Clouds

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In the last section, we said that white surfaces are key reflectors of solar radiation. In fact, we called out two particular surfaces, snow and clouds. While snow is generally seasonal and restricted to specific geographic regions, clouds are ubiquitous in our atmosphere. Check out the global satellite image below, it’s very rare to find spots on the Earth where there are no clouds nearby, and it’s even rarer for the sunny skies to remain day after day.

As you probably know from a previous science class, clouds are just condensed water. A simple explanation is that clouds are formed when warm, moist air rises and cools, causing water vapor to change from a gas to either a liquid or solid state.

As the moist, warm air containing water vapor rises, it cools down due to the decreasing air pressure at higher altitudes. As the air cools, it reaches a critical temperature called the dew point. The dew point is the temperature at which the air becomes saturated with moisture, meaning it can no longer hold all the water vapor in a gaseous state. When the air reaches its dew point, it becomes "saturated" with water vapor, and any additional moisture will condense into tiny water droplets or ice crystals, depending on the temperature. In most cases, air alone is not enough to initiate condensation. Tiny particles and aerosols in the atmosphere called condensation nuclei or cloud seeds play a crucial role. These particles can include dust, pollen, salt particles, and other atmospheric impurities. Water vapor condenses around these particles to form small water droplets. Once condensation begins around the condensation nuclei, it results in the formation of countless tiny water droplets or ice crystals (depending on the altitude where they are at and whether the temperature is broadly above or below freezing) suspended in the air. Collectively, these suspended water droplets or ice crystals form what we see as clouds.

Clouds can vary in appearance, shape, and altitude depending on factors such as temperature, humidity, and atmospheric conditions. They will be important in the context of the water cycle, which we’ll talk about soon, but it’s important to understand that this diversity in cloud properties can have a big role on their albedo and how much energy they scatter and absorb. Think about very thin wispy clouds you may see on a fair, otherwise sunny day. These are cirrus clouds which are made of ice crystals that are 100−200 µm in size and about 30,000 ice crystals per cubic meter (i.e., if you had a box that was one meter for each dimension, you’d have about 30,000 ice crystals in that box inside a cirrus cloud). This makes these types of clouds very transparent to sunlight – they don’t put up much of a fight when confronted with solar beams!

Cirrus Clouds

Credit: PublicDomainPictures from Pixabay is licensed under the Pixabay Content License

Conversely, we can have clouds that appear whiter in the sky. For example, cumulus clouds can also occur during general fair weather, but they are far more noticeable to you and me. That is because they are composed of water droplets that are roughly 10 microns in size. Wait, that is much smaller (by a factor of 10-20x!) than the cirrus clouds particles. How are cumulus clouds more “reflective” than cirrus clouds? The answer lies in the density of droplets that exist within that hypothetical one cubic meter box. The average cumulus cloud has somewhere around 65 million droplets per cubic meter! That high density of cloud particles makes it far more likely for sunlight to be reflected than if it was passing through a cirrus cloud. Now it makes sense when you climb through a cloud deck on your way out of an airport, and you suddenly can’t see anything but white outside your window.

Stratocumulus Clouds

Credit: Dense stratocumulus clouds by Nicholas_T is licensed by CC BY-NC-ND 2.0

So, the albedo of a cloud essentially depends on three things

1. Mass of water in the cloud
2. Droplet size
3. Thickness of cloud

Numbers 1 and 2 are closely linked to the droplet density we discussed above. If we have a cloud with a lot of water and very small particles, it means a high number concentration, which reflects sunlight easily. On the other hand, a cloud with just a little bit of water tied up in big particles is relatively transparent to solar radiation. We didn’t talk about #3, but it’s pretty logical – if I take two of the same cloud and stack them right on top of one another, the thicker one will do a better job preventing radiation from being transmitted.

Now, we just talked about two types of clouds here, but there is a tremendous diversity of different cloud types we can’t cover in this class.