To figure out whether contrails contribute more to cooling or to warming, the scientists first needed to isolate contrails from other clouds. But contrails are tricky to identify because they do not always maintain a neat, linear formation that makes them easy to detect.
On one hand, contrails must be large enough and persist long enough to be seen in satellite data. They decided to include only contrails that persisted long enough to be visible in satellite data, yet maintained a linear shape. After running the contrail detection algorithm, the results were matched to known flight paths. This allowed the researchers to find approximately million pixels from linear contrails that they were searching for. Previous studies had produced false alarms, erroneously identifying a variety of features as contrails.
The researchers found that contrails have an overall warming effect, acting like a light blanket. However, Bedka and Spangenberg said that the effect is still quite small. Likewise, if contrails form in otherwise clear skies over a hot desert, or over warm low clouds, they will have a much greater radiative effect. Yet while contrails may not increase atmospheric warming much on a global basis, the researchers discovered that contrails produced more significant regional warming.
For instance, over the United States alone, more than 5, aircraft may be in the air at once.
Many of these flights route through the large East Coast hubs, and generate a mesh of contrails that waft across the skies toward Europe. And even though their research supplied a low estimate, it is still the first study that firmly quantifies the contrail effect using satellite data.
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Duda, T. Chee, and R. Geophysical Research Letters —, doi: Duda, D.
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Minnis, K. Khlopenkov, T. Greenbelt, Maryland USA. Spangenberg, D. Bedka, R. Palikonda, D.
Duda, and F. Water vapour emitted by the aircraft engine will also turn into droplets of water in the same way. At such high altitudes the temperature is far below freezing and the droplets will quickly form ice particles, making up a white contrail. Being made from ice, contrails are harmless to health. Eventually, when conditions become dry enough lower humidity levels , the ice particles will evaporate, meaning that they do not reach the ground.
Condensation trails | Airservices
The length of a contrail depends on atmospheric conditions. If humidity is low, when there is little water vapour in the air to turn into ice, the contrail will be short-lived. The few ice particles that are formed will quickly evaporate and mix into the surrounding atmosphere. However, if the humidity is high as is often the case in the east and north of Australia , the large amount of water vapour present in the atmosphere will readily condense on the sulphur particles emitted along the track of the jet aircraft.
The newly-formed ice particles will grow in size by taking water from the surrounding atmosphere. As a result, these contrails can last for hours, extending long distances behind the aircraft. Air turbulence, caused by winds or by other aircraft, can lead to the contrails spreading out, sometimes reaching several kilometres in width and metres in height.