RV Weather – Types of Clouds to Watch For

One of the easiest ways to identify the approach of severe weather is to be able to recognize the most obvious visual cue: the presence of the types of clouds that spawn thunderstorms and tornadoes.

When severe weather is approaching, you may first notice a lot of the usually friendly-looking, fluffy white cumulus getting a lot thicker and starting to develop gray bottoms. These are called cumulus congestus.

Cumulus congestus cloudsThese occur when typical fair weather, fluffy, white cumulus develop dark bottoms and become numerous and close together.

As the cumulus congestus continue to grow and develop, with the heat of the day causing surface moisture to evaporate and rise into the air, you may notice some of the clouds growing into cumulonimbus, or thunderheads. These are the ones that look like big heads of cauliflower. As they keep growing, they begin to develop “towers” that pile high above the others around them. These are about to go supercellular.

Dramatic late afternoon lighting on towering cumulonimbus

As those towers rise, they hit the very cold, dense layer of air at the tropopause. They generally can’t rise above that layer, so they flatten out against it. Now they have become supercells. The most dramatic-looking of these can actually resemble a mushroom cloud, such as is seen in a nuclear blast. They’re quite intimidating, even scary.

A classic spreading flattop or anvil cloud. This is a mature supercell.
You can see another in the background beginning to flatten out on top.
This is how a spreading tower looks from its edge. You can see the bottom, the central tower, and the underside of the anvil head. Many people liken this type of appearance to a hydrogen bomb cloud. This is a dangerous storm.

If you find yourself underneath an anvil-head supercell, you won’t be able to see the flat top. All you will see is a large, flat, dark expanse of threatening-looking cloud, at least at first.

Directly under the anvil. That clear area to the lower right is called the dry slot, and it’s typically where rear flank downdraft is occurring. You can see in the lower right of the shot dust being kicked up by some downdrafts to the right, just out of frame. This is NOT a place you want to be in your rig, ever.

Beneath the anvil, you may see—on a very violent storm—a smaller area dropping down quite low from the middle. This is called a wall cloud, because from a distance, it just looks like a solid slab. In reality, it’s the bottom of the very dangerous, rotating core or mesocyclone of the supercell. If a tornado is going to form on this cloud, its funnel will drop from the wall cloud. Some of these have such clearly separated layers, they look like an upside-down birthday cake.

The bottom layer is the wall cloud. The layer right above it is called a shelf cloud, for obvious reasons. The inflow to the storm is sucking up dust from this dry field. If you ever see this, get the heck away from it, even if you have to abandon your rig. It may very well be getting ready to drop a funnel, and an RV is a death trap in a tornado.
In fact, this image shows the closest that supercell got to actually spawning a tornado. The wall cloud is extremely low and prominent here. It cycled for 11 hours over three states, before exploding in an amazing severe thunderstorm, complete with hail, mammatus clouds (see last image in this post) and a stunning rainbow.
If you see clouds with this aqua blue color in them, run! This indicates a heavy hail core. Nothing else causes this color in a storm. That gauzy gray area you see at the bottom of the cloud is an intense rain shaft and likely contains hail. Sometimes these hide tornadoes: You won’t see the condensation funnel we normally associate with tornadoes. These are called rain-wrapped tornadoes, and this kind of supercell is called a high-precipitation or HP supercell. If this storm had one and overran you, you would feel the tornado but not see it…and likely not live to tell about it. If you did survive, your rig would almost certainly not.

If you do need to run from a storm, try to discern which way it is headed, and drive away at a 90° angle, rather than running in the opposite direction. If you simply keep driving away from it in the direction it is headed, it could speed up and you may possibly not be able to drive fast enough to stay in front of it. If that happens, and you can’t find a road that will take you in another direction, you will get hit.

If you can’t tell which way the storm is headed, stand and face the storm. If you get hit by warm wind, it’s headed in your direction. If the wind is cold, it has most likely passed you already, and you’re experiencing rear flank downdraft (RFD). If it looks like it’s standing still but seems to be getting larger, it is headed straight for you and you usually have less than 10-15 minutes to get away.

If you see amazing-looking clouds like this at sunset, you are likely due for a storm that night. The sun sets in the west, and these thunderheads are between you and the sun. They break up the sun’s light and cause those “glory shafts” that are actually called crepuscular rays, since they usually occur toward twilight. In the continental U.S., most weather comes from the west (but not always), so it’s a pretty good bet this storm is headed your way. Soon after dark, you should start seeing distant lightning.

Sometimes—especially in late afternoon when most severe storms initiate—the sun is at such an angle, and there is so much moisture in the air, that a storm will cause light to refract and make the sky appear green. This is a very common occurrence that accompanies severe thunderstorms and especially tornadoes. If you see a green sky, you need to seek shelter. Do not stay in your rig until the storm has passed.

That squarish-looking part under this line of clouds may or may not be a wall cloud.
That gray area is not a tornado. It’s a microburst or downburst. The winds are rushing downward fast enough to bring down a jetliner (and they have). Afterward, the winds will rush outward, away from where they first hit the earth. These outrushing gusts are a form of straight-line wind, and can do as much damage as a tornado. You do NOT want to have your rig anywhere near this.
Though these clouds may look forbidding, they’re just loose, low clouds that zip along ahead of or behind a storm. They’re referred to as “storm scud.” That dark blue sky in the background, though, is the main storm.
These are called mammatus clouds, and they indicate extremely unstable air. They usually accompany severe storms, but may appear before or on the back end of it. Usually, it’s after the danger has passed, but you can’t count on that. If you drive under these and you haven’t yet been stormed on, you may well be driving right into a severe storm. It’s a good idea to pull over soon, check the radar on a good smartphone app, and either drive away from the storm or seek shelter if there’s no time to get away.

It’s great to be able to see all this online and in-depth. But what about when you’re out in the field, and either don’t have your smartphone with you, or can’t get cell reception? In that case, your best friend is an old traditional standby: a field guide.

This handy guide to cloud identification has sold hundreds of thousands of copies over the years, and contains a formula for using clouds to predict the weather with an amazing amount of accuracy.

This one’s called The Weather Wizard’s Cloud Book, and it’s all you’ll need to predict the weather the same way pioneers and farmers did way back when. The Weather Wizard’s Cloud Book offers a foolproof three-step system for predicting the weather. With amazing accuracy, this simple system can account for swiftly changing local weather developments more effectively than weather maps or official area forecasts, which are issued well in advance of weather conditions. Includes more than 120 photographs.

Next, we’ll review Nature’s most intense and powerful storm, the tornado.


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