We had such nice weather last week and a few of the days were almost cloud-free. Well, they were cloud-free except for some clouds that formed behind airplanes.
On a few of those days, we saw airplanes and a trail that was being left behind them. These planes flew all around the Alleghenies and some people called in to our newsroom asking what they are spraying. To clear things up, let me explain what contrails are.
Basically, the contrail is a trail of condensation that occurs behind an airplane.
According to the National Weather Service, here’s exactly how these are formed…
Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. Vapor pressure is just a fancy term for the amount of pressure that is exerted by water vapor itself (as opposed to atmospheric, or barometric, pressure which is due to the weight of the entire atmosphere above you). The mixing occurs directly behind the plane due to the turbulence generated by the engine. If condensation (conversion from a gas to a liquid) occurs, then a contrail becomes visible. Since air temperatures at these high atmospheric levels are very cold (generally colder than -40 F), only a small amount of liquid is necessary for condensation to occur. Water is a normal byproduct of combustion in engines.
Contrails aren’t new at all, either. They have been around as long as planes have. They have been reported all over the globe, as well. With more air traffic, they are becoming more frequent.
So why do they spread out like they do? That has to do with the wind speed and direction at different levels in the atmosphere. The wind direction and speed is often very different at different heights in the sky. The time the contrail stays in the air and visible has to do with how much moisture is in the sky. With a more humid atmosphere, the contrails will last longer than in a drier atmosphere.
According to the website I found authored by the NWS, sailors have been known to use the contrails in the sky to predict the coming weather. When the contrails disappear quickly or are non-existent, they can expect nice weather to continue. With a contrail that stays in the sky for a long time, that means increased moisture and occasionally warmer air aloft. That will point towards a more turbulent atmosphere and a change in the weather.
Check out the example below…
While it’s a little complicated, it explains why sometimes we see these contrails appear to ‘shut off’ and then ‘turn on.’
Imagine two locations, we’ll call them A and B. Imagine a plane traveling between these two pockets of air. When the air is unsaturated, the contrail will not form. However, when the air is saturated, the vapor from the plane will not be able to go into that atmosphere and has no other choice than to cool, condense and form a cloud.
So, in the example to the right, if a plane were to travel from location A to location B, the plane would not form a contrail at first but would as soon as it crossed that imaginary blue line into a saturated atmosphere, it would create clouds, or contrails. Then, when it passed back into unsaturated air moving towards location B, the clouds would essentially ‘shut off.’
Thanks to http://cimss.ssec.wisc.edu/wxwise/class/contrail.html for the description.
Some people believe this off and on is a result of a pilot turning on and off a chemical spray. These people believe contrails are actually chemtrails and are chemicals released by airplanes, as in chemical trail. While we can’t be 100% certain that chemtrails are not happening, it appears unlikely. A lot of the reasons people give to explain these chemtrails can be explained. One of the most common is the fact that they appear to be turned on and turned off. I debunked that myth with the information above.
Here’s the information from Wikipedia regarding the, as they call it, the ‘Chemtrail Conspiracy Theory.’ (I know this isn’t the best source for information when writing official papers, but this has some good information.)
The chemtrail conspiracy theory holds that some trails left by aircraft are actually chemical or biological agents deliberately sprayed at high altitudes for a purpose undisclosed to the general public in clandestine programs directed by government officials.
As a result, official agencies have received thousands of complaints from people who have demanded an explanation. The existence of chemtrails has been repeatedly denied by government agencies and scientists around the world, who say the trails are normal contrails.
The United States Air Force maintains that the theory is a hoax which “has been investigated and refuted by many established and accredited universities, scientific organizations, and major media publications”. The British Department for Environment, Food and Rural Affairs has stated that chemtrails “are not scientifically recognized phenomena”. The Canadian Leader of the Government in the House of Commons has stated that “The term ‘chemtrails’ is a popularized expression, and there is no scientific evidence to support their existence.”
Here’s the link to the NWS site that I found a lot of information on… http://www.wrh.noaa.gov/fgz/science/contrail.php?wfo=fgz
Here’s the link for the Chemtrails Conspiracy Theory Wikipedia site… http://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory