Why do some planes leave white trails, but others don't?

Wednesday 22nd July 2009
Submitted by Robert Matthews
Rosemary Whittington, East Lothian

Known as ‘contrails’, the white trails are caused by water vapour ejected from the back of jet engines. Their visibility depends on whether the water vapour condenses into droplets or ice crystals. This in turn depends on the humidity and temperature of the surrounding air – the drier and colder the better. For aircraft cruising at altitudes of 10,000m, temperatures have to be below around -50°C to guarantee contrails.

Can you simulate 'freefall'?
previous qanda Article
How long would a pebble take to reach the bottom of the Mariana Trench?
next qanda Article
Q&A Tabs

A lunar month is slightly shorter than a calendar month, so every two to three years, there is an extra lunar cycle above the usual 12. This additional moon is called the ‘blue moon’...

Your DNA is arranged into chromosomes, which are grouped into 23 pairs. When a sperm cell is manufactured, the father’s genome is split in two, so that each sperm receives one chromosome...

Phlegm is the mucous secretion of the respiratory passages. The cilia cells that line these passages are continually driving the phlegm upward to the throat, where it triggers the swallow reflex...

Actually, not all babies cry with their first breath after being born. But all babies will cry within a few seconds if they are not immediately reunited with their mother. This is a simple...

You sometimes hear very large estimates for the biomass of certain animal groups, like ants or nematodes, but these consist of thousands of different, specialised species. The single species with...

All female mammals have a clitoris, the sole purpose of which is to react to sexual stimulation, and presumably this stimulation has evolved to be pleasurable for most species. But establishing...

To create a sound, we have to set matter - whether it's a gas like air, a liquid or even a solid material - in regular motion, creating a wave of specific frequencies, which we hear as a sound of...

Mirrors don’t reverse left and right either – that’s just our interpretation of what happens. Your reflection in the mirror is actually reversed front to back – if you have...

Discovered by an American student named Gary Flandro in the mid-1960s, the slingshot manoeuvre usually involves spacecraft briefly 'coat-tailing' a planet orbiting the Sun, extracting some of the...

The ice disappears because the wind blows away water molecules that have evaporated or 'sublimed' from the ice, so the ice slowly shrinks in size. The molecules that escape are those with the...

We use cookies to improve your experience of our website. Cookies perform functions like recognising you each time you visit and delivering advertising messages that are relevant to you. Read more here