Billow Talk

I think flags that serve as banners for professional baseball and football teams are a little vain. For starters, why is it that these flags always seem to be promotionally photographed in their most flattering of profiles, mugging for the camera with their colorful team insignias fully unfurled and gently rippling in the wind? And they assume such pompous poses regardless of wind direction, so I consider any sports flag to be nothing more than a vain vane.

Curiously, the literal meaning of the verb "flag" is to " hang down or droop"-not a very complimentary profile for flags in general. But there's no need for me to create any big flap over the glaring inconsistency between a flag's image and its droopy namesake verb. After all, I was never one to make waves.

Clouds are not as gracious as I tend to be. Apparently resentful of the high-profile image of flags, envious clouds cannot just look the other way. Indeed, under special atmospheric conditions, billow clouds closely imitate flapping flags as they unfurl across the sky in wavy, rippling patterns. A wondrous photograph of billow clouds (previous page), taken over Mt. Shasta in northern California by photographer and artist Kay Ekwall, is one of the best I've ever seen.

"Waves of Ascension"

Billow clouds sometimes form over Mount Shasta in northern California. Courtesy of Photographer Kay Etwall (she calls this photograph "Waves of Ascension").

Ekwall calls the photograph "Waves of Ascension" because the billows gracefully ascend Mt. Shasta (at an elevation of 14,162 feet), apparently in concert with moist air rising up the sloping terrain. For readers with an appreciation of such unique and striking sky photographs, I highly recommend checking out her Web site.

In Ekwall's photograph, the billows formed on the upwind side of majestic Mt. Shasta, making the image even more special to me (remember, folks, I'm an Easterner, and such scenes are not commonplace here among the beautiful Appalachian ridges of central Pennsylvania).

That I liken billow clouds to flapping flags might seem like a stretch to some readers. Just consider their fabrics. The cloth of a flag must be strong enough to withstand continual torment by the wind, while the atmospheric threads from which rippling billows are woven-tiny water drops or ice crystals-seem too flimsy to withstand the wind and unable to hold a flapping pose long enough to be photographed.

Billow Embroidery

If the truth be known, cloud droplets are only embroidery for billows-the stiff cloth from which billows are woven is statically stable air (more on this in a moment). Besides, Grenci, wouldn't it have made more sense to compare billow clouds to ocean waves breaking on a beach? There's no denying a definite likeness.

Although billow clouds may resemble breaking ocean waves, such a comparison, albeit understandable, is unfortunate and misleading. For the record, ocean waves result from the give-and-take between the forces of gravity and buoyancy (submerge your rubber ducky in your bathtub and release it to observe the upward force of buoyancy). Once the wind ruffles the ocean surface, the give-and-take between buoyancy and gravity allows ruffles to grow into waves, with water oscillating up and down in a circular path (picture a piece of floating driftwood-it will bob up and slightly backward and then fall down and slightly forward as a wave passes).

Billow clouds, on the other hand, depend on a give-and-take between vertical wind shear and static stability, so any attempt to liken billows with ocean waves is scientifically doomed to fail. In the case of the give-and-take forces for billows, vertical wind shear translates to a rapid increase in wind speed with altitude. Static stability is a measure of the atmosphere's resistance to vertical displacement, which ultimately depends on the difference in temperature between a parcel of air and its environment.

If a parcel is given a slight nudge upwards and it becomes colder than its environment, it will resist further vertical displacement. Under these conditions, the atmosphere is said to be statically stable. When the give-and-take between vertical wind shear and static stability in a layer of the atmosphere results in billow clouds, shearing instability, formally called dynamic instability, is said to be at work.

Shear Energy

"Wait a minute, Grenci," you're probably muttering under your breath. "You just wrote the words "stability" and "instability" in consecutive sentences. Which one is it, man? Make up your mind, for gosh sakes!" Actually, folks, I'm not contradicting myself here. I hope to prove to you that static stability is needed for shearing instability (and billows) to exist. Read on.

We can clear up the confusion arising from static stability and shearing instability if we first confront the more familiar problem of flapping flags.

In the case of rippling flags, shearing instability results when winds of different speeds blow on either side of the cloth (granted, the case of a flapping flag is an example of horizontal wind shear, not vertical wind shear, but, once we're ready to tackle billows, I'll just turn the argument on its side). Another key observation of flapping flags is that their ripples form at right angles to the wind (please file this away for future reference).

Acting alone, horizontal wind shear is not sufficient to keep flags flapping in the wind. Indeed, the ripples that horizontal wind shear generates are encouraged and restrained by just-the-right stiffness and strength of the cloth. A flag made of steel wool, for example, would be too stiff and thus barely ripple, if at all. And a flag made from the little white fuzzies of past-bloomed dandelions would be too delicate and rapidly rip apart in the wind.

A Give-And-Take

The principle of give-and-take between horizontal wind shear and the strength and stiffness of the cloth of flapping flags can now be applied to the formation of billow clouds. Recall that I intend to turn our discussion on its side-instead of dealing with changes in wind speed on either side of a flag, we must switch to the case in which the wind changes speed with altitude (speed increases). To repeat the lesson we already learned from flapping flags: ripples form at right angles to the horizontal shear of the wind.

If we follow suit and apply the spirit of this observation to the case of a lofty layer of air that's subjected to vertical wind shear, any ripples that develop will form at right angles to the shear (picture the lofty layer of air as a flag that is unfurled parallel to the ground at some altitude, with the wind blowing faster above the flag than below it-if the flag flaps, the ripples will be oriented up and down like those in Ekwall's photo).

But not so fast. I said "IF the flag flaps." The only way that the lofty layer of air can imitate a flag is if it is made from "cloth of proper stiffness." Here's where static stability comes in. If static stability is relatively low, the layer's resistance to vertical displacement is also relatively low. It's as if the fabric of the layer is made of dandelion fuzzies. That is, the static stability (stiffness) is unable to prevent the vertically sheared layer's natural inclination to wildly ripple. In rapid fashion, shear-generated ripples grow to the point where order breaks down into the helter-skelter world of turbulent flow.

In terms of flags made of dandelion cloth, growing ripples would rapidly break down into a chaos of white fuzzies that would disperse in the wind. In the atmosphere, any vertically sheared layer of air with low static stability is considered dynamically unstable, meaning the static stability is too small to resist wind shear (resistance is futile).

In the other extreme, if the vertically sheared layer were highly statically stable (tantamount to a steel-wool flag), it would greatly suppress ripples and waves. In short, the vertically sheared layer of air would be highly dynamically stable, meaning that static stability has the controlling hand over wind shear and thus ripples are suppressed (resistance is not futile).

But near the transition from dynamic stability to dynamic instability, where wind shear and static stability are more on equal footing (but with a slight nod to dynamic instability), there exists an "atmospheric cloth" that is ideal for ripples or waves to form and amplify in a vertically sheared layer of air. Such an equal footing is all that's needed for billows to form, provided, of course, that there is adequate moisture to achieve net condensation in the crests of the otherwise invisible ripples.

In spite of the rather complex process by which billows form, there can be no flap over the their absolute beauty. Billows are truly "Waves of Ascension."