Weather Watching Guide - Contrails

4:50 pm Monday February 1st, 2021

Man made contrails can be useful for understanding the weather. I have written about the pollution aspects of jets in "Blue Sky Blues". This article will take us back to my very keen early days as a meteorologist when everything I saw made me ask "why". I envisioned thought experiments in my mind in an attempt to figure out the problems that I had assigned myself. Much of this effort played out on quiet night shifts. This one was about a contrail I witnessed back in Shearwater when the  skies were blue and advection fog was not messing with my forecasts. The contrail was sharply defined on one edge and not the other... "why"? I figured the answer out in 1979. It came back to me as I was watching a recent sunset. 

Every cloud has a story to tell… even the aircraft contrail. Here are the postulates I use for reading the stories written in the clouds.Also see "Weather Watching Guide for Everyone…" 

•       Every shape is generated within the atmospheric frame of reference.

•       The differential motions of the relative winds do the shaping.

•       Gravity waves are everywhere perpendicular to the relative winds in a stable layer. The spacing of these gravity waves is related to wind speed just like waves on the ocean. Larger waves are separated by larger spacing and are created by stronger winds.

•       Cloud streets and Langmuir streaks are everywhere parallel to the relative winds in an unstable layer.

•       Differential tangential changes in wind speed cause rotation –swirls.

•       Differential perpendicular changes in wind speed cause convergence -  DeformationZones.

This one contrail in the Singleton sky resulted from a jet probably leaving Toronto’s Pearson International Airport and headed to Europe. The postulates can be applied to the moisture patterns to decipher the weather story whether they be natural or man-made in nature.

The approach that I prefer is to start with the obvious clues and assemble as many puzzle pieces as possible. One can infer any missing clues since the pieces will only fit together in a few ways while still obeying the laws of nature.  The best solution will have the most puzzle pieces fitting meteorologically together into a plausible and consistent weather story.

These clouds are all ice crystal cirrus clouds. The layers of cirrus cloud are certainly at different heights above the ground but that does not impact on the weather story.

The sunset timing reveals that the view is westerly. The exact timing of 4:50 pm on Monday February 1st, 2021 would yield the exact angle but that too is not important. (244.5 degrees courtesy of https://www.sunearthtools.com/)

The train track optical illusion makes the streaks of cirrus appear to converge in the distance for the same reason as crepuscular rays. The Langmuir streaks of cirrus are actually parallel. This can be verified from the satellite view of the patterns.

Point A is located along one of the Langmuir Streaks. The ice crystals of the Langmuir streaks occupy a significant layer of unstable atmosphere in order for their formation. The bottom of this layer is the stable layer that is responsible for the gravity waves incorporated within the Langmuir streaks. I inferred the east wind (blowing toward the sun) because ascent is required in the atmosphere for cloud formation. I was playing the warm conveyor belt conceptual model in my mind.

Point B is located at gravity waves in the contrail. The jet had a significant head wind on his way to Europe. These gravity waves of the contrail are at a lower level than the Langmuir streaks but are in the same wind field. The contrail is persistent implying that the air mass is moist which is also consistent with the warm conveyor belt conceptual model

Point C is located at smaller gravity waves within the larger gravity waves of the contrail. These are what caught my eye and why I did the painting. I can explain...

The contrail has a sharp northern edge (to the right in the image) and a diffuse southern edge. Why?

The dispersion of the ice crystals can be explained mathematically … but not today. The pattern is very common and has a simple explanation not requiring differential equations. The Gaussian profiles can be simply approximated by two outward point vectors. Consider Hansel and Gretel leaving a broad trail of equally scattered bread crumbs. Atmospheric dispersion caused by turbulence will spread these crumbs outward in an even broader path. The particles are originally clustered along the path of the jet. The green concentration curve in the following graphs is concentrated with a narrow and high peak. With time the particles disperse outward and the concentration curve shift to the red, the green and eventually the broad and flat orange curve. 

In Graphic 1, the atmospheric dispersion of the contrail can be simplified by the blue arrows in order to avoid the complex mathematics. Ice crystals in column b can stay where they are while those in columns a and c move to the shoulders of the normal distribution with time.

In Graphic 2, the atmosphere relative winds are depicted by the large block arrow which moves the entire contrail. This relative wind will also move the ice crystals in the three columns. None of the ice  crystals have been moved yet by those vectors.

In the final graphic 3, the vector addition of the atmosphere relative winds to the dispersion vectors enhance the downwind convergence while increasing the upwind divergence. The crystals in column c are moved to the right by both the dispersion and the relative wind vectors. The crystals in column b are only moved by the relative wind vector. The crystals in column a do not move much as the dispersion vector is in the opposite direction of the relative wind vector. 

Note: The number of ice crystals and the area under each of the coloured distributions is intended to be the same.

The next time you view a contrail in the sky, now you will know how to hear what it is saying. If the drift is from the south which is likely the case for a persistent contrail in a moist air mass, you can expect warmer weather and perhaps even a low pressure system. The drift from the south will produce a sharper, more defined northern edge. 

If the contrail is drifting toward the south with the sharper edge on the south side of the contrail, you can expect cooler and drier weather. The southward drifting contrail is less likely to be persistent.

Keep you paddle in the water and warmest regards,

Phil the Forecaster