Weather Queries

By Schlatter, Tom

Q How do the planetary (long) waves in the Earth’s atmosphere relate to jet streams and the formation and deepening of surface low pressure centers?

Lee Ann Simone

Fall River, Massachusetts

A Most readers are familiar with the jet stream, a ribbon of strong winds in the upper troposphere (at altitudes between 20,000 and 40,000 feet) that circles the globe in mid-latitudes. The jet stream is stronger in the winter hemisphere than in the summer hemisphere. Sometimes the ribbon is very well-defined, and sometimes it splits in two: a subtropical and a polar jet stream. At other times, the jet stream is not well-defined, but one can almost always find a west-to-east current of air in the high troposphere, whether it is flowing at high speeds or not.

The jet stream meanders in a wave-like pattern around the hemisphere, sometimes taking an excursion toward the equator (troughs), and sometimes arching toward the pole (ridges). Cold tropospheric air favors the troughs, and warm tropospheric air favors the ridges. The figure shows a sample flow pattern at 500 millibars for November 1, 2006.

Jet streams lie over frontal zones. In the mid-latitude winter, if you are located on the pole side of the jet stream, your weather is probably cooler than normal. If you lie closer to the equator, your weather is probably warmer than normal. Whenever a strong jet stream passes overhead, you can expect big temperature changes, because the surface front associated with the jet separates two different air masses. Mid-latitude, low-pressure systems form and deepen along fronts.

In the figure, note the wave patterns around the hemisphere, the prominent troughs marked with bold red lines. The distance between troughs is a rough measure of the wavelength, and some waves are longer than others. The longest wave is over North America, and one of the shortest waves is over far northeast Asia. Typically not very fast-moving, long waves span many thousands of miles. When the long waves shift their position, big changes in the weather often occur over large areas. Conversely, short waves are more progressive; they move faster than long waves and, consequently, often move through them.

The formation and deepening of a lowpressure system is called cyclogenesis. Cyclogenesis is common near the base of long-wave troughs. Short wave troughs sliding toward the equator around the back side of a long wave trough tend to induce a spin (vorticity) in the air column ahead of them. As part of this process, they also tend to reduce surface pressure during their approach. If a strong, low-level front is already in place along the path of the short wave- and it often is along the east side of a long wave trough-this drop in surface pressure typically concentrates near the front, and cyclogenesis occurs: the pressure drops at the surface and a cyclonic circulation develops (counterclockwise in the Northern Hemisphere). This surface low shunts mild air toward the pole ahead of itself and drags cold air toward the equator in its wake, thus reinforcing the cold air already in the trough and the warm air in the ridge downstream. The track of the low follows its parent short wave, usually toward the equator in the direction of the base of the long wave trough, where deepening is most likely, then toward the pole again on the east side. In the case of a deep long wave trough over central North America, a weak surface low might move southeast from Alberta into the mid-Mississippi Valley, then develop strongly and move toward the Great Lakes.

In summary, movements in the jet stream define the short and long waves in atmospheric flow. When a short wave trough approaches the base of a long wave trough, cyclogenesis is favored, especially if a strong frontal zone stretches from the base of the trough northeastward.

Q It is unclear to me how the double rainbow in Loveland, Colorado, featured in the September/October 2006 issue of Weatherwise, can occur. For the radii to be different, don’t there have to be two light sources or some other phenomena? When I have seen double rainbows, they have had concentric arcs, and the sequence of colors in the outer band is always the inverse ofthat in the inner band.

Bruce Cummins

Mansfield, Ohio

A This first-prize photo is very special. The primary rainbow is the brightest, arching over the large tree at right. The secondary bow is quite faint, most easily seen near the back edge of the camper near the left edge of the photo. It is concentric with the primary bow. The unusual feature is a very bright reflected primary bow, which inter sects the non-reflected primary bow at the horizon. The reflected primary bow has the same sequence of colors and the same radius as the non-reflected primary bow but a different center.

Robert Greenler explains this phenomenon in his book Rainbows, Halos and Glories, to which I often refer in this column. The reflected bow is made possible, in this case, by a lake or reservoir behind the photographer. The water must have been very still to produce such a good reflection. The diagrams show the geometry of the situation. Facing away from the sun, the photographer sees two rainbows, one coming from drops at 42 angular distance from the antisolar point (the point directly opposite the sun, below the horizon), and another coming from drops at 42 angular distance from the reflected antisolar point, at an equal distance above the horizon. Because the primary and reflected light is refracted within raindrops by the same angles, the radii of both bows is the same. Moreover, the closer the sun is to the horizon, the closer together the primary bow and its reflected counterpart will be.

To get a brilliant reflected bow like the one pictured, the water surface must be mirror-like. Any breeze causing ripples would dim the reflected bow.

Contributing editor THOMAS SCHLATTER is a meteorologist at NOAA’s Earth System Research Laboratory in Boulder, Colorado. He is also affiliated with the Cooperative Institute for Research in Environmental Sciences, University of Colorado. Readers are encouraged to submit queries to the author in care of Weatherwise; 1319 18th St. NW; Washington, D.C. 20036; or by email to ww@heldref.org. Submissions without full names and addresses will not be answered. Due to the volume of questions received, personal replies should not be expected.

Copyright Heldref Publications Mar/Apr 2007

(c) 2007 Weatherwise. Provided by ProQuest Information and Learning. All rights Reserved.