The Science of Clouds

I am sure all of us at school were taught that rain is formed by the cloud droplets merging, until they get big enough to fall to the ground. Not so...

For rain to fall ice must be formed. Once ice is formed the surrounding water is incorporated into these ice crystals which grow as they attract more water. This ice, not the liquid droplets, grows. The ice generally melts on the way down so we get a lot of rain.

So far so simple BUT the water in clouds is pure having been evaporated from oceans and rivers. So it does not freeze anywhere near 0, but -36.5°C! It never gets that cold even in clouds so how is the ice formed?

There exists a very select group of tiny particles- rain making particles- ice making particles. These must be lofted into the clouds for the ice to form at temperatures warmer than-10°C. The vast majority of these particles are biogenic, from bacteria, pollen and fungi.

It was already known that there are a very low number of ice making particles in the oceans, but rivers had never been investigated. Roger Strawbridge (a local fisherman) and I set out to sample Fishguard Bay... The next day we sampled the River Gwaun.

Analysis of the marine samples confirmed the low number of rain/ice making particles in the oceans. However, the river samples, (the first ever to be analysed), were a revelation, containing thousands of times more than the marine samples.

The scientific community were initially sceptical about results obtained from a tiny Welsh river. However similar results have recently been demonstrated in rivers worldwide (e.g. Mississippi).

The discovery, in Fishguard, of huge numbers of freshwater rain making particles, has initiated international efforts to determine If ice making particles from rivers, are more important for rainfall than those from the oceans.

This research, initiated in Pembrokeshire, on the River Gwaun, has important implications for several aspects of climate change, including river management, rainfall prediction, and rainfall control.