How clouds form
Teaching Guidance for 14-16
Clouds form in the atmosphere when warm, wet air is pushed up. As the air goes higher its pressure decreases and it gets colder. The colder air cannot contain so much water vapour and the air becomes supersaturated with vapour. Given the right conditions, some of the vapour condenses to form water droplets in a cloud.
The droplets need something to form around. An extremely small droplet of just a few molecules cannot form a cloud all by itself. Its molecules would escape from each other again. If the air contains attractive particles (usually minute particles of salt) that the water can wet, a droplet will start to form. The salt particles serve as condensation nuclei on which larger drops can grow.
A similar process is put to use in a cloud chamber containing alcohol vapour. Ions in the air can serve as excellent condensation nuclei. The alcohol molecules are electrically ‘oblong’ with positive and negative charges at the ends, so they can cluster easily around a charged particle. The other small particles can be cleared out. This stops droplets forming into unwanted clouds (even when the air is supersaturated). Instead, the droplets form on the trail of ions left behind by ionising radiation, typically an alpha particle.