My colleague Richard Gilham was unable to control himself when he read my earlier article about contrails. In fact he literally exploded! He wrote to tell me about the critical role of nucleation in cloud formation.
In order to form a cloud – or as Richard calls it: an aerosol – at some point, the water molecules have to form droplets. The first stage of droplet formation is when just a few molecules cluster together – a process called nucleation. Richard is fascinated by how water vapour makes the transition from being a vapour to being a collection of droplets or crystals because – despite the fact that it happens every day and is critical to understanding climate and weather – it is still not completely understood. Richard wanted to tell me something really important about this process but before he could tell me the important thing he needed to tell me some jargon.
First of all he wanted to tell me about the two types of aerosols:
- A primary aerosol is when the material comprising the particles enters the air as particles e.g. dust.
- A secondary aerosol forms out of gaseous material already in the air
So dust particles form a primary aerosol, and clouds are secondary aerosols. In order to form a secondary aerosol – a cloud to you and me – the air must be supersaturated. This is when there are enough water molecules to form liquid droplets or solid crystals, but the droplets or crystals haven’t formed yet. There are two ways they can begin to form:
- Homogeneous nucleation is when the clusters of water molecules form spontaneously. This is really unlikely unless there is a very very high concentration of water molecules.
- Heterogeneous nucleation is when the water molecules cluster around a seed particle (part of a primary aerosol) which forms a ‘meeting place’ for water molecules. Water molecules stick to the surface of the particle and then stick to each other and clump together to make a droplet or a crystal. This is much easier and quicker than homogeneous nucleation, but it needs a seed particle.
So, eventually Richard got around to what he wanted to say: which is that contrails don’t form because of the additional water alone. Frequently they rely on particles that come out of the back of the engines. These are Richard’s favourite type of particles – nano particles.
Richard then told me that the temperature and pressure inside the combustion chamber of the jet is chosen to be hot enough to ensure almost complete combustion of the fuel (and so extract every joule of energy from the fuel). But running the engine too hot is bad for the engine and also causes reactions between the oxygen and nitrogen in the air creating various oxides of nitrogen (NOx). So there are always some nano-particles of partially-burned fuel which provide the necessary nucleation points for water droplets/crystals in the contrail.
And I thought that was what he had wanted to me. But at every end, it turned out that what he really wanted to tell me about was the OPAL project: which features a climate and weather survey including a contrail survey to which people could contribute.