Heavy rain and thunderstorms1/11/2024 Let us say that in a really moist atmosphere, about a quarter of a one-by-one-by-10,000-metre column of air is close to having 100 per cent humidity, and the average air temperature of this part of the column is about 20 C. While the amount of moisture within this column of air is not vertically uniform, we can still make some back-of-the-envelope calculations to get an idea of just how much water can be available in the atmosphere to form rain. We also have to remember that the atmosphere is vertical, with heights during the thunderstorm season typically reaching 10,000 metres. This would mean that a storm system forming on a hot summer day may have up to six times as much water to work with as one that formed at temperatures near 0 C. At 0 C that amount increases to around five grams at 20 C it is around 17 grams, and it jumps all the way up to 30 grams per cubic metre at 30 C. At -20 C the air can hold about one gram of water for every cubic metre. As you probably already know, warm air can hold a lot more moisture than cold air. As we have already seen this April, while we do not necessarily need thunderstorms to bring heavy rain events to our region, generally, most of our really big rainfalls are associated with thunderstorms. With the storm system that brought heavy rains and thunderstorms to parts of southern Manitoba at the end of last week, I thought we should start our look at severe summer events and specifically at what causes heavy rains. Thunderstorms can bring a wide variety of severe weather with them: heavy rains, hail, high winds, lightning and, on some occasions, tornadoes. In the last issue we went into some detail on how thunderstorms develop and how they can go from your average-summer-day thunderstorm to a severe thunderstorm.
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