In my last article I said that we’d continue our discussion on cold weather precipitation. But before diving into that icy topic, I want to take a look at the weather so far this fall and see if the long-range winter forecasts have changed.
After experiencing a cooler- and wetter-than-average September across pretty much all of agricultural Alberta, October did an about-face. All three of the locations I use to develop a snapshot of monthly weather across Alberta (Calgary, Edmonton, and Peace River) saw temperatures that were between 2.2 C and 2.8 C above the long-term average. Mild weather at this time of the year is often accompanied by dry conditions and October was no exception, with all three locations reporting below-average amounts. (It should be noted that the Peace River region was just barely below average.)
Will these mild and dry conditions continue into November and December?
According to Environment Canada they will, along with near-average amounts of precipitation. The Old Farmer’s Almanac forecast goes in the opposite direction as it is calling for a below-average November and then a well-below-average December. On the precipitation side, it is calling for near- to slightly above-average amounts in November and then below-average amounts in December. The Canadian Farmer’s Almanac appears to be calling for cooler-than-average temperatures as it mentions cold weather several times. As for precipitation, it mentions rain or snow several times in November and then snowy and stormy conditions several times in December, so I think that would mean above-average amounts of precipitation. As for my forecast, I am still leaning towards a warmer- and drier-than-average period with occasional blasts of cold weather. The trouble with this kind of pattern is it can produce some fairly strong storm systems and at this time of the year, one strong storm can easily bring a month or two worth of precipitation.
OK, now on to cold weather precipitation.
In the last issue I discussed the general process that produces most of the precipitation in our region, and that was the Bergeron process. This is the process that allows for the rapid development of ice crystals that in turn leads to the development of snowflakes. While snow is the most common form of winter precipitation, it is not the only frozen form of precipitation we see. Before getting into the details on these forms of precipitation, let’s remove one type of frozen precipitation that just doesn’t belong (or form) in the winter — hail.
Hail, while being ice, does not form during cold weather. For hail to form you need strong up- and downdrafts in the atmosphere that can take an ice crystal repeatedly between the liquid and frozen areas of a cloud. This allows the buildup of ice into layers to create a hailstone. In order for this to happen, you need a warm section of cloud that contains large amounts of liquid water, along with strong updrafts, which are only found in thunderstorms. While hailstones are not a winter- or cold weather-type of precipitation there is a close relative that is similar to and often confused with hail — ice pellets.
Ice pellets, also known as sleet, look somewhat like hailstones as they are made of solid ice, but they are not large (tending to be only a few millimetres across) and do not form in the same way. Ice pellets are formed when falling snow encounters a shallow layer of warm air that either partially or entirely melts the snowflake. This melted flake then re-enters a layer of cold air near the ground where it freezes again forming the hard ice pellets. Ice pellets are hard enough that they will bounce when they hit the ground and are difficult to break, making them seem like hailstones.
The next form of frozen precipitation is the snow pellet. These form when a snowflake partially melts and then refreezes, creating a small layer of ice overtop of a small pellet of snow. These snow pellets can be hard enough to bounce when they hit the ground, but are soft enough to break easily. As well, where ice pellets will typically be clear, snow pellets are white due to the inner layer of snow.
Next issue we’ll take a break from our look at cold weather precipitation and instead look at some weather-related Christmas gift ideas.