EVERYONE has seen the lines that follow planes across the sky, but few people are able to explain what they actually are.

Depending on who you listen to, you could be told the truth, or you could even be told a conspiracy theory.

However, there's a very simple explanation for the lines and what they are, regardless of what some may believe.

Contrails, short for condensation trails, are mostly made of water, in the form of ice crystals.

They are formed when the water vapour in aircraft engines, that is created when fuel is burnt, mixes with the low temperatures that are found at the high altitudes where planes cruise.

However, the trails aren't always seen following behind planes flying overhead.

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That could be because of the amount of moisture in the air.

According to the Met Office, if the air is very dry, the ice crystals will "sublime" which means they will change from solid to gas and become invisible.

Instead, the air needs to be humid, so the water droplets or ice crystals will stay where they are for a while, or even spread out, leaving a fluffy trail where the aircraft has passed.

Some trails will disappear quickly, while others can last for hours, leaving the sky criss-crossed with lines.

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This is what some people often falsely believe to be "chemtrails" which is the conspiracy that the trails contain chemicals that can negatively affect humans, or manipulate weather among a number of theories.

This claim has been widely dismissed by scientific communities and has little to no evidence to back it up.

As well as contrails, there is also a similar effect called “wingtip vortices”.

These effect is seen when plane wings create lift, causing a "vortex" to form at the tips of both the wings and the flaps.

These "vortices" stay in the air after the plane has passed and a reduction in both pressure and temperature can cause water to condense.

When this happens, it generates a thin line of water droplets that look very similar to a contrail.

The vortices can be seen behind the wing flaps of airliners during take off and landing.

This is the main difference between them and contrails.

Wingtip vortices are seen at low altitudes, with the planes moving relatively slowly after take off, or before landing.

They also trail behind the wingtips and flaps instead of the engines.

They also evaporate very quickly, just a few metres behind the aircraft, unlike the contrails, which can linger much longer in the air.

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