Cloud Shapes and Lines in the Atmosphere

#2216 

"Singleton Winter Contrails Cirrus and Deformation" Pixels Link

This is the grown up version of#1533 "Long Reaching Deformation Zone". The small panel begged for a larger rendition because of the design and the meteorology. I hope that bigger is better in this case. This painting also tells the story of why clouds are shaped the way they are. It’s all in the spin that you put on the moisture. 

Cloud are born, live and die within the atmosphere. We have to view them from our fixed location on the surface of the earth. It is our only option and we see some wonderful patterns. If we want to really understand the cloud shapes though, we need to examine them from the atmospheric frame of reference. When we examine clouds from the atmospheric frame the complex patterns become much simpler.

To view the clouds from the atmospheric frame we need to move with the average motion of the clouds. Let’s consider an area of interest and average all of the winds observed from the earth’s surface to calculate the average surface observed wind within that box. Every observed wind can now be expressed as the sum of this overall average wind and a wind difference. This wind difference is the wind in the atmospheric frame of reference and it shapes the clouds. In other words the atmospheric frame of reference is an observational platform or coordinate system moving with the mean flow of the atmosphere. Some graphics might help. 

In the above graphic our simple atmosphere consists of four points. In this simplest case for the purple earth frame. These four points are all moving in the same direction and at the same speed. The average of these four identical motions is the same purple vector. In the green atmospheric frame of reference the four points are stationary. They are not moving relative to each other. The winds measured in the purple earth frame of reference are simply the addition of the mean motion of the earth frame winds plus the wind difference measured in the green atmospheric frame of reference for each of the four points. The shape of any cloud will not change if all of the points within this cloud are moving with the same velocity which must also be the average velocity for the box of atmosphere we are observing. 

In the second graphic above we have changed the winds in the atmospheric frame of reference very slightly. Two of the points in our simple four point atmosphere are approaching each other at the same speed. The average of these two atmospheric frame winds is zero so they do not change the average wind in our atmospheric box at all. If we add these atmospheric winds to the mean wind, the winds as seen in the earth frame of reference are the solid purple vectors within the purple box. Earth born meteorologists would simply observe a slight increase followed by a slight decrease in the westerly winds – if we add the points of the compass to our simple graphic (north being upward on the page). Such a minor variation in the west wind would be a challenge to observe. 

However something almost magical is revealed in the atmospheric frame of reference – a line is born in the cloud field! Any moisture within our simple atmosphere must be stretched parallel to the green double headed arrow which is the symbol for the deformation zone. The deformation zone is my favourite meteorological tool and I helped to discover the mysteries behind it. The shape of cloud line in the sky and the deformation zone can reveal everything about that portion of the atmosphere and the weather. 

The variations in the wind speed is wind shear and this causes the atmosphere to rotate like a child’s pinwheel. By convention the red “X” is considered to be positive or cyclonic (counter-clockwise in the northern hemisphere) rotation of the atmosphere. The blue “N” is considered to be negative or anti-cyclonic rotation cyclonic (clockwise in the northern hemisphere). The right hand rule makes this easy to remember. Pointing your right thumb upward at an “X” also points your fingers in the direction of the cyclonic rotation of the atmosphere. Pointing your right thumb downward at an “N” points your fingers in the direction of the anti-cyclonic rotation. 

Rotation in the atmosphere is very important to the weather but it is even more important in shaping the clouds. The mean wind in the atmosphere may move the clouds around but it is the rotation caused by subtle wind shear that really shapes the clouds. In addition and with admittedly great simplification, rising air and thus clouds are more likely located in the quadrants of the deformation zone conceptual model occupied by the red “X”. The opposite and descending cloud free air is more likely in the blue “N” quadrants. 

The observation of a line of cloud in the atmosphere can reveal the subtle relative motions within the atmosphere itself. The power of this conceptual model and the understanding of cloud shapes is remarkable. I have made a career out of unravelling these truths which were once mysteries. There is much more to come. 

There are other analogies that can be used to explain frames of reference. One can think of the atmospheric frame as a box car of a train. All of the box cars are moving at the same speed as part of the train. Simple patterns can be made within each box car through uncomplicated relative motions that are easily understood within each of the cars. Observing and explaining these motions and shapes from the siding outside the train is much more problematic and full of twist and turns. 

The take away truth is that if you really want to understand the clouds and atmospheric motions, you need to live in the same world as the clouds. The secret is to look for lines. 

I will place other simple motions within the atmospheric frame and hopefully, open the beautiful world of clouds up to you… much more to come along these lines… so to speak.