How do clouds form? Cloud formation occurs due to condensation onto existing particles, called condensation nuclei, which leads to the formation of liquid droplets. These droplets grow through the condensation of water vapor. When the water vapor decreases, the droplets stop growing and may evaporate. Cloud droplets can undergo chemical reactions, such as the formation of sulfuric acid, contributing to acid rain. Clouds and fog have a significant impact on climate and the composition of the atmosphere.

The interaction between light and crystal structures is not only beautiful but also fundamentally important in materials science. This relationship, described by Bragg's Law, allows us to deduce crystalline structures through the diffraction of known radiation. The intensity and direction of the scattered beams depend on the position of each atom in the unit and the orientation of the crystal. W.L. Bragg derived a simplistic description of coherent scattering from an array of periodic scattering sites, such as atoms in a crystalline solid. This understanding allows us to conduct experiments to determine various properties of crystalline solids. So, while appreciating the dance of light in a crystal chandelier or an icicle, remember the atomic scale interplay of wavelength and crystal lattice described by Bragg's Law.

How do fog and cloud droplets form in the atmosphere? 

Well, firstly, they never form through spontaneous condensation of gases - from thin air. 

Instead, they form by condensation onto existing particles, such as dust, called condensation nuclei. When humidity increases, these particles take up water and eventually turn into a liquid droplet with a particle within. This is called coalescence. The droplets grow rapidly if there is more water vapor around to condense.

If the water vapor decreases to below saturation - by a change in temperature or a decrease in humidity in the surrounding air - then the droplets stop growing and start evaporating. Only about 1 in 6 cloud droplets that form grow large enough to actually precipitate, the rest will evaporate and become a solid particle again. Condensation nuclei can go through several of these cycles where they become droplets and evaporate again. 

A cloud condensation nucleus is only hundreds of nanometers in size, while the cloud droplets are around 100 times larger - in the range 10 microns,  which is still smaller than the diameter of a human hair at about 50 microns. But a rain droplet, just before hitting the ground, might be another 10 to 100 times larger at 1 millimeter in size.

From the perspective of a chemist, we can zoom into an individual cloud droplet and consider it as a tiny chemical reactor. 

While there can always be chemical reactions at the surface of a particle happening, once a particle becomes a droplet and we have an aqueous phase, a myriad more chemical processes can take place. We can have soluble gases that dissolve into the liquid. We can have the aerosol components on which the droplet forms that go into solution and all these species can react as in the droplet we can have solution chemistry in water. 

A classic example is that SO2, a gas that is emitted by coal combustion, can integrate into cloud droplets. In the droplet it can react and be oxidized to become sulfuric acid, leading to an acidic pH in the droplets. That cloud chemistry process is a main cause of acid rain. 

That sulfuric acid cloud can also be neutralized by ammonia. Ammonia from the atmosphere integrates into the droplets. In this case, when the droplet evaporates, we will be left with a dry residue that now contains ammonium sulfate, solid material formed in the droplets from a gas phase species. 

So clouds can impact the atmosphere directly by forming acid species -  as acid rain or by  transforming gas species into solid material. These are just examples as a multitude of these processes might happen.

Clouds are very common in the atmosphere. When they touch the ground we refer to them as fog. At any given moment about 50% of the earth surface is covered in clouds, which has important implications for climate and ~15% of the volume of the lower atmosphere is occupied by clouds. In every case, clouds hold a phenomenal capacity to influence the composition of our atmosphere, so understanding them is vital.