Why Airplanes Leave White Lines in the Sky

Have you seen white lines behind airplanes? These lines are called contrails. They happen when planes fly through very cold and wet air. The plane engines make small dirty particles. Ice forms around these particles and makes thin clouds.

Some contrails go away fast. Others stay for a long time and get bigger. These clouds can trap heat and make Earth warmer. But scientists are not sure how much this happens.

The UK and Google are doing a test called Operation Blue Skies. They will ask some planes to fly higher or lower to avoid making contrails. The test will last 30 months. They want to learn if small changes can help reduce these white lines.