June 29, 2015: The conditions in the atmosphere this afternoon brought some interesting weather to the Stateline. We saw funnel clouds of a few types around our area. To help with any curiosity or confusion over what we saw today, here’s the difference between cold air funnel clouds and tornadic funnel clouds (basically, two very different set of conditions in the atmosphere):
First off, what does a cold air funnel look like? A few viewers provided snapshots of some of the cold air funnels in our area today.
Notice a couple things about the funnel clouds. First off, see how high they are in the sky? And how small and puny they look? These are some of the distinguishing characteristics of a cold air funnel cloud. They are “high-based”, as we meteorologists like to call them, or that they form pretty far off the ground and high up in the storm or clouds. Two, they look like a much bigger problem, but only get to be about that size, and remain small, weak-looking, and are slowly rotating. Cold air funnel clouds rarely reach the ground, and if they do, there is minimal to no damage. They only appear threatening, but are basically harmless.
How do they form? There has to be a shallow layer of cold air, BEHIND a cold front (this is a key difference from tornadoes, in that tornado-producing storms usually form along or AHEAD of a front). There also has to be a little wind shear, or winds changing direction as you go up away from the ground. As the air from the surface rises, it spins a little in the weak shear, and if that air makes it to the cloud and fully condenses, you see a little, weak rotating cloud under the storm.
This is different from a tornado, in that a tornado needs much stronger wind shear, as well as plenty of warm, moist air to raise the instability in the atmosphere. Unstable air can rise very quickly, getting the base of the storm to be lower. This allows a much stronger rotation to be close to the surface, causing damage winds.
In summary, a cold air funnel forms much higher in the sky, is weakly rotating, and doesn’t pose much of a threat. A rotating funnel cloud spinning much faster and is much closer to the ground is most likely going to result in a tornado.
We saw both of these conditions today- the air near Rockford was cooler and weakly sheared, while the air in Lee Co. where we had a tornado warning for a while was much more humid, a little warmer, and had better shear.
So, how do you know the difference, and what should you do if you see a funnel cloud? Treat all funnel clouds with respect, and keep plenty of distance between you and them. The best advice is if you see a ROTATING (sometimes clouds hang low off of the storm, look like a funnel, but are harmless because they don’t rotate) storm cloud, check in with us online, on Facebook or Twitter, on-air, etc. and etc., or check to see if you weather radio is going off, your phone has an emergency alert on it, etc. We or the National Weather Service will let you know if that funnel cloud poses a threat or not. And remember, conditions can change in a hurry, or vary from location to location. Earlier in the day, the cold air funnels to the north did not pose a threat, but later in the afternoon there was a different set of conditions that sparked a potential tornado in Lee Co. When in doubt, play it safe, get inside, and check in with us.
This post was written by Alex Kirchner on June 29, 2015