The 10 Types of Clouds and How to Spot Each One in the Sky
How clouds form, the ten main cloud types in three height groups, a checklist to recognize them from the ground, what clouds cannot tell you and a sky diary to try this week.
By Polarzinho ·
There are ten main types of clouds, and three questions tell them apart: how high is it, is it made of lumps or a flat layer, and does it let light or rain through? By height they sort into three groups: high (cirrus, cirrocumulus and cirrostratus), middle (altocumulus, altostratus and nimbostratus) and low (stratocumulus, stratus, cumulus and cumulonimbus).
I am Polarzinho, the panda mascot of Polar Games, and I spend more time looking up than my neck recommends. Below: how a cloud forms, the ten types with the weather each one goes with, a checklist for naming them from the ground and a sky diary to start today. Names and heights come from the International Cloud Atlas of the World Meteorological Organization (WMO), the international reference for naming clouds.
How a cloud forms
A cloud needs two things: water and tiny particles for it to condense on, like smoke, sea spray or windblown soil. NOAA's JetStream site explains the rest: air cools as it rises, because it expands where the pressure is lower, and once it reaches its dew point the water vapor condenses onto the particles and forms droplets. A cloud grows while more water condenses than evaporates, and melts away when the balance flips.
And a cloud is not vapor. Vapor is invisible. A cloud is made of tiny droplets of liquid water or tiny ice crystals, and that is exactly why you can see it.
Types of clouds: the three height levels
The WMO International Cloud Atlas splits the sky into three levels, and the limits change with latitude. These are the approximate heights it gives:
- High (cirrus, cirrocumulus, cirrostratus): 6 to 18 km in the tropics (20,000 to 60,000 ft), 5 to 13 km in temperate regions (16,500 to 45,000 ft) and 3 to 8 km in polar regions (10,000 to 25,000 ft).
- Middle (altocumulus, altostratus, nimbostratus): 2 to 8 km in the tropics (6,500 to 25,000 ft), 2 to 7 km in temperate regions (6,500 to 23,000 ft) and 2 to 4 km in polar regions (6,500 to 13,000 ft).
- Low (stratocumulus, stratus, cumulus, cumulonimbus): from the surface up to 2 km (6,500 ft), everywhere.
The levels overlap, and some clouds will not stay in theirs: altostratus often reaches higher, nimbostratus usually stretches into the other two levels, and cumulus and cumulonimbus have low bases but tops that often climb to the middle and high levels. The numbers shrink from the equator toward the poles.
High clouds: cirrus, cirrocumulus and cirrostratus
Up there it is so cold that these clouds are mostly made of ice crystals. In these three sections, the descriptions follow the WMO Atlas and the weather notes come from NOAA's JetStream site.
- Cirrus: delicate white filaments, patches or narrow bands, with a hair-like look or a silky sheen. Thin ones barely dim the Sun.
- Cirrocumulus: a thin white layer of very small grains or ripples, with no shading. It is short-lived.
- Cirrostratus: a transparent, whitish veil, fibrous or smooth, covering part or all of the sky. It often makes a halo around the Sun or the Moon.
Middle clouds: altocumulus, altostratus and nimbostratus
- Altocumulus: white or gray patches, sheets or layers of rounded masses or rolls, generally with shading. It often shows up in several layers and can have a corona, a colored ring around the Sun or the Moon.
- Altostratus: a grayish or bluish sheet, striated, fibrous or smooth, covering part or all of the sky. The Sun shows through vaguely, as through ground glass, and there is no halo. Virga, streaks of rain, may hang from it, and it occasionally gives very light precipitation.
- Nimbostratus: a gray layer, often dark, thick enough to blot out the Sun. Rain or snow falls more or less continuously and in most cases reaches the ground. This is the cloud behind a gray, steady, no-drama rain.
Low clouds: stratocumulus, stratus, cumulus and cumulonimbus
- Stratocumulus: a gray or whitish patch, sheet or layer, almost always with dark parts and made of big pieces. Its pieces look bigger than half a fist at arm's length.
- Stratus: a generally gray layer with a fairly uniform base. It may give drizzle, snow or snow grains, and when the Sun shows through, its outline is sharp.
- Cumulus: separate, dense clouds with sharp outlines, growing upward as mounds, domes or towers. The sunlit parts are brilliant white and the base is darker and almost flat. Over land it typically forms on clear days and fades by evening. This is the classic cartoon cloud.
- Cumulonimbus: a heavy, dense cloud shaped like a mountain or huge towers, with a top that often spreads out like an anvil. It can bring precipitation, hail and tornadoes. Its base is low, but its top can climb to the middle and high levels. This is the thunderstorm cloud.
How to recognize clouds from the ground: a checklist
Step outside, look up and answer in order. The tests come from the WMO definitions.
- Lumps or layer? Separate clouds with sharp edges point to cumulus. A smooth sheet points to the stratus crowd: stratus, altostratus, nimbostratus and cirrostratus.
- Size up the pieces. Stretch out your arm: according to NASA, your pinky covers about 1 degree of sky and your closed fist about 10. Pieces smaller than a pinky: cirrocumulus. From a pinky up to about half a fist: altocumulus. Bigger than that: stratocumulus.
- Test the Sun, without staring at it. If its outline looks sharp through the cloud, think stratus. If it looks vague, as through ground glass, think altostratus. If it is blotted out, think nimbostratus.
- Look for rings. A halo points to cirrostratus (altostratus shows none). A corona, the colored ring, can point to altocumulus.
- Look at the overall shape. White mounds with a dark, flat base are cumulus. A tower with an anvil on top is cumulonimbus.
What clouds do not tell you
A cloud is a clue, not a forecast. Cirrostratus often makes halos, and an old saying links halos to rain on the way, but it only sometimes holds. A name does not tell you how much rain is coming or when, either: nimbostratus describes the cloud, not the amount of water. Before an outing, check the forecast from your country's weather service.
Common mix-ups
Fog or stratus?
The NWS defines fog as water droplets suspended in the air at the Earth's surface. Stratus is that gray layer with a fairly uniform base up in the sky. So the practical difference is where the bottom is: fog touches the ground, stratus floats above you.
Is the white line behind a plane a cloud?
Yes: a contrail, short for condensation trail. NASA explains that airplane exhaust adds water vapor to already moist air, and that vapor condenses and freezes into tiny ice crystals in the cold air at cruising altitude. Some trails fade. Others spread out and merge, and then they start to look like natural cirrus.
Sky diary: a week of looking up
- Pick a spot and a time. The same window, at the same hour, every day, so you compare like with like.
- Write the date and how much of the sky is covered: clear, a little, half, almost all or all.
- Run through the checklist and write your best guess for the cloud's name and its level (high, middle or low). “Not sure” is a perfectly good entry.
- Later, write what the weather did: dry, drizzle, rain or thunder.
After seven days, look for patterns. Which cloud showed up before the rain? Did the same name always bring the same weather? If the answer is “not always”, you have just found out why forecasting is serious work.
Clouds in Little Ninja Dash
Little Ninja Dash is a ninja action platformer you can play free in your browser. You hold to slow time, drag to aim and release to dash. It has 50 levels across five worlds of ten stages each: gardens, ice, lava, clouds and a fortress.
Two screenshots fit this guide. In the cloud world, the ninja jumps between grassy stone platforms above a thick white layer of clouds. In real life, do not step on a cloud: it is only droplets and ice crystals.
PolarzinhoUp here the clouds are scenery. In real life nobody steps on one: it is all droplets.
The storm setting is much darker: night, rain and heavy clouds. The game does not say what type of cloud that is.
PolarzinhoRain and a dark sky: my guess is nimbostratus. The game never confirms it, so it stays between me and the ninja.
FAQ
What are the types of clouds?
There are ten: cirrus, cirrocumulus and cirrostratus (high); altocumulus, altostratus and nimbostratus (middle); stratocumulus, stratus, cumulus and cumulonimbus (low). The WMO calls these ten genera.
Which cloud brings rain?
Nimbostratus gives continuous rain or snow that in most cases reaches the ground. Cumulonimbus can bring rain, hail and tornadoes. Stratus may give drizzle, and altostratus at most very light precipitation.
What is the difference between cumulus and stratus?
Cumulus grows upward in mounds with sharp outlines and a darker, almost flat base. Stratus is a gray, uniform layer with no mounds.
Is fog a cloud?
It is the same kind of thing: water droplets suspended in the air, only at ground level, as the NWS defines it.
What are clouds made of?
Tiny droplets of liquid water or tiny ice crystals. Never vapor, which is invisible.
Your turn
Look out the window now. Lumps or layer? Where is the Sun? Name the cloud in a minute, then see tomorrow whether the weather agreed. I will be on the windowsill, watching for cumulonimbus.