With an early-summer heat wave impacting our region as I write, I thought we should take a short break from severe summer weather and re-examine this topic. I went into a fair bit of detail about summer heat waves last July, so I will only do a short summary this time around. When you think about it, we are not really taking a break from our topic, as heat and heat waves are a form of severe summer weather.
To start off, we need to define what a heat wave is, then look at the criteria that Environment Canada uses to define heat events. The best definition I could find is the same one I used last year:
“A basic definition of a heat wave implies that it is an extended period of unusually high atmosphere-related heat stress, which causes temporary modifications in lifestyle and which may have adverse health consequences for the affected population. Thus, although a heat wave is a meteorological event, it cannot be assessed without reference to human impacts.” — On the Definition of a Heat Wave by Peter J. Robinson, department of geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.
This basically states that while a heat wave is a meteorological event (abnormally warm temperatures), the impact that the heat has on people is key.
If you check Environment Canada’s website for a list of criteria for public weather alerts, you would find the following heat-related alerts for the Prairie provinces. In southern Manitoba and Saskatchewan, a heat warning or advisory is issued when two or more consecutive daytime maximum temperatures are expected to reach 32 C or warmer, and nighttime minimum temperatures are expected to remain at or above 16 C — or, when two or more consecutive days of humidex values are expected to reach 38 C or higher. Southern Alberta has the same criteria as for southern Manitoba and Saskatchewan, but without the mention of humidex values as this region rarely sees high humidity.
Over more northern regions, for Manitoba, the values are two or more days with daytime highs warmer than 29 C (and 16 C or warmer at night) or humidex values greater than 34 C. In northern Saskatchewan and Alberta, it is 29 C during the day and 14 C at night.
We now know the criteria — but what conditions need to come together to give us a heat wave? There are several different scenarios that can come together to give us conditions that will prompt a heat warning, but what I want to look at are the conditions that lead to not just a couple of days of heat, but the big heat waves that last for several days.
To get the long-lasting, intense, record-breaking heat waves, a couple of meteorological events must come together. First, we need a blocking pattern to develop, and the most typical pattern is the omega block. This pattern has upper-level lows sitting to our west and east with a ridge of high pressure in the middle. This is important because this pattern, as the name suggests, can become fairly stable and tend to sit in one place for several days.
The ridge of high pressure allows for a couple of things to happen. First, descending air inhibits the growth of clouds; this in turn means plenty of sunshine, and in the summer, sunshine means heat. On their own, sunny skies do not mean a heat wave; we see plenty of sunny days in a row without experiencing a heat wave.
The next part has to do with the strength of the high. When the high is strong we get very strong subsidence, or sinking air. As this air is pushed downward, it hits the ground and is compressed. Now, anyone who has used an air compressor, or even just a hand pump, knows that as you compress air you are forcing the air particles closer together; this in turn increases the rate of particle collisions, and these collisions transfer energy which we feel as heat. Don’t believe me? Grab a bike pump, give it 20 or so pumps, then feel the bottom of the pump — it’s hot, due to the compression of air.
So, when there is a strong ridge of high pressure over us, the compression of sinking air can dramatically heat the air and give us some truly warm days. Now, if the upper high is not that warm, then all this compressing and heating of the air won’t do that much to give us record-breaking temperatures. If the upper high is warm to begin with, this compression of air, combined with the additional heating of the sun, can really push the temperatures up.
In this issue’s forecast it looks as though another omega block is trying to develop across the central U.S.; I’m just not sure how far north the heat will push. Currently, the models are keeping us on the northern edge of this heat, but as with any type of weather system or pattern that can bring us adverse weather, we will need to keep an eye on this as there is the potential for record-breaking heat if it pushes farther north.
In our next issue we will continue our look at severe summer weather by going back to thunderstorms, where we will try to understand why they sometimes produce extreme wind gusts and much-dreaded hail.