BLOG: If you see a shelf cloud, brace yourself!

BLOG: If you see a shelf cloud, brace yourself!

CHARLOTTE, NC (WBTV) - My latest blog is based on the best question I had earlier today. It is rather long, but very descriptive. Read the following, then I will answer David's question:

I thought I would pass on some unusual cloud photos that I took while delivering a boat on an offshore passage from Palm Beach, FL up to Charleston, SC on Friday and Saturday. 

We were traveling about 30 miles offshore and these photos were taken when we were abeam Cape Canaveral or no more than 10 miles north.

I had never seen this sort of cloud formation and thought you would find them interesting. As we traveled north the clouds overtook us and the ambient temperature dropped from 82 down to 58 over a period of no more than 2 minutes as shown on the boat's weather instruments and wind speed increased from 21 knots to 43. 

There was no rain and within 20 minutes the sky had cleared and sun was back out. I had never experienced this degree of temperature drop over such a short period.

You will note that the cloud formation was curved with layered clouds and a consistently curled perimeter edge. It looked very much like the edge of a hurricane in that it was a perfectly defined edge with clear sky outside of the cloud area.

Any wisdom on your take on the cloud type is appreciated.

Best regards,
David Sidbury

Hi David,

Great shots, and what you saw and experienced can be just as easily observed over land. Those are called shelf clouds, or roll clouds. They form on the leading edge of an outflow boundary being ejected from the base of a shower, or more typically a thunderstorm.

As the rain falls in a storm, it not only drags down cold air from aloft, but it also loses heat on the way down from evaporational cooling. With all this cool air sinking and slamming into the surface, it spreads out, most prominently in the direction the storm is moving, thus the term, forward flank downdraft.

As the cool air spreads out, the leading edge acts like a mini cold front, and bulldozes the warm air ahead of it upward. As that surrounding warm air is rapidly hoisted upward, it cools, its water vapor condenses, and forms that shelf cloud.

Indeed the shelf cloud is a very good indicator and locator of where those winds are surging, so as that cloud passes over you, you will definitely notice the cool wind associated with it. So no surprise you experienced both the temperature drop, and the increased wind.