Today driving to work I saw a small, isolated horse's mane cloud at about 2000 feet. Directly above it was a small strato-cumulous cloud at 4000 feet and and they were moving in opposite directions. The horse's mane was moving north and the strato-cumulous was moving south.

How could this happen? I imagine there must have been two layers of air, each moving in a different direction. In fact there must have been three layers of air, because the strato-cumulous would have been sitting on one layer and moving with the layer above.


This indicates that there was wind shear. Wind can change direction and speed with height.

Since air is a fluid, it's wind vectors can change with height. On a large scale, this is called the thermal wind balance. In the ocean, it is Ekman balance. On smaller scales, there isn't really a term that I know of that describes the balance, because it is rather turbulent.

As far as "How could this happen": What you saw was the mean flow over the height of one cloud, followed by the mean flow over the depth of another cloud.

  • $\begingroup$ If the atmosphere was homogeneous, with a single horizontal vector, then why would clouds be at different heights at all, and why would global atmospheric models calculate up to 40 different atmospheric layers? There is no mystery here. It is well known that the atmosphere is complex, with many interacting layers, many of which have entirely different air currents. $\endgroup$ Jun 9 '16 at 15:39
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    $\begingroup$ The Ekman balance in the ocean has little or nothing to do with the thermal wind balance. The Ekman balance assumes constant density, while thermal wind considers the horizontal gradient in density. So both things you mention are wrong! $\endgroup$
    – arkaia
    Jun 9 '16 at 15:40
  • $\begingroup$ The atmosphere is not homogeneous. That statement was not used in the asking of the question. Depending on your definition of homogeneous, hypothetical atmospheres aren't homogeneous. Ekman balance is the balance between friction, horizontal pressure (density) gradient force and coriolis force. The vertical variation of Ekman balance due to vertical variation in the frictional force provides a good analogy of how wind can vary within a fluid over a depth. This creates the spiral in z, showing how the fluid can change velocity with depth. $\endgroup$ Jun 10 '16 at 15:09
  • $\begingroup$ Anyway, the scale that the clouds were observed are too small for at least the Ekman and perhaps the thermal wind balance to truly be valid as an approximation. I used those as examples of how the atmosphere is not homogeneous. $\endgroup$ Jun 10 '16 at 15:11

Pressure gradients cause movement of air, i.e. wind. Wind blows from high pressure to low pressure. Wind can be looked at as a vector quantity. It has magnitude and direction. What you saw was due to the fact that the direction of the wind at 2000 feet was different from the direction of the wind at 4000 feet. This is called wind shear (vertical wind shear in this case since we are concerned with the difference in the vertical direction.) Wind shear can be caused my many different phenomenon, such as: A frontal passage (sharp gradient in the temperatures of the air in the horizontal direction). Temperature inversion (temperature usually decreases as you increase altitude, but in some case, over a layer of the atmosphere, it can increase) enter image description here

Warm air has lower density (molecules have more energy and are bouncing around with greater intensity and speed) than cold air, a warm air pocket next to a cold air pocket causes a horizontal pressure gradient and wind to blow, if there's a pocket of hot air (due to a temperature inversion) above the cold air pocket, the reverse will begin to happen (over a layer, you will see that the red has move to the right and blue to the left) and you will see wind blowing in the opposite direction due to the new pressure gradient.

Thermal wind is a meteorological term (not referring to an actual wind), but a difference in the wind between two pressure levels; in essence, wind shear.


The previous two answers have pointed to wind shear, which is certainly something that happens.

There's also a perceptual issue, where an object moving one direction in the foreground can cause the illusion that an isolated background object is moving in the opposite direction.

Thirdly, if you were driving, your own velocity contributes to perceived motion. If you were driving north, the clouds would appear to be moving south, and lower clouds would appear to move faster.

Fourth: are you sure of your description? "Horse's mane" cloud sounds like cirrus, which would be much higher in altitude (>6000m) than stratocumulus (<2000m).


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