While there are actually numerous ways to get an idea of what clouds are harbingers of good or bad weather, some reliable, some little more than old wives tales, it takes very little to acquaint yourself with techniques to pick bad weather clouds very quickly and easily. It would be a stretch to say you can read this article once and immediately identify every bad weather cloud which passes within your line of vision, but picking half of them out within a few days is reasonable. Over the course of a good active weather month you can increase that percentage even more until you appear to be a wizened old sage at the game.
While it is true some people have bodies which are sensitive enough to pick up on slight changes to the electromagnetic charge in the air created by impending bad weather, not all can. Color is an indicator as well but can’t be wholly relied upon since some bad weather clouds come dressed as good weather clouds as clouds constantly evolve and devolve into various forms at each of the three heights; low, middle, and high, it takes a keen eye to watch for changes. There are however some very easy tips to watch for to pick out the most basic bad weather clouds with little effort.
The first cloud to watch for is often called a cauliflower cloud (Cumulus congestus) because the tops of it simply look like a head of cauliflower. To help further identify this, or any cumulus cloud for that fact, hold your arm out straight and make a fist. If your fist blots out at least 85% of the cloud it is cumuliform. Move your fist down slightly so you just see the top, if it looks like cauliflower you can expect moderate to heavy rain showers.
Staying in the low clouds we move to cumulus clouds of vertical development commonly known as cumulonimbus. There are too types of cumulonimbus clouds to be aware of which are the two varieties of low cloud nine. By first employing the “fist technique” described above you can insure you are dealing with a cumuliform cloud. Next look at the base cloud. While each variety will have what appear to be hanging sacks at the base of the cloud, cumulonimbus mammatas will further develop an anvil top. The anvil top is immediately noticeable and associated with the most severe types of weather: rain, hail, snow, lightning, and most notably tornadoes.
Nimbostratus clouds also belong to the low cloud family and you can forget identifying them via the fist technique as these are huge, so much so in fact the tend to blot out the entire sky to the point you can’t even discern the edges. These will appear dark and ragged in formation almost as if patch-worked together. Nimbostratus clouds mean prolonged rain or snow.
In the middle cloud family we have the altocumulus cloud to deal with. Going back to the fist technique, amend it so that this tie you use only your thumb. If your thumb can blot out almost the entire cloud, then you have an altocumulus. For further clarification these clouds reside between 2,000-7,000m vertically. An altocumulus will appear grayish white as if a dirty and clean cotton ball were pressed together. The appearance of this cloud on humid mornings means afternoon thundershowers.
In the high cloud family (5,000-13,000m) we look for cirrostratus clouds which look like long thin white sheets that seem to span the entire sky. Due to the high content of ice crystals in these clouds the sun or moon shining through it will look like it has a halo which is an easy way to pick these out. If you hold your hand up at an arms length the halo will be about the width of your hand. The presence of cirrostratus clouds indicate rain or snow of varying intensity within 12-24 hours, particularly if there is a large amount of middle cloud activity.
The final cloud we look for in the high cloud family is the cirrocumulus. These look like long rows of cotton balls lined up side to side, or more accurately like fish scales, hence the name mackrel sky. While that is easily identifiable, if you have any confusion hold your arm out straight and this time measure the width of a “scale” against the tip of your pinkie finger. If it is about that size you undoubtedly have a cirrocumulus cloud. While cirrocumulus clouds don’t actually bring bad weather, they are a sure sign of colder weather.
With these few tips of what to look for in the sky you should be able to pick out the most important bad weather clouds with relative ease. Take a look at the sky a little each day, practice identifying each cloud types particular signatures, and in no time you will have it down pat.
Reference:
USAF/USN/USMC Weather Observers Manual, Block 1 Instruction
