Powerful convection in summertime thunderstorms explains why raindrops can become ‘trapped’ in the cumulus clouds, recycling the water at high altitudes, where temperatures are below zero – even today.

That should -theoretically- however not be cold enough to explain the large and damaging hailstones, which may also accompany the tornadoes that presently cause so much devastation in the US Midwest.

Scientists of Louisiana and Montreal State Universities at Tuesday’s annual meeting of the American Society for Microbiology state the required temperatures in ‘clean air conditions’ for all water to freeze would be around minus 40 degrees Celsius, much colder than typical air in thermal thunderstorms.

Largest of aerosols

If however the concentration of certain aerosols, serving as condensation nuclei would be sufficiently high, water could freeze at much higher temperatures [up to -2°C]. The size of the aerosols seems to be of influence as well, suggesting larger aerosols could be much more effective.

Larger aerosols are typically not chemically created gases or fluids, but solid material, and organic of origin. They could even be alive.

To test this, the scientists scraped different layers of ice off large collected hailstones – and separated these samples. And indeed concentrations of bacteria turned out to be highest in the very core of the hailstones, suggesting summer hailstorms require powerful convection to carry micro organisms to the high and freezing altitudes. It also means our planet’s meteorology would be different without biology.

Intelligent design?

One of the studied bacteria is called Pseudomonas syringae, which has a water-attracting protein on its surface. Some researchers suggest this may be a deliberate strategy for the organism: when ice forms around it and it drops to the ground as a hailstone, these stones can damage crops and plants – allowing the bacterium easy access to the plant cell interior, where it can feed.

© Rolf Schuttenhelm | www.bitsofscience.org