Basic SLC Definition
Scary-looking clouds are non-tornadic or non-funnel cloud formations or fragments or precipitation that appear to resemble true funnel clouds or true tornadoes. Meteorologically, tornadoes and funnel clouds are two different animals and these terms should never be interchanged to describe the same weather phenomena (more on this later).
Most scary-looking clouds are located at the bottom of storm clouds. Due to hills and trees blocking your field of view, they may even appear to touch the ground (see Jerry's picture below). These kinds of clouds look scary to some people who might call them in as funnel clouds, or even tornadoes, to the 911 Dispatchers at the local Sheriff Department. This results in false funnel cloud or false tornado reports being relayed to the National Weather Service.
In other cases where the scary-looking clouds are located at higher levels in non-storm clouds, the clouds are scary because they are dark or have a weird shape or color. On occasions, even precipitation cores or rain shafts extending from cloud base down to the ground may be scary to some people, depending on the colors or darkness of the rain and/or nearby clouds and sky.
Combining all of these possibilities, scary-looking clouds imply to some people that bad storms or funnel clouds or tornadoes must be right around the corner. So these people call 911 Dispatchers or the NWS.
Below is an excellent example of a Scary-Looking Cloud (hint - it wasn't rotating and there was no damage!).
Science Behind SLCs
1. SHELF CLOUD BASICS - Most false tornado and false funnel cloud reports are associated with shelf clouds. They are a low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). You can view many shelf cloud pictures in the gallery. As indicated in the image below, unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. The storm in the image below is moving left to right.
2. SHELF CLOUD SPECIFICS - Usually there is no persistent rotation on a vertical axis within shelf clouds or within individual cloud fragments that extend downward from the shelf cloud. The lack of persistent rotation means the lack of tornadoes! Shelf clouds often resemble snowplows, big waves, or tsunamis. Sometimes they may found only a couple hundred feet above the ground. Shelf clouds are the result of abundant moisture in the atmosphere and sufficient rising motion in the column of air between the ground and the predominate cloud base. The shelf cloud develops in response to the rain-cooled air associated with the downdraft under-cutting and rapidly lifting up lighter, warm, moist air found ahead of the storms. This lifting of moist air quickly results in visible condensation or cloud formation. These visible cloud fragments subsequently form the shelf cloud.
Shelf clouds can extend horizontally for many miles in length and are your visual indication that the downdraft portion of the thunderstorm line is approaching (behind the shelf cloud, relative to the storm motion). In lines of thunderstorms the updraft is on the forward side and the downdraft is on the backside of the line. The downdraft consists of three things: gusty winds, rain, and possibly hail. These downdraft phenomena are found in the purple-colored area below the base of the storm in the image in Part 1 above. On occasions, the shelf cloud itself may resemble a stack of pancakes! Some examples are in the gallery.
Tornadoes rarely develop under or near the shelf cloud because of the lack of persistent, organized, rotation on a vertical axis on the front side of the line of storms. What most commonly happens behind shelf clouds is a mixture of outflow gusty downdraft winds, rain and maybe hail. The strongest of downdrafts are called "downbursts" which can produce hurricane-force, straight-line winds of 75 mph to over 100 mph at ground level, torrential rains, and near-zero visibility. The resultant damage can resemble damage associated with tornadoes!
In rare situations, atmospheric winds along and ahead of a line of storms will allow for the setup of a vertical rotation beneath and in the updraft which is on the forward side. This results in a tornado development. These kinds of tornadoes often are rain-wrapped and very difficult to see, even in daytime.
3. OTHER SCARY PHENOMENA - There are three other phenomena that might resemble tornadoes or funnel clouds, but are not:
A) dark rain shafts, or narrow columns of heavy rain,
B) the white color of a hail shaft, a column of hail extending from the ground to the cloud base, may generate a light-dark contrast with surrounding rain, resulting in what might appear to be a funnel cloud or a tornado to the untrained eye,
C) non-storm clouds in which wind currents or wind sheer combined with sun/shade angles and atmospheric moisture create weird cloud structural effects that resemble funnel clouds.
4. CASE STUDY - Below are a couple pictures of non-rotating cloud fragments taken in Marathon County by Paul Nellas. Looks scary, doesn't it? The funnel-shaped appendage on the left is just scud associated with cool, moist, outflow air seen in the background (rain shaft). Just to the right of the center of the picture on the left you can see some smaller, disconnected scud fragments that are also low-hanging. A close up of these smaller, non-rotating fragments is found in the second picture. These smaller fragments might look scary to some people and they may call the 911 dispatcher to report a tornado or funnel cloud. Hopefully, you will not be that person!
5. HOLDING BACK YOUR EMOTIONS - So, will you be fooled by scary-looking clouds? Trained severe weather spotters understand that they have to look at the SLC feature to determine if it is actually rotating itself. They have to take a deep breath, relax, and observe for a minute or two. The name of the game is to be 100% accurate - and not be the first person to call or radio in a report. Of course, if they are the 1st person to call or radio in a true funnel cloud or true tornado, then more power to them! Trained severe weather spotters have been told that if they can't figure out what they are looking at, then they shouldn't report!
Below is another good example of a scary-looking cloud, taken by John Romadka in Dane County. What thought enters you mind when you first see such a cloud feature? Are you thinking funnel cloud or a tornado? The SLC that John was looking at wasn't rotating, therefore it wasn't a funnel cloud or a tornado. But, it's scary - isn't it? Keep your emotions in check - take a deep breath and relax!