The Solution to Cloud Swirls Can Be Found in Your Hands

#2361 "Singleton Sunset Swirl"

In the atmospheric frame of reference, every cloud shape can be considered in terms of a swirl. If you align the fingers of your hand in the direction of the cloud curvature, your thumb must point either up or down. Why and what does it mean? 

A movie played in my mind when I was attempting to explain this stuff years ago, arms flailing and pencil and paper flying. My friends at COMET turned some of those into animations and graphics. I will try again with a different approach. 

Imagine yourself at the equator with you back to the rising sun. Life is good with the warmth on your back. You can’t feel it but you are in a non-inertial frame of reference. The Earth is spinning as it orbits the “stationary” sun. Fictitious forces that result from the rotation of the Earth include the Coriolis and centrifugal forces. These forces make weather even more interesting as they shape the lines in the sky. 

Imagine I am at the equator at sunrise looking west...

Now stretch your arms outward and curl your fingers out as well. Your right arm must point north while your left arm is directed toward the south. The fingers of both hands are curled cyclonically. Both thumbs of your cyclonically curved hands must point upward. 

The previous entries in the Weather Blogs all mentioned the “right hand rule” because the selected weather examples were in the northern hemisphere.  When I participated in the Solar Car Race in the Australian Outback in 1996, I employed the “left hand rule” instead. Both rules describe cyclonic rotation. 

The effects of the Coriolis force at the equator are not noticeable - in fact, they are zero. An observer stationary with respect to the sun, could watch you standing at the equator. They would see that your back does not turn at all as the Earth does it daily spin. 

If you could stretch your arms gradually until your hands reach the poles, the Coriolis force gets really interesting. The same solar stationary observer, would see your right hand complete one entire, cyclonic rotation each and every day. Gumby standing at the equator with his right hand stretched to the North Pole would not feel any difference but Gumby is within the non-inertial frame of reference. An observer looking down on the North Pole would see Gumby’s hand rotate cyclonically counter clockwise. At the same time, Gumby’s left hand would also complete one full cyclonic rotation every day. An observer above the South Pole looking downward would see Gumby’s left hand rotate clockwise. 

My Friend Gumby

The impact of the fictitious Coriolis force increases dramatically from the equator to the poles. This explains why the swirls and deformation zones are more obvious and dynamic over the higher latitudes. 

Everything that I explained in the earlier Weather Blogs can now be employed in the Southern Hemisphere remembering to employ the “left-hand cyclonic rule" south of the equator. Cyclonic rotation has the same meteorological implications in both hemispheres but you have to use the appropriate hand. 

Another visualization is to shoot arrows from the Poles. An archer at the North Pole aiming toward the south (the only available direction) would see the arrow deflected to the right as the Earth rotates counter-clockwise underneath. The fingers of the archer’s right hand will curl counter-clockwise as well when the thumb points up. Similarly an Antarctic archer at the South Pole shooting northward (an impossible to miss shot) would see the Coriolis force deflect that arrow to the left. From the South Pole perspective the Earth is rotating clockwise in the direction of the fingers of your left hand with the thumb pointing upward. 

There are many ways to look at the fictitious Coriolis created by the spinning Earth. The take away messages are:

• The Coriolis force increases from zero at the equator to a maximum at the poles.
• The deflection is always perpendicular to the motion and to the right in the Northern Hemisphere but to the left in the Southern Hemisphere.
• The rotation of the Earth defines the cyclonic sense of rotation. Cyclonic is counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• Differential advection of cyclonic rotation with height results in upward vertical motion as indicated by the direction of your thumb (Trust me on this one).

Rotation creates the swirls in the atmosphere. Simple conceptual models based on these swirls can explain everything you need to know about the atmosphere and the weather. The following graphic explains the weather painted in #2361 "Singleton Sunset Swirl". You can do it as well with just a few lessons and lots of sky gazing practice. 

Keep you paddle in the water and warmest regards,

Phil the Forecaster Chadwick