As we slowly work our way toward winter and several locations across the Prairies have seen snow, I thought it might be time to revisit the icy topic of snow. Instead of jumping straight into our discussion we’ll look at the process that creates precipitation in cold clouds, which, in our part of the world, is the predominant method of precipitation formation whether it is summer or winter.
Before we look at the precipitation process in cold clouds, we need to explore the idea of super-cooled water. All of us at some point have experienced freezing rain. Under most occurrences of freezing rain we find temperatures just slightly below 0 C. This means that the surfaces onto which the raindrops are falling and freezing are just a little below 0 C. Now, if you have ever dropped some cold water onto a freezing surface you would notice that the water does not freeze instantaneously (unless the surface is very cold). So why, then, does the raindrop falling from the sky freeze as soon as it hits a solid surface? This happens because that falling raindrop was super-cooled — the liquid water in the raindrop is actually below the freezing point!
How is this possible? Well, we all learned that water behaves differently than most other substances on Earth. While other substances are most dense when they become solid, water is most dense at +4 C. If water didn’t behave in this way, we wouldn’t be here. Just think what would happen to rivers, lakes and oceans if ice were heavier than water! Well, the uniqueness of water doesn’t end there. Strangely enough, when we are looking at water in the atmosphere, it doesn’t normally freeze at 0 C.
For atmospheric water to freeze, it has to have something onto which it can freeze. Just like water droplets need something to condense onto, ice crystals need something to freeze onto. The problem is that in the atmosphere, there are large numbers of particles for water to condense onto (condensation nuclei) but very few particles for water to freeze onto (ice nuclei). For ice to form (at temperatures just below 0 C) you need a six-sided structure, and there are not many of those around. Ice crystals themselves are six sided, but where do we get the ice crystal in the first place? Because of this, if the cloud temperature is warmer than -4 C, the cloud will be made up of super-cooled water. If we cool the cloud down to around -10 C, ice crystals will begin to form even if there are no ice nuclei, so at these temperatures the cloud will consist of a mixture of ice crystals and super-cooled water. Once temperatures fall to -30 C, the cloud will consist almost entirely of ice crystals, and if we are colder than -40 C the entire cloud will be made up of ice crystals.
Riming and aggregation
OK, now we know that within cold clouds we will usually have a combination of ice crystals and super-cooled water. How does this tie into the creation of precipitation in cold clouds?
In warm clouds, rain develops through a process known as collision and coalescence, where water droplets collide and grow together until they are big enough to fall to the ground. In a cold cloud we have a similar process (although it’s called something different) but before this can occur another process has to work its magic: the Bergeron process.
The Bergeron process relies on another unique property of water: If there is just enough water vapour in the air to keep a super-cooled water droplet from evaporating, then there is more than enough water vapour in the air for an ice crystal to grow larger! Because the saturation vapour pressure over ice is lower than that over water, ice crystals will attract water vapour more readily than water droplets will.
Our cold cloud now has ice crystals in it and these ice crystals are growing. As the crystals grow they pull water vapour from the atmosphere. As the amount of water vapour in the atmosphere drops, our super-cooled droplets will begin to evaporate to help make up the difference. These droplets evaporate and the ice crystals continue to grow at the expense of the super-cooled water droplets. After a while, the cloud consists mostly of ice crystals. This process by itself would only result in light amounts of precipitation, though; for heavier precipitation, we need the second process to kick in. In a cold cloud we call this second process riming and aggregation.
As I pointed out earlier, this second process is much like the collision and coalescence process in warm clouds. Ice crystals fall and either collide into super-cooled water and grow larger (riming), or collide into other ice crystals and grow larger (aggregation). Aggregation occurs best when cloud temperatures are only slightly below 0 C, as the warmer temperatures allow the ice crystals to have a wet surface that helps other ice crystals stick to them. This is one of the reasons we see large snowflakes when it is relatively warm.