Coming from the UK, I have an unhealthy fascination with the weather. The never ending, will it – won’t it rain tomorrow? Will we get a day off school if it floods? Might it snow? Will we get snowed in? Talking about this is practically a national pastime, if weather worrying were an Olympic sport, we’d get gold every time.
Despite this early ‘training’ I only recently learned a little about how rain and snow actually forms. An important lesson is that water can’t condense onto itself to form rain droplets or snow flakes. There needs to be a seed particle present and these seeds are known as aerosols. An aerosol being a particle of one phase (such as a solid or liquid) suspended in a different phase (such as a gas). So our seed particles are solid or liquid particles suspended in air. The air we breathe is full of these aerosol particles. There are thousands of types of aerosol in air, coming from both natural and man-made sources. Examples might be tiny dust particles, sand, sea salt, bacteria, tree and plant emissions, car exhaust fumes to name but a few.
Aerosols are very difficult to study and understand because there are so many types of aerosols in the air, and they are constantly changing over time. Despite the challenges, there is a lot of research being done on atmospheric aerosols because they are very important factors in climate change and our health. But, back to rain and snow…
It isn’t just temperature that dictates whether clouds form as snow or rain. Specific seed particles are good for making one or the other. Dust particles from deserts for example, are known to be very good at nucleating ice (making snow). Experiments in laboratories can be used to mimic the conditions leading to cloud formation and this is how a lot is learned about the snow or rain forming potential of different aerosols. But, there are also plenty of field experiments where rain or snow is collected – on the ground or by flying aircraft through the clouds and collecting samples – and the non-water components analyzed to determine what the seeds were.
A good example is the CalWater field campaign which studied aerosol impacts on clouds and precipitation over the course of 3 consecutive winters In the Sierra Nevada. They found that during all major storms desert dust and biological residues were the most common seed particles. Using a variety of techniques including chemistry experiments on the ground, satellite data tracking the aerosols, measurements from aircraft in the clouds and detailed weather tracking the scientists were able to trace the dust and biological aerosols leading to increased snowfall to deserts in the Middle East, the Sahara in Africa and even as far afield as China*. It’s amazing to think that dust storms in Asia can affect the weather thousands of miles away. I wonder who we can blame for all the rain in Wales?
Photos courtesy of Katy Jenkins of Glynhir Mansion (Proof that it doesn’t just rain in Wales)
For more info on CalWater see:
*Science 339, 1572, 2013