Today was unfortunately for me and I'm sure for countless other storm lovers across South Africa, another quiet sunny day. Don't get me wrong, I love sunshine, boring as it is. But there has been way too many sunny days in the past few weeks and I've quite frankly had enough of them. In the meantime, I'll post another storm from the past that blew me away. This is from the 22 November 2012.
And wow, what a day that was. It was a hot day on a Thursday and was the day of the Centurion Nikerun at Centurion mall. I saw this shelf cloud forming later that afternoon just after arriving in Centurion and decided to get snapping. The storm itself wasn't very severe in terms of it's overall intensity, but the shelf cloud was quite a beauty to watch. It was nowhere near as dramatic as Monday's shelf cloud, but still beautiful nevertheless.
 |
| Shelf cloud over Centurion in Pretoria, South Africa on 22 November 2012 |
It took on many forms as it slowly over Centurion, crossing the N14 highway over the hill and the Gautrain viaduct, the long grey structure cutting across the landscape. The thing about it that really caught my attention though was the odd looking lowering to the left of the images. I didn't know what it was but it quite a spectacle to watch. It appeared to be rotating and kept lowering and rising back into the cloud almost like a tornado. It certainly wasn't a tornado though, since shelf clouds don't produce tornadoes. At one point it got down so low it seemed to almost touch the trees in the distance.
It was quite an experience indeed standing there watching this beauty make its way across the Centurion landscape. The lightning was also quite impressive, streaking and dancing across the sky and the several number of Gautrains moving back and forth on the viaduct didn't seem to be much affected by the storm.
 |
| A wide view of the shelf cloud, also showing the size of the storm. |
So what exactly is a shelf cloud and how does it form? According to general descriptions, a shelf cloud is a low, horizontal wedge-shaped cloud, usually attached to the base of a severe thunderstorm. To understand How a shelf cloud forms, think of a cold front and how it's formed. Now think of it on a smaller scale, but with the same properties. That's pretty much how shelf clouds form. http://weathersavvy.com/Q-Clouds_ShelfCloud.html.
Shelf clouds are part of a family of clouds called arcus clouds, which also includes roll clouds. http://en.wikipedia.org/wiki/Arcus_cloud
Although they usually share the same general shape, as you can see from the three images below, shelf clouds still come in many different forms, sizes and intensity.
 |
| Shelf cloud over the city of Enschede in The Netherlands (Courtesy of John Kerstholt) |
 |
| Shelf cloud approaching Sydney, Australia (Courtesy of Nick Moir) |
 |
| Very odd looking, but menacing shelf cloud in Crawfordsville, Iowa (Courtesy of Nick Nolte) |
Shelf clouds are very often mistaken for wall clouds and mesocyclones, and I know, I've made that mistake myself on numerous occasions. They may look similar in appearance by they couldn't be more different. Firstly, they form in different areas of thunderstorms. Shelf clouds form at the front, or leading edge of a thunderstorm and are associated with downdrafts and outflows. Wall clouds on the one hand form at the rear of severe thunderstorms, an area that is usually rain free and where the updrafts occur and are usually associated with inflow. http://vermilionweather.com/wallshelf.php
No comments:
Post a Comment