How to break a drought

In a dry spell, storm systems don’t have much moisture to offer up as rain

Much of Manitoba last week had the temperatures and conditions favourable for thunderstorms to develop — but was short on humidity.

Sometimes I really hate my early deadline — and this is one of those times. As I write this on May 20, I’m watching the weather to see just how much rain we might see from the trough of low pressure moving through the province over the May long weekend. So far, the first part of this rain event has been a bit of a bust, with most regions reporting amounts less than 10 mm. There has been a lot of talk about this system being a “billion-dollar” rain event. For that to come true, most areas would need to see a good slow soaking rain, with amounts pushing 50 mm. I am just not seeing that, but hopefully the couple of shots of moisture expected over the weekend will still bring some significant rainfall.

The one downside to this “wet” pattern shift that has taken place across our region is that the summer heat has been put on hold. That means the chances for severe summer weather are greatly diminished. If we do experience significant rainfall, once the heat returns, we will have the humidity and with it the possibility of severe weather.

This brings me to a weather topic that periodically surfaces, especially during droughts — that is, the hypothesis that dry weather promotes or aids more dry weather, and wet weather promotes or aids more wet weather.

There is some truth to this. We had the temperatures and conditions favourable for thunderstorms to develop last week but we were lacking humidity. I know I said we would continue our look at severe summer weather, but I think I will spend the rest of this article looking into this topic a little closer.

Feedback loop

Humidity in the air comes from several sources. The first major source is the oceans, and in particular, the Gulf of Mexico. Since we’re nowhere near an ocean this humidity must travel a long way. If a storm system develops between us and the source of the humidity, that storm system can “rob” us of the moisture and have it fall as rain elsewhere. If conditions are dry between us and the source of moisture, a lot of that moisture is used to moisten the dry air, leaving less moisture when it finally arrives.

Another source of humidity comes from evaporation and transpiration, or evapotranspiration. Evaporation is water evaporating from surface water sources, and transpiration is water being released from actively growing plants. Under normal conditions we do not have surface water sources for evaporation. If there was a lot of snow in the winter resulting in spring flooding, there can be a lot of surface water available to help humidify the air. One thing that is missing every spring is active plant growth. I do not think I have to point out that plants are amazing water pumps, pulling moisture out of the soil, then transpiring it through their leaves and into the air. For example, an acre of corn, under ideal conditions, can transpire over 11,000 litres of water a day. Since we do not have active plant growth until late May or early June, we do not currently have this as a source of humidity.

OK, where was I going with this? Oh yes — rain brings rain and drought brings more drought. If we are in drought conditions such as we have been this spring, and if these drought conditions are widespread, which they are, then you can start to see how it can be difficult to have large amounts of atmospheric moisture. Any systems or storms that do develop do not have a lot of moisture to work with, which makes it difficult to produce significant rain. Not much rain and it gets drier; the drier it gets, the tougher it gets to have a supply of available atmospheric moisture; and the cycle continues. This is known as a positive feedback loop.

On the other side of the coin, if we have lots of water sitting around and plants are actively growing, systems can tap into this extra moisture, which may lead to more rain, which then keeps it wet, which leads to more moisture in the air, and so on. The one thing we must keep in mind with this scenario is that for rain to fall, the right atmospheric conditions must be in place. Without these conditions, the moist air mass will move off and its moisture will be transported somewhere else, where it will eventually fall as rain. If we have enough days in a row without the right atmospheric conditions for rain, eventually we dry out and we are back to drier conditions. As one of my university professors pointed out, if wet surfaces promote rainfall, then it should always be raining over the oceans. Some areas and times it is almost always raining, but other areas and times, the conditions are not right for rain and it is not raining.

So, has the dry weather resulted in less rainfall and an aggravation of dry conditions? To a point, yes, but with the right conditions in place we can still get rain and lots of it. Otherwise, we would never break out of a drought cycle.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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