The evidence jets sometimes leave in the sky of their flight paths - the contrails - can apparently affect how we on the ground experience the weather.
According to research from Pennsylvania State University, jet contrails can influence ground temperature, reducing the difference between the day's highs and lows.
Geography professor Andrew Carleton said there was some research done regarding contrails, or rather, the lack thereof, during the days after 9/11 when no commercial jets were flown. Taking it a step futher, Carleton said longer research needed to be done on the effects of the trails of condensed water left in the sky.
First, here's a bit of information from the National Oceanic and Atmospheric Administration on how contrails form and persist in the atmosphere:
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.
This cloud formation is very similar to the process that occurs when you breath on a cold winter day and you can see your own breath in the form of a "cloud". You may have noticed that on some days this "cloud" you produce lasts longer than on other days where it quickly disappears. The length of time that a contrail lasts is directly proportional to the amount of humidity that is already in the atmosphere. A drier atmosphere leads to a more short-lived contrail, while an atmosphere that has more humidity will lead to longer-lived contrails. However, if the atmosphere is too dry, no contrails will form. Occasionally a jet plane, especially if ascending or descending, will pass through a much drier or more moist layer of atmosphere which may result in a broken pattern to the contrail, with it appearing in segments rather than in one continuous plume.
Carleton and graduate student Jase Bernhardt identified weather stations in the U.S. - one in the South in January and one in the Midwest in April - that they knew were frequently under contrails or not under contrails and proceeded to record daily temperatures at these locations. The researchers made sure the stations were in similar area in terms of land use, soil moisture and other conditions.
Like "ordinary clouds," a news release from the university stated, contrails seemed to decrease the daily maximum temperature in an area and raise the minimum temperature:
In the South, this amounted to about a 6 degree Fahrenheit reduction in daily temperature range, while in the Midwest, there was about a 5 degree Fahrenheit reduction. Temperatures the days before and after the outbreaks did not show this effect, indicating that the lower temperatures were due to the contrail outbreaks.
"Weather forecasting of daytime highs and lows do not include contrails," Carleton said in a statement. "If they were included in areas of contrail outbreaks, they would improve the temperature forecasts."
Watch PSU's video for more details about the research:
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