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==Contrails and cloud formation==

==Contrails and cloud formation==

[[image:Ct1905-9.jpg|Photo of clouds before 1905<ref>Clayden, Arthur W. Cloud Studies (1905) Verlag: KESSINGER PUB CO Nachdruck 2008 ISBN-13: 9780548985267 ISBN-10: 054898526X</ref>|300px|thumb]]

[[image:Ct1905-9.jpg|Photo of clouds taken before 1905<ref>Clayden, Arthur W. Cloud Studies (1905) Verlag: KESSINGER PUB CO Nachdruck 2008 ISBN-13: 9780548985267 ISBN-10: 054898526X</ref>|300px|thumb]]

[[image:contrail.jpg|Meteorological conditions for the creation of contrails|600px|thumb]]

[[image:contrail.jpg|Meteorological conditions for the creation of contrails|600px|thumb]]

[[image:contrail2.jpg|thumb|600px]]

[[image:contrail2.jpg|thumb|600px]]

Contrails (also called Ice-SuperSaturated Regions(ISSR)) are visible clouds of liquid (condensed) or frozen water droplets which form when hot exhaust streams of airplanes meet cold air under particular weather conditions. Decompression causes cooling and condensation, as can be seen when a bottle of sparkling wine is opened. "Fog" appears above the liquid. The same process plays a major role in the creation of clouds and rain in the atmosphere.

Contrails (also called Ice-SuperSaturated Regions(ISSR)) are visible clouds of liquid (condensed) or frozen water droplets which form when hot exhaust streams of airplanes meet cold air under particular weather conditions. Decompression causes cooling and condensation, as can be seen when a bottle of sparkling wine is opened. "Fog" appears above the liquid. The same process plays a major role in the creation of clouds and rain in the atmosphere.

Some forms of clouds caused by planes appear without the interference of any exhaust gasses, for example ''winglet contrails'' which can be observed by flight passengers at the wing tips of planes under certain weather conditions, either shortly before landing or after take-off. Typical turbofan exhaust temperatures are at 600&nbsp;K approximately.

Some forms of clouds caused by planes appear without the interference of any exhaust gasses, for example ''winglet contrails'' which can be observed by flight passengers at the wing tips of planes under certain weather conditions, either shortly before landing or after take-off. Typical turbofan exhaust temperatures are about 600&nbsp;K approximately.

The formation of contrails can be explained physically: Generally, water becomes visible in the air when relative humidity (number of water droplets in relation to air temperature) exceeds 100%. The warmer the air is, the more water vapour it can hold. Modern planes travel at high altitudes where temperatures are very cold. Cold air can only hold small amounts of water. Water is created chemically when kerosene is burnt, and exits the engines as an invisible gas. The surrounding cold air rapidly cools down the hot exhaust gas, and after a few metres (depending on temperature and humidity) causes the water to crystallize, forming the contrail. The contrail remains visible until water crystals are dispersed to the point where humidity drops below 100%. In a high pressure area with low humidity this happens rather fast resp. there will be no contrail at all when temperatures are high enough in the summer, like it is often the case in southern Europe for example. In the Arctic and Antarctic, however, contrails can sometimes form even at ground level or during take-off.

The formation of contrails can be explained physically: Generally, water becomes visible in the air when relative humidity (number of water droplets in relation to air temperature) exceeds 100%. The warmer the air is, the more water vapour it is able to hold. Modern planes travel at high altitudes where temperatures are very cold. Cold air can only hold small amounts of water. Water is created chemically when kerosene is burnt, and exits the engines as an invisible gas. The surrounding cold air rapidly cools down the hot exhaust gas, and after a few metres (depending on temperature and humidity) causes the water to crystallize, forming the contrail. The contrail remains visible until water crystals are dispersed to the point where humidity drops below 100%. In a high pressure area with low humidity this happens rather fast resp. there will be no contrail at all when temperatures are high enough in the summer, like it is often the case in southern Europe for example. In the Arctic and Antarctic, however, contrails can sometimes form even at ground level or during take-off.

Given certain weather conditions, contrails can expand, remain visible for hours, or gather more clouds to the point where it starts to rain. Supporters of the chemtrail hypothesis then claim the plane ''made'' the bad weather. In reality it is exactly the other way around: Persistent contrails indicate upcoming bad weather, because low pressure areas typically come with high humidity, and the additional water (water vapour from the exhaust gas) is more than the air can absorb, and thus it will remain visible as fog or clouds. Sometimes chemtrail supporters will wonder why "spraying" has stopped. A quick look at the weather map helps: Usually there is a stable high pressure area with low humidity. Then, as soon as a low pressure area approaches, the ''chemtrail reports'' will come in again. Generally contrails appear at temperatures below -35°C, depending on altitude and humidity.

Given certain weather conditions, contrails can expand, remain visible for hours, or gather more clouds to the point where it starts to rain. Supporters of the chemtrail hypothesis then claim the plane ''made'' the bad weather. In reality, the exact opposite is true: Persistent contrails indicate upcoming bad weather, because low pressure areas typically come with high humidity, and the additional water (water vapour from the exhaust gas) is more than the air can absorb, and thus it will remain visible as fog or clouds. Sometimes chemtrail supporters will wonder why "spraying" has stopped. A quick look at the weather map helps: Usually there is a stable high pressure area with low humidity. Then, as soon as a low pressure area approaches, the ''chemtrail reports'' will come in again. Generally contrails appear at temperatures below -35°C, depending on altitude and humidity.

*'''Short contrails''' form at an altitude of about 10kms, if temperatures in the tropopause (10-15&nbsp;km) are about -50°C with a humidity below 70%. Ice crystals form, but sublimate after a few seconds and become invisible.

*'''Short contrails''' form at an altitude of about 10kms, if temperatures in the tropopause (10-15&nbsp;km) are about -50°C with a humidity below 70%. Ice crystals form, but sublimate after a few seconds and become invisible.

During daytime, contrails will cover about 0.7% of the sky over Europe during the course of the year, at night (when contrails have an insulating effect) they will cover about 0.25%. On average they will cover about 0.5% of the sky.

During daytime, contrails will cover about 0.7% of the sky over Europe during the course of the year, at night (when contrails have an insulating effect) they will cover about 0.25%. On average they will cover about 0.5% of the sky.

When a plane flies through the not always straight and level (but wavy) border between warmer and colder air, it may appear as if contrails are turned off or on suddenly, even repeatedly in case of turbulences. Air warms quicker over large dark (tarmac) surfaces than over wet meadows, and begins to rise, forming bubbles or "chimneys" of warm air. Different thermal phenomena can exist next to each other. This results in the typical hops and bumps pilots of small airplanes experience. When contrails persist long enough, their form will get changed by winds and turbulences - just like regular clouds. Different colours are the result of differing refraction and reflection of sunlight. Chemtrail believers' sites display these aspects as unnatural and thus an evidence of chemicals. But they can actually be explained naturally, and in consistence with our knowledge of meteorology and physics, requiring no additional hypotheses or conspiracy theories.

When a plane flies through the not always straight and level (but wavy) border between warmer and colder air, it may appear as if contrails were turned off or on suddenly, even repeatedly in case of turbulences. Air warms quicker over large dark (tarmac) surfaces than over wet meadows, and begins to rise, forming bubbles or "chimneys" of warm air. Different thermal phenomena can exist next to each other. This results in the typical hops and bumps pilots of small airplanes experience. When contrails persist long enough, their form will get changed by winds and turbulences - just like regular clouds. Different colours are the result of differing refraction and reflection of sunlight. Chemtrail believers' sites display these aspects as unnatural and thus an evidence of chemicals. But they can actually be explained naturally, and in consistence with our knowledge of meteorology and physics, requiring no additional hypotheses or conspiracy theories.

Such pictures of clouds which, according to supporters of the chemtrail hypothesis, were evidence of sprayed chemicals, are photographically documented since well before the first powered flight of the Wright brothers, for example in the 1905 book ''Clouds studies''.<ref>Clayden aW: http://contrailscience.com/files/Cloud_Studies.pdf</ref> The first actual contrails were recorded shortly after World War&nbsp;I,<ref>http://docs.lib.noaa.gov/rescue/mwr/049/mwr-049-07-0412c.pdf</ref><ref>Everett Wells: Article in Scientific American, ”Clouds formed by Airplanes“, 7.6.1919, page 60</ref> when more powerful planes reached ever higher altitudes. The first report of a contrail is based on a flight of pilot Franz Zeno Diemer at an altitude of 9,300&nbsp;m over Munich in 1919.

Such pictures of clouds which, according to supporters of the chemtrail hypothesis, were evidence of sprayed chemicals, are photographically documented since well before the first powered flight of the Wright brothers, for example in the 1905 book ''Clouds studies''.<ref>Clayden aW: http://contrailscience.com/files/Cloud_Studies.pdf</ref> The first actual contrails were recorded shortly after World War&nbsp;I,<ref>http://docs.lib.noaa.gov/rescue/mwr/049/mwr-049-07-0412c.pdf</ref><ref>Everett Wells: Article in Scientific American, ”Clouds formed by Airplanes“, 7.6.1919, page 60</ref> when more powerful planes reached ever higher altitudes. The first report of a contrail is based on a flight of pilot Franz Zeno Diemer at an altitude of 9,300&nbsp;m over Munich in 1919.