.jpg)
In the last issue we discussed warm and cold clouds, and we learned that most of our local precipitation is produced in cold clouds, which means that most of it starts off as snow, even in the summer.
This week we are going to look at the different types of precipitation we experience and examine how each type forms.
Next on our list of precipitation types is sleet or ice pellets.
This form of precipitation is created when raindrops freeze in the air before they reach the ground.
It does not occur that often, and when it does, it can sometimes be mistaken for small hail (remember, hail forms in thunderstorms).
Sleet begins as snow and then melts into rain as it encounters warm air.
Usually, once a snowflake melts into rain, the air below the cloud will remains warm and we will only see rain, but sometimes there is a lower layer of air that is colder than 0 C.
Once the raindrop encounters this layer, if it is deep enough, the raindrop will freeze and hit the ground as a transparent or semi-transparent ice pellet. These pellets are usually less than five millimetres in diameter.
If the same process occurs but the low layer of cold air is not deep enough, the falling raindrop will not have enough time to freeze.
As the drops fall through the layer of freezing air, they become super cooled — water that is below zero but still a liquid.
When the drops hit the surface, they form a thin film of water that freezes very quickly, creating that smooth continuous coating of ice that creates really nasty driving conditions. This is another way that freezing rain can form. We examined the other way in last week’s article.
The next two types of precipitation are graupel (snow pellets) and hail. Both types of precipitation form in similar ways but need much different conditions for us to see them fall to the ground.
These forms of precipitation begin their lives as a snowflake or ice crystal that grow by the process of riming, which is layers of ice forming around the ice crystal.
Prairie rain was a mixed bag – This week’s weather map shows the per cent of average precipitation that has fallen across the Prairies over the 30 day period ending on April 23. It was a fairly wet period across much of southern and central Alberta, with several regions seeing more than 200 per cent of average. A band stretching across the northern agricultural regions from northwestern Alberta southeastward to the Manitoba Interlake regions saw well below average amounts.
These snow pellets then fall to the ground as hard white little balls that kind of look like miniature hailstones (graupel).
Since the centre of the pellet has a large amount of air in it, the pellets are fairly light and rarely reach velocities greater than 10 km-h.
Under the right conditions (thunderstorms), the graupel can undergo a series of trips within a storm, where it is carried upwards into a freezing region of the cloud and then back again into a warm region.
Each time, it gains a layer of water in the warm region and then that layer freezes in the cold region. In very strong thunderstorms, hailstones can grow to three centimetres or more in diameter, which can give you a final velocity of more than 110 km-h.