A tale about the tail end of a storm
Some of you might be thinking, what, more cloud photos? Yes, but I’d rather look at them as science lessons in the sky.
Last Tuesday, I received an email from Cyndie Mackinnon. Attached were six incredible photos taken on Tuesday, June 9. Cyndie wanted to know what caused this unusual, not to mention ominous, cloud over Middle River Cape Breton, N.S. I immediately checked the past weather data for Cape Breton Island. Hourly observations indicated that a cold A front had come through between 8 and 9 p.m. There was a significant temperature drop, the wind veered from the south to the west and the barometric pressure started to rise.
I went back to some of my Facebook posts from that day and located my screen capture of the storm cells over Cape Breton that afternoon. When Cyndie snapped the photo, the thunderstorms had all but dissipated and that’s the key to the mystery. Before I explain what you see in the photo, let me back up a little and talk about the mature stage of a thunderstorm. During the mature stage, the heaviest rain and occasionally hail fall from the storm. As long as the updraft can keep feeding the thunderstorm warm, humid air, it will continue to grow and intensify. However, the downdraft will usually end up killing off the thunderstorm as it cuts off the updraft’s supply of warm, moist air. Once this happens, the storm goes into its last stage — the dissipating stage.
During dissipation, when this photo was taken, the updraft is very weak and the downdraft is the main dominant force in the thunderstorm. You can think of a downdraft as a pocket of cool air in the cloud that is rapidly falling toward the earth’s surface. Downdrafts can change the
temperature rapidly in an area in a short amount of time. Downdrafts cause moist warm air to rise, cool below its dew point and form a cloud. If there is sufficient moisture in the air, a lowaltitude cloud called an arcus cloud can form. The arcus cloud is almost horizontal and does not touch the ground.
And that’s what Cyndie and many of her neighbours saw overhead on June 9.