Chemistry, Physics

CLOUDS CLOUDS CLOUDS! Why are some flat on the bottom and fluffy on the top?

Clouds are cool. Like, they NEVER fail to amaze me (maybe I’ve just been in lockdown too long…). Sure, they are incredibly annoying when you *just* want to observe the moon/jupiter pairing but missed it due to the entire weekend being overcast, but seeing how they form layers, and how by looking at them you can predict the weather.. just wow! So, here are some common types of clouds and what they mean, followed by some of the coolest clouds out there! One of the things I’ve *always* wondered is- Why are they fluffy on the top and flat on the bottom? But first, a little primer on cloud types along with some pretty pictures (clouds are one of my favourite things to photograph!)- if you want to skip straight to the explanation, click here.

Clouds tend to be named by their shape and height in the sky, so you will probably notice some patterns as we go through the common ones! They also tend to be categorized according to height, although the interesting ones are usually exceptions.

What clouds might I see normally and what do they mean?

High Clouds

Cirrus: These are the thin wispy clouds that you might see high in the sky. They are actually made of ice crystals that are shaped by wind currents and form either on a nice day or can signal that a front is about to arrive! Fronts often bring rain and are very common in the UK

Cirrocumulus: The small patchy clouds that look like honeycomb or ripples high in the sky, associated with cold weather

Cirrostratus: These can cover the sky like a thin veil, often in winter, and usually mean snow or rain is soon to come. You’ll still be able to see the moon/sun through them, and it might be surrounded by a halo!

Mid Clouds

Altocumulus: Fluffy ripples fairly high in the sky, often on a nice day

Altostratus: A thin layer of cloud, often making it look hazy, as the sun might be just visible behind it.

Low-Mid Clouds

Nimbostratus: Low cloud that covers the whole sky like a thick sheet. Rain. Depressing Drizzle

Low Clouds

Cumulus: Pretty, fluffy, stereotypical clouds that dot the sky on a nice day, but can grow into cumulonimbus! 

Stratus: Like Nimbostratus but lower. Sometimes more interesting as they can bring fog! Spoooky

Stratocumulus: One of my favourite types of cloud! They form broken up sheets in the sky, almost like honeycomb but massive. They are of medium thickness, and look SO cool! They’re low down so you are often able to see the full thickness of them!

Storm Clouds

Cumulonimbus: This cloud will always bring some exciting weather- thunderstorms! The most interesting type of rain. They often grow from cumulus clouds, and tower into the sky, and are the only cloud type associated with thunder, lightning, and hail. They tend to be flat on the bottom, but often have cool features like the ones below!

Cool Clouds

Not that those clouds aren’t cool- they definitely are, and some of my best landscape photos are interesting because of these everyday clouds, but now for some more unusual ones! 

Noctilucent Clouds: The highest clouds in the atmosphere, and only visible when lit from below by a set sun, they are made of ice and are beautiful!

Lenticular Clouds: These look like UFOs, and form when winds flow across mountains.

Kelvin helmoltz clouds: These are incredibly rare, and form when 2 layers of the atmosphere are travelling at different speeds- if the higher layer is moving faster than the lower layer with the clouds, then the cloud may get pulled up into these peaks. 

Clouds that look like ocean waves | Earth | EarthSky
via Earth Sky

Anvil Clouds: These are the tallest cumulonimbus clouds, and occur when they grow so tall that the top reaches the next layer of the atmosphere (the warmer stratosphere) and stops, like hitting a ceiling, and spreads out.

Mammatus: Also usually forming on cumulonimbus clouds, these are little protrusions from the bottom of the cloud where turbulence has created a little pouch. They don’t last long, but can fill the sky!

What are mammatus clouds? Stunning photos here | Earth | EarthSky
via EarthSky

Fallstreak hole: A large circular gap amongst cirro/altocumulus clouds, where supercooled water has suddenly frozen due to sudden introduction of a nucleus site for the water to freeze around, such as an airplane passing through. They can also look like UFOs! (Almost like they messed up the cloaking shield or something lol)

Fallstreak hole - Wikipedia
via Wikipedia
So many cloud types in one photo!

Ok now back to the main question:

Why are clouds flat on the bottom but fluffy on the top?

First have a think- what actually are clouds? Well they are the interaction of rising, warm, moist air, and the cold air around it. We can see clouds because the warm air cooled down to the ‘dew point’ where the water condenses into little droplets. The temperature decreases mainly due to pressure, which decreases fairly consistently with altitude, so on a fairly calm day, most air at the same altitude will be of the same temperature and have the same humidity, so the clouds tend to form at the same height and with flat bottoms! Usually these ‘comic book’ clouds are cumulus clouds, although these can grow into cumulonimbus, and sometimes we see it in other cloud types. The humidity also dictates how low/high this lowest altitude is! The higher the humidity, the lower the boundary for cloud formation. But why does this first layer of cloud not just continue to rise up, creating square clouds? Well the air in clouds isn’t static, as convection is what made them rise and form in the first place, so this continues in the cloud. Air that was at the bottom gets pushed upwards, but not all air is rising at exactly the same rate, and this time there isn’t a barrier/boundary preventing them growing (not until they get really tall like anvil clouds), so this irregular upward movement and general turbulence makes them fluffy on top!

Heres a great graphic from ABC weather 
https://www.abc.net.au/news/2020-09-01/flat-bottom-clouds-and-the-science-behind-them/12591230

Are you a cloud watcher? What’s your favourite type of weather/cloud? Let me know in the comments! Follow my photography account for more photos!

Photo from when I visited the science museum! They have a whole exhibit about the atmosphere!

All photos are my own unless stated. I’ve guessed about the cloud types, but I’m pretty sure they’re right! Some photos have multiple cloud types, but I’ve either specified or its the most prominent one!

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