At certain altitudes, aircraft produce contrails -- the vapour wake caused when water in the chilly atmosphere is condensed by the plane's hot exhaust.

These contrails have a surprisingly big but also complex effect on the climate.

Because they are clouds, they trap heat that is emitted by the Earth's surface, creating a "greenhouse effect" that adds to warming.

Yet during daytime, these clouds have a cooling effect because they are white and thus reflect some of the Sun's energy back into space. In certain conditions, contrails can exist for several hours.

Meteorologists at the University of Reading, southern England, estimated the radiation caused by contrails at a busy flight corridor in southeast England.

Using high-resolution aircraft flight data and routine weather balloon data, they looked at "persistent" contrails -- wakes that remained for an hour or more after the aircraft has flown over.

Night flights account for only 22 percent of Britain's annual air traffic but contribute between 60 to 80 percent of the greenhouse effect from contrails, the scientists found.

"We also found that flights between December and February contribute half of the annual mean climate warming, even though they account for less than a quarter of annual air traffic," said lead researcher Nicola Stuber.

The study appears in Nature, the weekly British science journal.

Man-made global warming is mainly caused by unbridled burning of oil, gas and coal.

These fuels spew out carbon, in the form of carbon dioxide (CO2), that has been stored for hundreds of millions of years underground.

The CO2 hangs in the atmosphere like an invisible blanket, trapping heat that would otherwise radiate out into space and thus inflicting changes on the climate system.

Global emissions of man-made CO2 are between 6.2 billion and 6.9 billion tonnes per year. Added to this are around 1.5 billion tonnes from land use.

Commercial aircraft account for only a small contribution compared with power stations, industry and road traffic.

However, passenger travel is growing at the rate of around five percent a year, which means that this share will grow fast.

A 1999 estimate by the United Nations' Intergovernmental Panel on Climate Change (IPCC), the top scientific forum on global warming, found that the airline industry accounted for two percent of man-made CO2 emissions in 1992. But it would rise to as much as 15 percent by 2050.

Environmentalists are angry, complaining that airlines get a free ride when it comes to environmental taxes.

In addition to rescheduling night flights for the daytime, planes could diminish their contribution to global warming by changing their altitude.

A study published last year in the journal Transportation Research suggests that the regions of "ice-supersaturated" air where contrails form is only about 500 metres (1,650 feet) thick.

The goal would be to fit sensors on aircraft that could inform pilots where this layer lies, thus enabling them to shift altitude accordingly.