Weather 101 - Wave Clouds and the Jet Stream

Another in a series of cold fronts will pass through southwest Florida by tomorrow morning (Fig. 1). Prior to frontal passage, this area will experience a one-day warm-up (with temperatures rising to only about 5 degrees below average) and then it's back to temperatures well below expected readings for much of the rest of the week.

What's behind this incessant onslaught of cold air (it's been going on since early January 2010) is not the cold fronts themselves, but what is bringing the fronts so far to the south, so often.

The "jet stream"

And this means looking at winds at about 5 to 8 miles above the Earth, the so-called "jet stream" level. It is at this altitude that winds often blow at speeds of 50 to 100 miles per hour and more. It is here that aircraft fly, with airlines and pilots trying to capitalize on high winds to speed their travels and save on fuel. Since winds tend to increase with altitude up to about 6 to 10 miles above the Earth's surface (the altitude of the tropopause), airlines can fly with the wind at higher levels and fly against the wind at lower altitudes.

This year (and in late 2009), there have two active jet stream zones (a polar jet and a subtropical jet). Thanks to a large blocking upper level high pressure system over the western U.S. and western Canada (which has kept cold air away from Olympic venues in British Columbia), the two jet streams have "split" over the eastern Pacific and don't rejoin until they reach Florida (Fig. 2). This has resulted in some beautiful middle and high-level cloud patterns across our region (Associated Content slide show), as well allowing cold air from northern Canada to spill southward to the Gulf Coast for much of the winter.

In addition to blowing strongly in a horizontal direction, there are vertical air motions around the jet stream that favor clouds forming along and to the south of the jet stream (Fig. 3). For much of the past two months, the jet has actually positioned itself just to the north of the Naples - Fort Myers area in southwest Florida. That pattern is now expected to persist for the next week (Fig. 4), but be even stronger than it has been in recent days. Thus, there should be even more beautiful and more extensive cloud shows overhead.

The clouds will help lower daytime temperatures as they block incoming solar energy. On the other hand, they will keep temperatures from plummeting at night. Hence, even with cold air in place, freeze watches are only posted as far south as about Ocala, located well to the north of Orlando and Tampa (Fig. 1).

Cloud types with waves

Cirrus clouds (high altitude ice crystal clouds typically found above 20,000 feet or four miles above the Earth) often appear as streaks, wisps and sometimes bands. On February 13 in Naples, the cirrus clouds actually developed wave-like features indicative of Kelvin-Helmholtz waves. These are like waves coming ashore at the beach, making waves crests and then crashing and disappearing. My wife and I were not disappointed with how fast this happened as we watched the waves form and vanish within minutes.

When cirrus clouds fill the sky, they may merge and form an extensive cirrostratus cloud layer. The suffix stratus means "layer".

Sometimes the refraction (or bending) of sunlight through the ice crystals will lead to the formation of a ring around the sun or moon. This is known as a halo. At other times, especially with the sun lower in the sky, look for vertical colored bands at about the 3 and 9 o'clock positions around 23 degrees from the sun. These are known as "sun dogs."

When the cloud action drops down to about 2 miles above the Earth's surface, high winds (not of jet stream strength) can create some interesting, wave-like cloud formations. Bands of altocumulus clouds often in a herringbone patterning have been very prevalent. Cumulus clouds are puffy, while the prefix "alto" means high (but not as high as cirrus clouds). The bands of clouds signal rising air regions and the clear spots in between indicate sinking motions. Hence, there is a vertical wave pattern in these clouds, much like waves at the beach or ripple marks in the sand. If there are intersecting bands, then the pattern contains alternating places of wave reinforcement and interference (i.e., the herringbone pattern).

If the clouds are present around sunrise or sunset, look for red, pink or orange colors to appear.

And, without looking directly at the sun (block the solar disk with either your fist, or with a well-positioned street light or a leafy tree branch), look for colorations of pink, blue and teal near the sun (iridescence) or a ring around the sun (corona). At night, you can more easily see a corona around the moon.

Look up often to see the ongoing sky show

While we, at How The Weatherworks, are always looking up at the sky, taking photographs and educating people about the sky, try doing it yourself, any time there are clouds. Clouds are beautiful to behold and, as you can see here, they are filled with science.

Also, National Sky Awareness Week, a national celebration of the sky, is coming in late April. It's a great time of year to look up and enjoy the sky, to learn about the sky, and appreciate what the sky does for our very existence. But you don't have to wait for that event to look up to see the ongoing sky show. Anywhere, anytime works perfectly!