Skip to navigation | Skip to content

The contrail effect


(Pic: Dr Andrew Carleton)

US researchers have calculated that the water vapour exhaust trails, or contrails, left by cruising jet planes have a small effect on daily temperatures.

Dr Andrew Carleton at Pennsylvania State University and colleagues measured the effect of contrails during the three-day grounding of commercial aircraft after September 11 last year, when the terrorist attacks on the World Trade Centre and the Pentagon occurred.

Though the circumstances were tragic, the clear skies granted a unique opportunity to tease out the impact of contrails. Previously it was virtually impossible to measure their effect on climate because air traffic, especially over regions like Europe and North America, never stopped.

The researchers compared the average daily highest and lowest temperatures over North America for this period with records for the same interval between 1971 and 2000. Their results appear in this week's Nature.

In the absence of contrails, the range of temperatures — that is, the difference between the daytime maximum and the night-time minimum — was more than one degree Celsius larger than when contrails are present.

In other words, the contrails had an insulating effect, reducing the variation between day and night.

It has been known for a while that contrails formed high in the atmosphere behave like cirrus clouds under the right atmospheric conditions. These are the high, wispy, ice-crystal clouds commonly known as 'mare's tails'.

They are thin enough to allow a fair amount of solar radiation to pass through to the ground during the day, thereby warming the Earth's surface. But like other clouds, they still trap the heat energy (in the form of longwave radiation) the Earth gives off at night.

Other clouds, especially thick stratus-type clouds, let in less solar radiation — so that even though they trap longwave radiation at night (preventing the overnight temperature from falling too low) — they lower the daytime temperature more than cirrus clouds.

Studies in Western Europe, where contrails are very common, suggest that the contrail effect is at least as important as global warming due to greenhouse gases.

According to Dr Carleton, however, it is difficult to predict whether contrails will exacerbate global warming or counteract it.

"When contrails first form, or when you have lots of them in one area, they are quite thick, so this is when they tend to act more like stratus clouds and block more of the solar radiation, potentially leading to cooling near the Earth's surface," he said.

The ultimate effect on global warming depends on which kind of contrail — thin or thick — will predominate.

"Most researchers, however, believe that contrails will make global warming worse because they tend to spread out and become thin over time," Dr Carleton explained.

"It is these persisting contrails that have a climatic impact; not the transient ones that form and disappear within a few minutes."