Jet contrails can sometimes make for beautiful sunrises and sunsets as lines of clouds stretch from horizon to horizon, bending the light from the incoming sun.
But have you ever wondered why they form and why they seem to be more prevalent than they used to? First, consider the Meteorology of the atmosphere. Even in the Summer, temperatures are well below zero Fahrenheit at the altitude where commercial airliners fly. In fact, at the top of Summertime thunderstorms, temperatures can be as cold as -30°F!
Turbine jet engines produce condensation, much the same as we do when we exhale and "see our breath" on a cold Winter's day. Contrails are formed when hot, humid air from jet exhaust mixes with cold air with low pressure. The mixing of that air is a result of the turbulence behind the jet.
You'll probably see quite a few more jet contrails in the Upper Midwest today. pic.twitter.com/hiBIQxwqXn
— EricSorensen (@ERICSORENSEN) July 18, 2018
What are the conditions that produce them and why does it seem there are more of them? According to the FAA, more than 5,000 airplanes are flying over the United States on any given day and that number is steadily increasing.
Airline passengers in the United States (source: WorldBank):
- 798 million (2015)
- 720 million (2010)
- 721 million (2005)
- 665 million (2000)
- 534 million (1995)
- 465 million (1990)
- 372 million (1985)
- 295 million (1980)
- 205 million (1975)
- 163 million (1970)
So if it seems there are more contrails in the skies these days, you're exactly right!
We don't have to know much about the aircraft type or which flights are going overhead but if we pay close attention to the atmospheric conditions, we can understand when contrails will overspread the sky. Higher moisture levels are needed at flight level between 30,000 and 36,000 feet. While short contrails are possible in times of lower humidity, long contrails can last for minutes and hours when the relative humidity is around or above 70% at flight level. That occurs more often when low pressure (storm systems) lie upstream to our west. So if we have dry, high pressure in the atmosphere, jet contrails are a lot less likely. That's one of the reasons there are fewer jet contrails in the Desert Southwest, versus the Midwest or Southern States. (However, there are a lot more planes flying between the bigger population centers east of the Rocky Mountains.)
Another fact: Wednesdays have fewer jet contrails because that is the day with the least amount of passenger air travel. Higher passenger counts are seen on days that bookend the weekend and around holidays.
-Meteorologist Eric Sorensen