere.
The bacteria is Pseudomonas syringae, or P. syringae for short. It has long been known that this bacteria has evolved to trigger the freezing of sub zero water. It produces proteins on its surface that encourage water to start freezing. Ultra pure water freezes at some temperature below about -40 C, the exact temperature is not known as it is fiendishly hard to make water this pure. The slightest impurity causes water to freeze at temperatures higher than this.
Anyway, P. syringae can cause water to freeze at a relatively balmy -2 C. This may not sound very warm but you should appreciate that pure water will not freeze at 0 C, and you can have clouds of liquid water droplets that are a lot colder than – 2 C.
Also, ski resorts often need to make snow and they actually use this bacteria to help water freeze and make snow. So people make a fair bit of money from P. syringae.
All this has been known for a while, but work a few years ago by an American microbiologist, Brent Christner, has found P. syringae in the hearts of hailstones. There are nice BBC and Wired stories on this. The BBC one is recent, it covers work on some hailstones from a big shower that actually fell on his university campus. The suggestion is that the bacteria may start ice growing in the atmosphere, which then grows around them. The hailstone will then fall to ground, bringing the bacteria with it. So, starting an ice crystal growing may be a way of bringing a bacterium travelling in the atmosphere back to earth, where it can grow.