Sitting in a field with a calm, blue sky above, and watching the clouds is a really calming thing to do. But the art of reading clouds gives vital clues about the weather, which can be useful for navigation and planning.
The Secret World of Weather by author and natural navigator Tristan Gooley is packed full of content related to the art of reading weather signs in nature.
Weather has a tendency to change very quickly in the UK. In the space of half an hour, the clouds will have completely changed shape, or dissipated.
Because of this, studying the weather at a local level is often avoided by meteorologists. They will usually place their equipment away from areas such as valleys, hills and rugged terrain.
Cloud spotting is a past time that can be enjoyed on a local level. The fact is that reading clouds and the signs they provide is a brilliant way to tune the senses in to the local environment.
Finding these patterns and signs can also be a very beneficial way of working with the natural world, rather than against it. This article will outline the lost art of spotting and reading clouds in the local environment.
What is it about clouds that makes them so fascinating?
The simple answer to this is that they form on a localised level. They’re then carried across from their source by winds, and due to their locality, it is difficult for weather forecasters to judge exactly where they are.
In addition, a very small area of land, such as a local field, or a city centre is able to influence the shapes and sizes of the clouds, which can generate a telling report of what the weather will be like, hour by hour.
There are many different types of clouds, and each of them tell a story of how the weather will change.
The humble cumulus cloud is more than just a heaped, puffy cotton wool like shape. The shape, size, and height can all play a role in whether the local weather will be fair or unsettled.
Cumulus clouds are very common over stretches of land which contain darker features such as trees which make up woodlands, or forests. In contrast, they are less likely to form above open flat land.
Woodland areas tend to be darker green, so they absorb more of the Sun’s heat. This in turn raises the temperature of the air above the woodland area. The density of the air will decrease, and the air will start to rise.
The technical term for the rising air is called a thermal, but it is easier to think of it in terms of the heated air above an area with a greater ability to absorb the Sun’s radiation.
As the air rises, the temperature lowers towards the dew point. The dew point refers to the point at which the air has become fully saturated with water vapour. At this point, water vapour condenses, and water droplets can now stick to other particles within the air and form cumulus clouds.
The cloud forms because as the water vapour reaches the dew point, the heat absorbed from the land has nowhere to go. The result is that heat energy is released as latent heat.
This is because the law of energy conservation states that energy cannot be created or destroyed. It therefore must go somewhere.
If latent heat continues to be released, the cloud will grow taller and taller because the air above will be heated, which will cause it to continue rising.
A cumulus cloud that is growing taller, indicates that the air is unstable, and it is likely that bad weather is imminent. As the air grows more humid, the base of the cloud lowers, which is why clouds are lower over oceans than on land.
In contrast, a cumulus cloud that has stayed flat, indicates that the air is stable, and the weather will remain dry and settled.
Cumulus clouds often fail to form in the morning, but they will form around midday to mid-afternoon as the Sun continues to warm the land. But if the clouds do not stop growing after mid-afternoon, then bad weather is due that evening.
To summarise, a flat cumulus cloud is a sign of fair weather, and a tall one is a sign of wet weather to come. The best time to read cumulus clouds is mid-afternoon
The dull looking stratus cloud doesn’t necessarily mean that rain will fall. This is especially the case if the air is stable. When it comes to natural signs, stratus clouds indicate longevity.
Stratus clouds tend to be white to light grey and will often cover the entire sky in the local area. This makes reading other signs more difficult because stratus clouds will hide other cloud types. The solution is to use other signs, such as plants, animals and the wind.
When stratus clouds are overhead, the air is stable so the clouds cannot grow upwards, and any change in the weather will be very gradual.
Some stratus clouds may carry light rain or drizzle but they cannot carry heavy downpours. If the weather is said to be overcast, then stratus clouds are the likely explanation.
The cirrus clouds are the highest clouds in the sky. The clouds themselves are complex but every one has a purpose.
High up in the sky, the air is extremely cold. Ice crystals form to create a ‘head’, before falling and leaving a trail. These look like comma shapes, and the trail lines are called fallstreaks.
These are very rarely straight, because wind speed and direction changes with altitude. The fallstreaks map the difference in wind speed and direction between the head and the air below it.
The shape of the fallstreak matters too. A gentle wind speed or gentle change in direction indicates a more stable sky. Fallstreaks which are in the same direction as the moving head and have a very slight bend is an indication that the weather is good and will continue.
But, a sharp curve in the comma shape of the cloud indicates a mare’s tail.
The mare’s tail is caused by a sudden sharp change in wind speed and direction. This is what is known as wind shear. Wind shear is often a natural hazard in aviation, but in nature, it indicates bad weather is near.
The commas can also be used to compare wind direction at the base of the cirrus cloud with wind direction lower in the sky. Good weather will continue if the direction has not changed.
Several commas that form by the hour indicate excessive wind shear and high levels of moisture. Even if the weather appears fair and dry, change is due sixteen hours later. A low-pressure system is on the way, which will bring unsettled and wet weather.
If the jet stream (a fast flowing river of air) is blowing directly overhead, thin long lines of cirrus often appear that can be followed across the sky like ropes. This indicates an increase of winds is likely within twelve hours, and a warm front within twenty four hours.
Jet exhaust from jet engines as well as the tips of aircraft wings can result in the formation of contrails. If a contrail can be spotted, hold up a fist. If after an hour, the contrails have grown by two or three fists, rain will fall the next day.
A cirrocumulus is a high level cloud made up of lots of small faint puffs of cumulus. They often form what is known as a mackerel sky, with their stippled appearance. The appearance of cirrocumulus in wavy shapes often indicates unsettled weather in twenty four hours.
The cumulonimbus cloud is a very large storm cloud that is instantly recognisable. It is a huge sign of unstable air, because latent heat is being released faster than can be lost through the expansion of the air.
After a while, the cloud stops growing and gravity takes over. Friction is created by growing ice crystals and water droplets rushing up and down the cloud in what are called updrafts and downdrafts.
The friction results in positively charged ice crystals and water droplets, and a negatively charged cloud base. Because opposite charges attract, electric bolts are able to find the path of least resistance and strike, which is seen from the ground as lightning.
The sudden strike of the lightning bolt causes the air to expand incredibly fast, because the temperature around the bolt is five times hotter than the surface of the Sun. The air then contracts suddenly and violently, resulting in the sound of thunder.
The top of the cloud is wider, because the air is warmer than the air of the updraft. This part of the cloud is called the anvil.
The stratocumulus cloud is the most common cloud visible in the sky. It is also dull and bland and does not offer many signs.
It is a blanket cloud made up of cumulus shapes. It forms if a blanket of stratus breaks up, or a large amount of cumulus meets a stable layer of air.
The weather will be settled, no change is likely in the next twelve hours, and only modest winds.
The nimbrostratus cloud is the least cheery cloud because it simply means a blanket of rain. A nimbostratus cloud is very widespread and thick.
They differ from cumulonimbus clouds because they are so wide, and a clue is that rain has been continuously falling for more than thirty minutes.
They’re dark grey in colour, because they are thick, so little to no sunlight can penetrate through.
And because it is a type of stratus cloud, the sign of longevity applies, and it is likely that rain will fall for a prolonged period of time.
This cloud spreads over wide areas but are high up. They are very thin, but they are visible, and cannot hide the sun, moon or stars effectively.
The ice crystals higher up can play with the sunlight or moonlight creating halos, which are circles of light with a bright sphere at the centre. Halos can often be spotted around the Sun or the Moon. The physics of halos and cloud iridescence is complex and beyond the scope of this article.
The cirrostratus cloud is probably the most humble cloud in the sky because it offers a lot of information to those who get to know it.
The most common sign is that it is logically an indicator of moisture, and usually means bad weather is likely.
Another flat blanket cloud, but the prefix ‘alto’ means a middle level cloud that sits in between the high cirrus and the low cumulus.
It is thicker and lower than cirrostratus, as well as opaque and wide. This is a sign that the clouds are growing and getting lower. Rain is on the way.
This is another middle level cloud, but with a different, fairer story. It forms through the breakup of altostratus, or when pockets of moisture are cooled by gentle air turbulence.
Around mountains they can form as lenticular clouds which tend to look like flying saucers. These flying saucer shapes are often distinctive and easy to spot.
If it can be observed that the clouds are rising and forming small towers, then a thunderstorm is likely imminent. Otherwise, altocumulus generally indicate fair, dry weather.
There are many other types of cloud in the sky that have not been covered here. Some of these clouds are harder or rarer to spot, such as noctilucent clouds which form within a much higher layer of the atmosphere, and mammatus clouds which can form under a cumulonimbus given the right conditions.
Clouds which form very near to the ground are known as fog, while mist is just a term for a less thick fog. A fog tends to form overnight if heat from the day has escaped, and the air close to the ground has been able to reach the dew point.
Whatever cloud is visible in the sky on a given day, each one has a purpose and an explanation for why they form. This reason is often found on a local level.
In conclusion, clouds are one of the most fascinating and effective natural signs in the local environment, and the information they often hold can prove to be incredibly valuable for numerous situations.
Feature image, cumulus clouds west of Airthrey loch, credit: Sonny Bailey