Welsh 
Contrails
Related Demonstrations and Experiments
What are contrails?
We
have all seen white streaks in the sky like those in the
picture. What we are seeing are trails of ice crystals left
in the wakes of jet aircraft. These condensation trails
(known as 'contrails') sometimes persist for many minutes
or even hours. On other occasions, they disappear quite
quickly.
Once formed, contrails are distorted by upper winds and spread by diffusion, and curtains of ice crystals can sometimes be seen falling from them.
Persistent trails often form large patches of fibrous clouds that look like cirrus, cirrocumulus or cirrostratus. Thus, old contrails sometimes cannot be distinguished from these clouds.
Supercooling
Supercooling is a normal occurrence in the atmosphere. Clouds composed of water droplets can persist at temperatures well below 0°C, even at temperatures below -30°C. At temperatures below about -40°C, however, all cloud droplets freeze very quickly. On long-haul routes, commercial aircraft usually reach altitudes of 10 to 12 km, where temperatures are typically below -40°C. Planes on these routes therefore tend to leave contrails behind them. Over the British Isles, trails rarely form below about 8 km in summer, 6 km in winter. When the weather is as cold as it often is in mid-winter in Alaska, Siberia and central Canada, contrails can even form at ground level. Indeed, airfields in these regions have sometimes had to be closed when low-level clouds (ice fogs) composed of aircraft-generated ice crystals have proved persistent.
Condensation on Windows
Condensation can often be seen on windows and doors when a room is hot and the air outside is very cold. We can demonstrate this by heating a beaker of ice (right) with a bunsen burner and observing the outside of the beaker.
Questions:
- Why does condensation occur on the outside of a can of Coke (or a can of anything else, for that matter) soon after it has been taken out of a fridge and placed in a warm room?
- Why is there often condensation on the windows of cars that have been left standing out of doors overnight?
- Why does condensation occur in front of your face when you breathe out on a cold day?