Uploaded on May 17, 2009
CBS 29 News
This is an ongoing controversy with some people who insist a malevolent conspiracy is subliminally gassing Americans.
I have found no credible evidence of this. Condensation trails are common and form as a function of temperature. Condensation nuclei are produced by hydrocarbon exhaust and quickly form ice fog in the high atmosphere. The colder it is at flight level the more persistent the ice fog is.
There are really three types of fog (which are manifest as clouds aloft): 1) regular water droplet fog at temperatures above freezing, 2) water droplet fog at temperatures below freezing (which can result in icing conditions for aircraft), and 3) ice crystal fog (ice fog). Ice fog generally forms at temperatures below -22F. In arctic towns during the dead of winter it is not unusual to have ice fog form behind cars on the road. Ice fog dissipates very slowly because it must sublime from solid to vapor which requires a molecular heat transfer of about 540 calories per gram. At extremely low temperatures in the 30,000 to 40,000 foot range (between -60 and -100 degrees F), the change of state can take some time.
When I was a military meteorologist in Alaska back in the 1970s, one of the routine forecasts I put out was a contrails prediction. It was something that dated back to World War II. Our bombers needed to know at what altitude they must fly in order to NOT produce contrails. Of course, there was radar back then and the Japanese probably could spot an incoming B-29, but perhaps not. If contrails were visible 100 miles away then everyone would know. Nomograms were developed for the prediction. It was fairly straight forward. A critical temperature and pressure was required for contrail formation, usually something like -45F. Humidity came into the equation for relatively warm temperatures, but once it got below about -70F humidity didnt matter- ice clouds would form regardless. When you see jets in which the contrail forms for only a short distance behind the aircraft and then it quickly disappears- that means the jet is flying at an altitude close to the critical contrail temperature.
I can also tell you that the manner in which contrails disperse, or dont disperse reveals important information about atmospheric stability and diffluence. The same is true about lenticular clouds that form in Tehachapi. These mountain wave clouds dont automatically form when its windy. There are several other considerations, such as stability, humidity and the wind angle relation to the mountain range. In the same way contrails will expand in areas of diffluent flow (winds pulling apart over a horizontal area). Much of this also applies to steam or exhaust that is released from a common smoke stack in breezy conditions. If the smoke goes up or if it goes down, if it expands or if it stays uniform- all tell a story about the character of the atmosphere.
So when contrails form an X in the sky, it means there are two persistent contrails behind aircraft with tracks at 90-degrees to each other- nothing more.