Cloud Condensation Nuclei

Setting up the experiment
  1. put the smaller dish into the larger dish with something to elevate it a little bit (paper clips or blu-tack for example)
  2. pour a little bit of water in the bottom dish
  3. sprinkle a few salt grains on to the top dish
  4. cover the dishes with a transparent cover (a glass, a glass jar or cut off the top part of a plastic bottle)
  5. now wait…

After about twenty minutes, a layer of liquid water will start forming around the salt grains

6. now wait a bit longer..

If you wait long enough, the salt grains will be completely transformed into salty water droplets

Results of the experiment. From left to right: salt grains, salt grains coated with a thin film of water, water droplets


Water evaporates from the liquid in the bottom dish and condenses on to the grains of salt. Actually, water also evaporates from the salt grains but at a slower rate than from the water. As such, there is a net transfer of water from the liquid on to the grains. We can see that at first, a thin film of water forms around the salt grains, and given enough time, the grains will have dissolved and in their place there will be a salty drop of water.

Why is this important?

Salt particles are a type of cloud condensation nuclei (CCN) which means that they “help” with cloud formation.

Clouds consist of very small droplets of water (and sometimes of ice crystals, but we’ll focus on liquid clouds for now). At the very small scale, water molecules are constantly escaping from the droplets into the atmosphere (evaporating) or moving from the atmosphere into the droplet (condensing). The smaller the droplet, the fewer water molecules attract (hold) each other in place, and the easier it is for a water molecule to escape from the surface of the droplet. For droplets smaller than 1 μm, the rate of evaporation (molecules escaping from the droplet) exceeds the rate of condensation (molecules moving into the droplet). For a cloud droplet of radius 0.001 μm not to shrink, the relative humidity of the environment would have to be 300 percent! Such conditions do not exist in the atmosphere! In order for clouds to form, droplets need to grow, yet based on the physics above, a cloud would evaporate – literally vanish in thin air- before it has even started forming!

What if the droplets don’t have to start out as extremely small water droplets? What if water vapour condenses on a small solid particle, such as a salt particle or another type of condensation nucleus? Then a slightly supersaturated environment would be enough for the droplet to grow. If the condensation nucleus is soluble (such as salt particles) the rate of evaporation will be even slower, so that soluble particles smaller than 1 μm can serve as cloud condensation nuclei.

One of the goals of the Southern Ocean Clouds project is to better understand the sources and types of cloud condensation nuclei, and how they affect cloud formation in the Southern Ocean and Antartica.

Source: Clouds in a glass of beer – Craig F. Bohren

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