In the last article we discussed rotating storms and venting at the top of storms. This week we will take a quick look at something called caps, but before that, let’s take up a bit of space looking at the drought that is currently affecting a large part of Alberta.
I have included two maps from the government of Alberta Agriculture and Rural Development website (www2.agric.gov.ab.ca/app10/acis/).These two maps show the cumulative precipitation compared to the long-term average. The first map covers this growing season (April 1 to June 23). This map shows that most of agricultural Alberta has seen less precipitation than average, with a sizable area seeing low to extremely low amounts. This map indicates that amounts this low are only experienced once every 25 years, while some reports are putting the current dry spell in at a one-in-50-year event.
The second map looks to be almost the same as the first map, however, it is showing the same data but for the last 365 days. This shows us that this current drought is not simply the result of low snowfall and a dry spring; rather, it is turning into a long-term event which is making it much worse.
Just what is causing this dry weather? I have to admit there does not seem to be one simple cause. Cold weather over the winter and during the spring kept any significant moisture to the south. Ridging in the upper atmosphere over the West Coast, combined with a trough of low pressure over central and eastern regions, seems to be keeping most of the moisture either to the north or the south. When lee side lows do form they just don’t seem to have the moisture or the dynamics to create significant storm systems until they have moved further east.
Finally, while thunderstorms are developing, their moisture has been hit and miss at best, as once again, large organized thunderstorms just do not have the right conditions to develop.
When will we see an end to this? I wish I knew. With the possibility of an El Nino developing over the next several months we may see a shift in the weather to a wetter pattern. Typically, El Nino does not have much of an impact across Canada during the summer, but winters in Western Canada tend to be warmer. Precipitation is not as consistent, but let’s hope that it brings us more rain and snow.
BACK TO THUNDERSTORMS
Now to finish off this issue’s article, let us continue our look at thunderstorms, and the affect a cap can have on thunderstorm development.
A cap is a weather term for a temperature inversion in the middle part of the atmosphere. This inversion prevents rising air from going any higher. In essence this cap or inversion acts like a lid trapping all the heat and energy close to the Earth’s surface. If the cap is strong enough nothing will happen. We will have a hot humid day, but any and all updrafts will be snuffed out before they can develop into a thunderstorm.
If the cap isn’t that strong, or the surface layer gets really warm, a really strong updraft can sometimes break through the cap. When this happens it is pretty much just like what will happen if the lid on a pot of boiling water breaks its seal – all the trapped energy comes rushing through and all heck can break loose. Instead of several thunderstorms developing at different times sharing and dissipating all the stored up energy, only one or two storms develop at roughly the same time and develop very quickly, tapping into the large amount of stored up energy…but more on that later.