As we slowly work our way towards winter I thought it might be time to begin our look at the sometimes dreaded ‘S word’ — snow.
Instead of just jumping straight into a discussion about snow I think we’ll take a bit of a slower approach that will hopefully help lessen the pain for some of you. In this article, 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 (in both summer and winter).
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But we first need to explore the idea of super-cooled water. All of us have experienced freezing rain, which usually happens when the temperature is just slightly below zero. This means that the surfaces the raindrops are falling on and freezing to are just a little below zero.
But 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? 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 to freeze onto. Just like water droplets need something to condense onto, ice crystals need something to freeze onto. The problem is that while there are large numbers of particles for water in the atmosphere to condense onto (condensation nuclei), there are very few particles for water to freeze onto (ice nuclei). For ice to form at temperatures just below zero, 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 it is colder than -40 C, the whole cloud will be made up of ice crystals.
OK, so 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, and that is 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 collide into either super-cooled water and grow larger (riming), or they collide into other ice crystals and grow larger (aggregation). Aggregation occurs best when cloud temperatures are only slightly below zero, 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.
Next time we’ll continue our discussion by looking at different forms of precipitation.
