A new analysis has rebuffed an earlier study that had suggested aircraft contrails could have a major effect on the climate.
A study in 2002 had suggested that condensation trails left behind by jets could have a significant effect on daily temperature patterns.
Now, according to a report in Nature News, a new analysis now claims that altered US temperature patterns during the three flight-free days can be explained by natural variations in cloud cover, rather than the absence of planes.
Aircraft contrails can spread into cirrus-like clouds high in the atmosphere. Similar to natural clouds, they are thought to have an overall warming effect on the planet.
But they can also moderate daily temperature extremes by trapping heat that escapes from the ground and reflecting sunlight. This raises the lowest overnight temperatures and, to a lesser degree, reduces the highs during daylight hours, scientists have suggested.
With air traffic projected to grow by 2-5 percent per year in the near future - amounting to at least a tripling in traffic by 2050 - the effects of contrails are expected to become an increasingly important factor in climate change.
Two studies noted that when planes stopped flying on 11-14 September 2001, the average daily temperature range in the United States rose markedly, exceeding the three-day periods before and after by an average of 1.8 degree Celsius.
According to David Travis of the University of Wisconsin-Whitewater, who led both of the earlier studies, the unusual size of the shift implied that an absence of contrails gave the temperature range a significant boost.
But that idea, he says, was "more like a hypothesis" than a firm conclusion.
Now, research led by Gang Hong, an atmospheric scientist at Texas A and M University in College Station, suggests that this hypothesis is wrong
Examining patterns of cloud cover and temperature in early September at US weather stations from 1971 to 2001, Hong and his colleagues found that thicker, low clouds are the dominant influence on temperature extremes, whereas high clouds such as contrails have a minor effect at most.
They added that the 2001 temperature swings seem to be within the range of natural variability over those decades.
According to Ulrich Schumann, director of the Institute of Atmospheric Physics at the German Aerospace Center in Oberpfaffenhofen, near Munich, Hong's work doesn't prove that the contrails have no effect on temperature, just that they are unlikely to have a major role.