Originally, I was going to discuss the drought that has been impacting central and eastern parts of Manitoba, but thanks to some significant rains over the last week, the drought conditions, at least for the short term, have been alleviated. The rainfall did get me thinking about precipitation in general, so I dug back to see when was the last time I took an in-depth look at precipitation and was surprised to find that it has been over 12 years!
Surprisingly, very few clouds produce precipitation, so if this is the case, what has to happen within the cloud for rain, snow, sleet or hail to form? Over the next couple of issues we are going to look at this question and examine the different types of precipitation and how they form.
Before we can look at the different types of precipitation we need to examine why most clouds don’t produce any precipitation; after all, these clouds are filled with billions of tiny water droplets or ice crystals. When you think about it, you can’t help but wonder why they don’t fall to the ground, but instead stay suspended in the air?
As you may know, cloud droplets are incredibly tiny — so small that on their own, they will fall at a rate of about one kilometre per hour. If most clouds are located one to three kilometres high (at least), it would take a typical cloud droplet several hours to fall to the ground. During that time, if the droplet encounters air that is not saturated (100 per cent humidity) the droplet would quickly evaporate. Also, since the droplet is falling at such a slow rate, it wouldn’t take much of an updraft to keep the droplet from falling. Combine these two factors and you can see why these droplets don’t make it to the ground.
The question, then, is: how do we get the droplets big enough so that they can overcome any updrafts and not evaporate on the way down? There are two different ways this can happen. The first way is known as collision and coalescence. This type of precipitation formation is found primarily in warm clouds — clouds that are warmer than 0 C. In our neck of the woods, we only see this in the summertime and even then, only in certain clouds. A good example of rainfall from this process was the widespread soaking rains that fell on July 9.
In this precipitation-forming process, the name, collision-coalescence, almost says it all. Within a warm cloud we find billions of tiny water droplets. As they are blown around by wind and updrafts, some of the droplets collide and combine together (coalesce), getting bigger each time this happens. As the droplets get bigger, the chance of colliding with other droplets increases, so once a droplet begins to grow it tends to grow fairly quickly. To turn into a raindrop, our little droplet would have to make a lot of collisions, as a typical raindrop is about one million times larger (by volume) than a typical cloud droplet.
OK, so in warm clouds, droplets can collide together to eventually form a raindrop. These raindrops are large enough to now overcome any downdrafts and fall to Earth. This still does not explain why very few of these clouds produce rain — heck, so far, it sounds like all these clouds should produce rain.
The reason they don’t is tied into the moisture supply for the cloud. Since it takes about a million droplets to produce a raindrop we need to have a supply of moisture coming into the cloud to replace all those droplets that have been “sucked up” into a raindrop. If this doesn’t happen, or is not happening fast enough, a couple of things will happen. First, it won’t take long before no new raindrops are being formed. Secondly, of the raindrops that do form, they will begin to evaporate to replace the droplets that absorbed in the first place. This will cause them to shrink in size and they will no longer be able to fall to the ground.
Even if all of this comes together; we have a good supply of moisture, droplets are colliding and forming raindrops, and the raindrops are growing big enough to overcome any updrafts and fall towards the Earth, it still doesn’t mean it will rain. The final key to this puzzle lies within the air between the cloud and the Earth. If the air is dry, all the rain could evaporate before it reaches the ground. Remember that the raindrops typically have to fall over two kilometres, and for the typical raindrop that means a travel time of five to 20 minutes, which is plenty of time for the raindrop to evaporate.
This evaporation of rain as it falls happens more often than you would think. When it happens we give it the name virga, and we can often see it as a wispy trail coming down from the clouds that disappears before reaching the ground. When the air below the cloud is too dry to allow for raindrops to make it to the ground, it doesn’t always mean we won’t see rain from the clouds. The evaporating raindrops will eventually saturate the air below the clouds if the rain-producing clouds or system can stay around long enough. Once this occurs, the raindrops will make it to the ground — and we now have rain!
Next article we will continue our examination of precipitation by looking at how precipitation forms in cold clouds — the type we see around here most of the time.