Quelle: https://www.facebook.com/US.NationalWea ... istown.govThis photo was taken in Bean Station this afternoon and was submitted to us by the local media. At first glance it would be very easy to mistake this for a possible tornado. This picture presents a great educational opportunity.
First, if you observed this feature in real-time it would be pretty easy to tell if this were a tornado or not. You would want to look for signs of rotation, a wall clo
ud, and potential debris. In real-time you would also be able to note storm motion, which is also an important piece of the puzzle.
Trying to determine what this feature is, by picture alone, is much more difficult. Because this is a picture we can’t tell if it is rotating, we don’t know the storm motion, and it is hard to see if there is any debris. So we must look at other context clues to determine what this feature is.
First, there is no visible wall cloud in this picture. Second, we can tell that this is actually a shelf cloud. The arched blue line shows the shelf feature. This solves a large piece of the puzzle because shelf clouds are commonly associated with high winds and heavy rainfall but not tornadoes. We know that the heavy rainfall will always be behind the gusty winds of a shelf cloud and because of this we now know that the clouds are moving to the right in the picture (yellow arrow). This helps to explain why the feature is tilted the way it is (red arch). The winds are causing it to bow out.
So now that we know this is not a tornado, let’s explain the feature. As mentioned above, shelf clouds typically have gusty winds along the leading edge. These winds are produced by rain cooled air. So the gusty winds (air) along the leading edge of the shelf cloud are going to be much cooler and drier than the air out in out ahead of the storm. When air becomes saturated, clouds form. Cold air is easier to saturate than warm air.
In today’s case, there was enough moisture in the air to cause the air along the leading edge of the shelf cloud to condense and form a cloud along these outflow winds. However, it is rare that shelf clouds extend down to and come into contact with the ground.
This is why it is important to always look for other clues.
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