Monday, April 09, 2007
FAIR WEATHER FUNNEL
It all started with an email we received on Saturday April 7th; a tornado had been sighted. High pressure was in control and the closest thunderstorms were several hundred miles away, so what could it have been?
After a pleasant email exchange and a great description of the event, it was absolutely apparent that indeed something happened around 10:50 am that Saturday morning in Benton county. It was time to do some atmospheric investigating. Satellite imagery confirmed that we just had a few high clouds in place but shape-wise they were indicative of a strong jet stream. Archived upper air maps for Saturday, showed that not only were winds aloft in excess of 100 kts., but due to the positioning of the jet stream, NW AR was in a sweet spot for upper level divergence, inferring rising-motion at the surface.
On Saturday morning we tied records and temps were still in the upper 20s at the time the funnels were observed. With good breaks of sun earlier in the morning it is likely that strong heating of the ground, perhaps over a recently plowed field, set a "blob" of air rising. On a micro scale the atmosphere was very unstable with this warm pocket sitting below and being surrounded by cold air. A little eddy of spinning air, possibly originating from something as simple as a breeze blowing around a barn, hit that rising blob. At this point a process known as vortex stretching took place. As the spin rose and became elongated, due to the conservation of angular momentum, the speed of the funnel increased. As the rising air condensed and perhaps some dust and debris became suspended, it became visible.
After a pleasant email exchange and a great description of the event, it was absolutely apparent that indeed something happened around 10:50 am that Saturday morning in Benton county. It was time to do some atmospheric investigating. Satellite imagery confirmed that we just had a few high clouds in place but shape-wise they were indicative of a strong jet stream. Archived upper air maps for Saturday, showed that not only were winds aloft in excess of 100 kts., but due to the positioning of the jet stream, NW AR was in a sweet spot for upper level divergence, inferring rising-motion at the surface.
On Saturday morning we tied records and temps were still in the upper 20s at the time the funnels were observed. With good breaks of sun earlier in the morning it is likely that strong heating of the ground, perhaps over a recently plowed field, set a "blob" of air rising. On a micro scale the atmosphere was very unstable with this warm pocket sitting below and being surrounded by cold air. A little eddy of spinning air, possibly originating from something as simple as a breeze blowing around a barn, hit that rising blob. At this point a process known as vortex stretching took place. As the spin rose and became elongated, due to the conservation of angular momentum, the speed of the funnel increased. As the rising air condensed and perhaps some dust and debris became suspended, it became visible.
We were told that the main funnel lasted for over ten minutes and that a secondary one eventually intersected with the main. The picture above, which was toned down to better highlight the funnels was captured as part of this meeting took place. Shortly there-after, the funnels dissipated.
A big THANKS goes out to the fine folks who shared this neat meteorological occurrence with us.
Perhaps there were other viewers of this awesome sight too.
Posted by Ted Zarras at 8:09 PM
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