Condensation trails, or more simply contrails, form when hot, humid air from a turbofan jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing and cloud formation you see behind the aircraft is a result of turbulence generated by the engine's exhaust. This process is similar to the "cloud" you see outside on a cold winter day when you exhale and "see your breath." This process is called isobaric mixing. That is, the air from your breath is warm and contains moisture (like the exhaust of the engine). As you exhale your breath mixes with the environmental air that is colder and dryer. Essentially, you are mixing two unsaturated air parcels to produce one that is saturated without any change in pressure (isobaric).
Contrails that dissipate rapidly are a sign of turbulence in the upper flight levels. You can definitely see cases where the contrail is very long and persistent and retains sharp edges over time periods of 30 min or more as in the image above. This implies very smooth conditions at the flight level of the aircraft creating the contrail. At other times, if the contrail becomes ragged very quickly and dissipates within 5 minutes, this may imply moderate or greater turbulence in the flight levels.
The wind speed aloft impacts the rate of spreading of the contrail. Stronger winds will generally flatten out the contrail and also may aid in its dissipation. When the winds aloft are weak (less than 50 knots), contrails may persist for 30 minutes or longer.
The Skew-T log (p) diagram above near the Charlotte Douglas International Airport (KCLT) is valid at the same time and general location as the contrail pictured above. Notice that the environmental lapse rate (red line) is nearly constant with height and fairly close to the standard lapse rate of 2°C per 1,000 feet. This provides very stable conditions. Also notice that the winds aloft are generally out of the north and do not change direction with height (no backing or veering). Lastly, the winds in the flight levels are fairly weak and the velocity is generally constant with height. All of these aspects of the environment are consistent with a stable and nonturbulent atmosphere.
The tropopause is the intersection of the troposphere and the stratosphere. This is marked by a sharp shift in the temperature profile as shown above. It is located at approximately 49,000 feet MSL owing to a strong ridge of high pressure over most of the lower two-thirds of the U.S. on this day. Notice that there's a small dewpoint depression (temperature - dewpoint spread) that begins around 37,000 feet MSL to just above the tropopause. This is the most likely altitude range to see contrails form.
In some cases you may see contrails mentioned in the remarks section of a surface observation like the one shown below.
KNHK 081408Z 23019G26KT 10SM FEW100 SCT250 09/M07 A2979 RMK AO2 PK WND 23026/1401 CONTRAILS NE-SE-S
Below is a satellite loop that depicts a number of crisscrossing contrails in the central High Plains. Notice all of the contrails that are created during this loop are persisting for quite some time implying smooth conditions aloft. You can also notice that contrails that are perpendicular to the flow (east to west in this case) tend to spread out more than those that are parallel to the flow aloft.
Want to learn more about the Skew-T? You can order your copy of The Skew-T log (p) and Me eBook today.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
Founder, EZWxBrief™
CFI & former NWS meteorologist
