Making Chili Science

#0839 "Cold Air Mass Cumulus" 8x10

This is the next installment in knowing the weather through observing the wind. 

I would tend to dream on quiet night shifts playing thought experiments in my mind when examining satellite imagery. The simpler the explanation, the more likely it would be to bear fruit and to circulate among the team. Here is one of my favourites. 

Imagine large pencils rolling along a table. I used colloured pencils a lot when working the forecast desk. Friction provided by the table and even a gentle push at the top gets the pencils rolling. In today's reinvention of that nightshift, we were making Chili during a snowstorm, so I used cans – same idea but tastier. 

These hand waving thought experiments from night shift required both of my hands with me looking along the direction of the wind in the accompanying graphic. Bodies are biologically constrained so that the fingers must be curled in the same orientation as the rolling pencil or bean can.

Two Cans Rolling on the Floor

Now imagine any non-uniformity like a locally stronger push on one of the cans. That push is equivalent to a locally stronger wind. This situation is exactly analogous to what happens in the planetary boundary layer (PBL) of the atmosphere. Friction at the surface slows the wind while aloft, the stronger wind is never perfectly uniform. 

Venturi Meter and Bernoulli's Principle 

Now let’s invoke Bernoulli's principle which states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure. Bernoulli said a lot more but without getting into the math, the local decrease in pressure encourages air inflow towards the locally stronger wind speeds. Air must flow from higher to lower pressure. The pressure gradient directs air inward toward my thumbs where the wind speed is stronger. 

What happens when the stronger wind speed is applied to the rolling cans over a period of time?

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If I then turn my thumbs to align with the stronger wind of the speed increase, the thumbs must also align. The sense of rotation of the fingers from both hands, complement each other and a local updraft is created as you gaze down on your fingertips. Air convergence also creates an updraft – the air has to go somewhere! If the resultant lift is sufficient to reach the lifted condensation level for the air mass, a cumulus cloud is born. 

If we try to immerse ourselves in the atmospheric frame of reference, the local speed maximum must be shouldered by local speed minima, at least in a relative sense. On that night shift, I flipped my thumbs around to point in the opposite direction in order to simulate the relative speed minimum. This is a simple slight of hand. Do not rotate your body or move your arms.  Being biologically constrained once again, the fingers of my hand now circulated downward. The air in the local minimum was descending. If there was cloud, it was dissipating in the sinking air. 

The final step was to add the circulations formed with the local speed maxima to the speed minima. The circulations from my fingers merged together perfectly. The resultant pattern could be replicated as far as the wind regime permitted. 

A Locally Stronger Wind
Shouldered by Locally Weaker Winds

But why a line? On that night shift, I invoked the continuity equation. Air converging to a pressure decrease caused by a locally stronger wind speed can be self-sustaining. In an unstable environment that encourages the growth of cumulus cloud, the initial cumulus cloud will be guided by the wind. The conditions that caused the first cloud to form, will also result in a second and a third cloud. Before one can say “a mix of sun and cloud”, a line of cumulus guided by the wind has formed. On the bright side, a line of clear skies shoulders the street of cumulus. This pattern is simply copied and pasted across the landscape. Cloud streets happen every day... 

Snowsqualls are great examples of cloud streets

The wind rolling pencils or cans on a table top can create cloud streets parallel to those winds. This movie played out in my mind but was confirmed by the satellite image. More on these concepts to follow. 

Warmest regards and keep your paddle in the water,

Phil the Forecaster Chadwick