The month of September has started hot, with temperatures getting up into the 30s at times, making it feel more like the heat waves the region endured during July and August. One difference between those heatwaves and the one that just ended, however, has been the overnight lows.
While we have seen temperatures of 32.5° on September 3rd and 32.7° yesterday, the overnight lows were 9.5° and 10.8° respectively. In August the hottest day of the month, August 20th, saw a daytime high of 34.3° with the coolest temperature recorded that day being 19°. In July, the warmest day was the 25th, with a high of 39.1°, but the overnight low was 18.9°.
So, why is there such a discrepancy between overnight lows and daytime highs this month?
It all concerns the sun, humidity, and a few other factors, explained Natalie Hasell. She is a meteorologist with Environment and Climate Change Canada.
The sun is starting to rise later, and set earlier, which means fewer hours of the sun warming the ground. Then, with the source of heat gone, the ground starts to lose heat, radiating it into the sky. The principle of heat transfer means the air above the ground then cools, then the layer of air above that, until you reach a point there isn’t any more heat transfer. This ends up in what is called an inversion.
“But there are a couple of other things that we need to look at,” Hasell added. “If you had cloudy skies, then heat that gets radiated from the ground kind of gets reabsorbed by the clouds, and then re-radiated down, so that would allow for slightly warmer temperatures in the air.”
Another factor is the wind. This can mix things up so that the warmer temperatures aloft get mixed back down with the lower air layers. However, when the skies are clear, and the winds are light, it depends on things such as humidity.
“If you have a very humid air mass, your temperature when it falls, will reach dew point, and it will reach it faster if you have a very humid air mass because your dew points are high, so you’re going to reach that high temperature relatively quickly.”
The dew point is called that because that is the temperature where dew forms as the moisture in the air condenses into droplets on whatever surface it can find. When this happens, the cooling off stops because of a latent heat release, and that adds heat into the system.
“It kind of moderates the inversion a little but stops it at a cooler temperature than what we saw in August and July,” she continued. “Your air mass is still defined by this hot air, but right now it’s much drier than it was when we were talking about the high-heat events earlier in the year.”
However, the wind and clouds again could play a factor, as they cause the inversion to be stopped, meaning the temperatures may not even make it to the dew point.
“This weather forecasting business, there’s a lot of factors to pay attention to,” Hasell chuckled.
While the heat that the Weyburn area was enduring over the weekend has moved on, temperatures are expected to stay above seasonal for much of the week. There is also the possibility daytime highs could get close to the 30s by the time we reach the weekend, as well. Stay up to date with the latest forecast by visiting the Discover Weyburn Weather page.