Clouds have been man’s best friend since he woke up in Africa, well, perhaps not his best friend, but they’ve always been around. Humans throughout history have paid attention to them in an effort to be forewarned about what weather conditions might lay ahead.
As time has gone on and people have got cleverer with technology and more engrossed in Candy Crush, we’ve let people like Michael Fish worry about the ins and outs of the weather and we wait with baited breath for the experts to tell us what will happen in the skies next. It’s time to take the power back.
The neglected cloud has continued unabated and unwatched for decades but only recently has it come back in to favour, albeit for the wrong reasons. Nowadays if someone looks at a cloud that seems “weird” they scream “chemtrail!” And thanks to the birth of camera phones, any weird looking cloud can be documented and shared instantly. All of a sudden people are looking at the sky again, and drawing their own conclusions.
You can’t decide a cloud looks “odd” if you don’t know anything about normal clouds. You can’t assume a cloud is laden with brainwashing chemicals because it’s a strange colour if you’ve never paid any attention to what colour clouds normally are.
So I thought I’d attempt to redress the balance and have a bit of a look at our wizened and watery friends in a bit more detail. But before we start predicting the weather with clouds… what are they?
Yes, I know it’s a bit pre-GCSE but there’s nothing like a nice recap to get you started. And, on top of that, loads of people think clouds are made of cotton wool and/or anthrax so it’s probably worth a second look.
Clouds are suspended droplets of water vapour and/or ice crystals depending on how high up they are. In a cubic metre of cumulus cloud (the big fluffy ones) there’s about a marble’s worth of water; each cumulus is an average of 1 km long. That means that an average cumulus cloud weighs about as much as 100 elephants. All of that just floating above your head.
Clouds form due to the atmosphere becoming saturated, and that can be for one of two reasons:
a) Cooling of the air – cool air can’t hold as much water as warm air, so as it gets colder the water that it’s holding onto condenses out and forms clouds.
b) Adding water vapour – if more water enters the atmosphere, e.g. from a large body of water being warmed up by the sun and evaporating, clouds will form.
So here are some ways that you can predict or guess what might be about to happen weather-wise. These aren’t exact rules as Michael Fish will tell you, but at least it gives you an idea and a sense of what these gentle behemoths are up to…
Cirrus are the long feathery ones you see that live high in the atmosphere, they’re made mostly of ice crystals. The name comes from the Latin word cirrus meaning ringlet of hair. They sit at around 5000-6000 m up and if they’re in the jet stream they can stretch the length of a continent.
If you see just a few of these stringy “mare’s tails” in the sky it means you’ll probably have a warm spell. However, if you start to see them increase in number and steadily get lower and thicker, you’re in for some rain.
If they form like this next photo, with bumps on top of the tendrils it’s called cirrus castellanus. This means there is instability in the upper atmosphere and there may be stormy trouble ahead.
And if they look like this:
… they’re called Kelvin-Helmholtz waves and that means that there’s a high level of wind shear up there, i.e. wind moving in different directions.
Cirrocumulus are also high level clouds, they normally show up at between 5 and 12 km up. Unlike most high level clouds they do contain a bit of liquid water alongside the ice. But this water is in a supercooled state. Cirrocumulus are sometimes referred to as “mackerel clouds” because of their similarity to the skin of mackerel(?):
Cirrocumulus are more blotchy than standard cirrus. If you’re watching a patch of it amongst cirrus clouds, you may see it start to grow in size and steadily take over the cirrus clouds. If cirrocumulus do expand in number this often means rain will follow in the next 8-10 hours.
If you see “virga” pictured below, that’s also a good predictor that a rainy front is on its way. Virga are essentially shafts of rain that evaporate before they reach the ground. Despite the rumours they’re nothing new and nothing to do with HAARP at all.
But, if the small patch of cirrocumulus doesn’t grow, you’ll probably be OK and you can relax and enjoy the sun.
Cirrostratus is a high level sheet of cloud that looks pretty boring. They’re thin and made of ice crystals. As above, you might notice “halos” around the sun (or moon) as the light shines through. The shape and colour of the halos are dependent on what shape the individual ice crystals are within the clouds. They’ve been used for centuries as a warning that rain may well be on its way:
If this sheet of cirrostratus has formed from stringy cirrus, i.e. the strands have all spread out into a sheet, this probably means rain is coming in the next 12-24 hours (or sooner if it’s fast-moving up there).
Contrails from aircraft – the newest type of cloud in the atmosphere – can sometimes spread out to form these clouds. You may also notice other clouds hanging in the air lower than the sheet of cirrostratus becoming flattened. This is because the ubiquitous sheet of cloud keeps things constant and stops air from rising and falling willy-nilly.
These are mid-level clouds (2000-6000 m) and appear as medium-sized fluff balls. They are generally made up of water droplets rather than ice. If you see these altocumulus start to grow taller into altocumulus castellanus you might be in for some thunderstorms later in the day. This growth of the cloud shows that there is convection in the middle of the atmosphere i.e. air being moved about and changing temperature as it rises and falls.
Another type of altocumulus cloud to note is the lenticular cloud. This type of cloud generally forms over mountain tops and looks like a disc. No, it’s not a UFO. Air rushes towards a mountain and is forced up the side, as it rises it cools and can no longer hold as much water so it is released into the atmosphere as a cloud. As the air passes over the tip of the mountain and sinks back down the other side it warms back up and stops producing the cloud.
These clouds are often referred to as standing lenticular because they remain in the exact same spot, unlike other clouds. But they are not truly static, air is still constantly moving up and over the mountain, it’s just that the water is only released from the air (and therefore visible as a cloud) as it cools near the top.
Cumulus clouds are low-level clouds (about 2000 m) and they’re probably the cloud that most people would bring to mind of if you asked them to imagine a cloud. Cumulo means ‘heap’ or ‘pile’ in Latin. They’re generally taken as a sign of good weather, but you still need to watch them, they’re crafty…
If cumulus start rising up into the atmosphere and turning into cumulonimbus you’re in for thunder and/or hail. Or at least a downpour. Cumulonimbus clouds grow upwards as warm moist air is sucked up inside of them. As the warm moist air zips upwards it cools and condenses into clouds up to 20 km in height. As the water collects high in the atmosphere it forms droplets and nuggets of ice, these then bump into each other and form bigger masses. Once the mass of a droplet is enough to overcome the strength of the warm updraft, they begin to fall. This downfall causes the winds associated with storms.
If the updraft in the cloud is strong enough these particles aren’t released down to earth for longer periods, this means that they’re given the chance to bump into more and more ice crystals and become fairly large chunks of ice. Normally the ice melts as it falls and we experience rain, but if the ice balls created are big enough they don’t have time to melt on their way down to the ground and we get hail. The stronger the updraft in the cumulonimbus, the bigger the hail stones.
The impressive looking “cap” or “anvil” on the top of a storm cloud is formed when the warm air rises until it hits an area of warmer air and can therefore climb no more. This warmer air is generally the tropopause (the area between the troposphere – our normal atmosphere – and the stratosphere). The air can’t rise any higher so it just spreads out.
These boring low-level sheets generally mean that there’s not a lot going on, maybe some drizzle. You can relax, but you can’t sunbathe.
Fog is stratus at ground level and is formed by a temperature inversion: instead of air higher up being colder it’s the other way round and there’s a pocket of cold air below a pocket of warm air.
What’s the difference between fog and mist? It’s just a case of visibility. If you can see less than 1 km then it’s fog, if you can see further, then it’s mist.
There’s plenty more to telling the weather from clouds, and as I said it’s not an exact science. But the more you take note of the clouds around you, the more likely you are to guess what’s going on.
Eyes to the skies people.
