My original plan for this article was to dig deeper into the changing average or “normal” temperatures across southern and central Manitoba. Unfortunately, it is taking me longer than I thought to dig through the data and, I’ll have to admit, teaching has been a little more stressful than usual, which is cutting into some of my weather time. So instead, with the couple of rounds of mid-winter freezing rain that parts of our region have experienced, I thought it would be a good time to revisit this topic. Looking back, I think the last time I talked about this was in 2010.
Over the years I slowly accumulate questions that have been sent to me by readers or people I bump into (not much bumping into going on right now). One of the most interesting questions I received about freezing rain was the following:
Is it true that if water cannot expand it cannot freeze? Freezing rain occurs because the outer surface of a raindrop freezes first, which then prevents the inside water from expanding and freezing. Then, when this super-cooled droplet strikes a surface it breaks the ice sphere, and the water instantly freezes to whatever it strikes.
Looking at the question, I immediately said to myself, that’s not how freezing rain occurs, but I always find that I really have to dig deep to try and explain what is actually happening. I knew we can have super-cooled droplets of water, but how and why these droplets form usually leaves me scratching my head. So, whenever this happens I either jump to the internet or, even better, I go and grab one of the best textbooks on weather, called Meteorology Today, and as expected, it had the answer. Quoting straight from the textbook:
“Pure ice melts when the temperature is raised to 0 C, but water does not necessarily freeze until temperatures fall well below that value. Over large bodies of fresh water, ice ordinarily forms when the temperature drops just slightly below 0 C. Yet, laboratory studies show that, the smaller the amount of pure water, the lower the temperature at which it freezes. A cloud droplet about 25 micrometres in diameter freezes spontaneously at -36 C, while a droplet whose diameter is only a few micrometres does not freeze until the temperature is near -40 C.
“The freezing of pure water is called spontaneous (homogeneous) nucleation. For spontaneous nucleation to occur, enough molecules within the water droplet must join in a rigid pattern to form a tiny ice crystal, or embryo. Once the embryo grows to a critical size, it acts as a nucleus. Other molecules in the droplet then attach themselves to this nucleus, and the whole water droplet freezes. The chance of an ice crystal growing to a critical size decreases as the volume of water decreases. Tiny ice crystals form at water temperatures just below freezing, but, for small volumes of water, molecular motions remain large enough to weaken their structure. The ice crystals simply form and break apart. So, for these smaller droplets, lower temperatures are required if the ice crystal is to grow to critical size before being broken apart by thermal agitation.” (Ahrens 1988; Meteorology Today Third Edition p. 234; West Publishing Company, St. Paul, Minn.).
Because of this, a cloud at around -10 C will only have about one ice crystal for every one million liquid droplets! The water is still in liquid form at temperatures well below 0 C. What actually happens, when the super-cooled droplets hit the ground, is that they spread out and combine, which allows for nucleation to occur rapidly, and the water freezes almost instantly — so now we know!
When you really think of it, if the outer part of the water drop froze first, it wouldn’t be strong enough to contain the expanding water inside; after all, freezing water is capable of breaking rocks and cement!
I should point out that the question, or rather the proposed answer, was not entirely wrong. Under laboratory conditions it is possible to prevent water from freezing at subzero temperatures by exerting outside pressure, but on that same note, if you create enough pressure (around 800+ MPa), we can have solid water at temperatures above 0 C — kind of weird.