Tales Clouds Narrate

Been meaning to write this for some time, but did not have enough pretty pictures to put in, so kept delaying it. Now that I think the pictures are pretty enough, here we go.

Clouds have fascinated people of various inclinations – from poets to physicists to farmers to nature lovers. They symbolize freedom, fertility and make sunsets magnificent. Apart from the metaphorical tales that poets imagine clouds to carry, they do tell us a lot of things, some of which I hope to convey through pretty pictures.

Clouds are among the most important factors driving our climate and definitely the least understood. The fact that they are so important derives from that fact that the climate is determined to a large extent simply by the amount of solar radiation that is trapped on earth, and clouds play a huge role in determining that amount. The atmosphere is almost completely transparent to incoming sunlight. The only thing that can reflect sunlight back to space are clouds, cooling the earth. Interestingly, clouds also absorb almost all the infrared that the earth’s surface emits, thus heating it. Depending on what type of cloud it is, clouds can either cause a net heating or cooling of the earth.

As a fluid dynamical problem, clouds are in a league all of their own. They display a nice interplay between dynamics and thermodynamics and are insanely difficult to model. If you look at the IPCC reports, the uncertainty due to clouds dominates any other factor. Of course, you can just numerically integrate equations of motion to see cloud development, but qualitative understanding is a recent thing.

Another useful thing about clouds is that they are the only opaque object in the atmosphere, and can be used to understand the state of the atmosphere. Since air is a fluid like water, it supports waves which carry momentum from one place to another and the best place to look for wave signatures is in clouds. The main waves in the atmosphere are Rossby and Internal Gravity waves. Rossby waves are huge, with wavelengths in thousands of kilometers, whereas Gravity waves are small enough to be seen. They are seen as regular patterns of cloud-no cloud and easy to spot, especially during the evenings.

Textbook picture!
Textbook Gravity Wave signature

The above is about as good a picture as you will find. Gravity waves normally will occur when denser air from the bottom will rise up and eventually come down due to their weight as compared to their surroundings. When I say small wavelength, it still is in the order of hundreds of meters! A couple more, but not as distinct.

Scattered, but still visible!
Still fainter, but nice looking cloud :)

Stratus clouds are thin (relatively!) and very large in spatial extent. They are a sign that vapor is unable to rise high due to strong density differences in the atmosphere, i.e, the atmosphere is strongly stratified. Thus, instead of rising high, it simply spreads out to a thin, large layer. This shows that the atmosphere is stable to rising motion – which is a bad thing! A stable atmosphere discourages convection of water vapor, which means less rain.

A more familiar sight in tropical areas like ours are cumulus clouds, which are normally associated with convective activity and rainfall. On nice and windy days, they can ‘march’ in step, as shown below.

Cloud March near home!

These can also develop into the thunderstorm version, which are called cumulonimbus.

Eye Candy!

Ok, that’s not a cumulo-nimbus tower, but you get the idea. Also, the sunset picture was too good to not put into the post :D Cumuli indicate either strong convection/ a weakly stratified atmosphere (which are actually cause and effect).

The height at which clouds start forming is another indication of the stability of the atmosphere. Cloud formation implies water vapor must condense, and this implies that the temperature must be low enough. If the clouds are low, it means that there are low temperatures at a low altitude, thus the atmosphere is quite stable. Higher up means the atmosphere has been well heated by the surface, like a kettle on a stove.

All in all, literally having your head in the clouds is good time pass!


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