I greatly enjoy reading Weatherwise, and especially enjoy following atmospheric phenomena of the arctic areas of the world. I am also an advanced amateur astronomer with at least 40 years experience. So, one of the items that caught me in Ann Posegate's article, “Observing Weather at the Bottom of the World: The South Pole,” in the September/October 2010 issue, was the declaration that there is “six months of complete darkness” at the South Pole. She later asserts that “there's either daylight or no light at all.” That there are 6 months of darkness at the north and south poles each year is a common misnomer, perpetrated in textbooks and urban legend. Neither the sky nor the outdoor environment attains darkness the moment the sun sets! This is just as true at the poles as it is anywhere else on the Earth. And, actually, there are more than 6 months of daylight at the poles, due to strong inversions causing refraction of the solar image from below the horizon—all very well documented in the polar literature. It is not uncommon for the sun to “appear” above the horizon many days before it physically rises, or many days after it has set.
As an amateur astronomer, I am very well aware of the twilight time when I wait to observe the sky–including both civil and astronomical twilight periods. The average person would not consider it dark outdoors until well into the second half of the civil twilight period after sunset, for example. Astronomers stretch that a bit more. Also, the ground cover at/near the poles has a very high albedo and thus the amount of illumination scattered about and available is much more than the average urban/suburban environment.
Therefore, it is obvious that as the sun spirals slowly down below the horizon about two-thirds of a solar diameter each day, it will become only gradually darker day by day. There should be a period of at least many weeks after polar sunset and a few weeks before polar sunrise where it would be quite bright enough to read without any extra illumination—given a relatively clear sky, of course. Without going through the calculations, I could say with fair certainty that there is actually less than 3 months of actual, full darkness at either pole each year. This would be the time when the sun is at least 18 degrees below the horizon. Even then, there are the lunar illumination and the aurorae to consider.
Since this is such a common misunderstanding, maybe some small note could be placed in a future issue of Weatherwise, to keep your readers well informed.
Jeffrey A. Corder
Mr. Smith is to be very much commended for writing a book (Warnings; Greenleaf Book Group Press, 2010) that calls attention to the history and value of severe weather warnings; the book is interesting and unique. But the line, “In 1955 the Weather Bureau failed to provide adequate notice for a series of tornadoes known as the Blackwell (Oklahoma)–Udall (Kansas) tornadoes, and 102 people died” in the derivative article that appears in Weatherwise (“Weather Warnings: History in Action,” July-August 2010) does not fully convey the entire meteorological reality of the evening.
As Mr. Smith notes on pages 72 and 73 of his book, a SELS (Weather Bureau) tornado watch was indeed in effect for parts of northern Oklahoma and southern Kansas several hours before the devastating tornadoes occurred. The watch was noteworthy not only for its long lead time, but also given the rudimentary understanding of severe weather that existed at the time. What is tragic is that the exemplary forecast was not properly communicated to those whose lives might have been saved, due to problems with radar and communication processes at the local Weather Bureau office.
Steven Corfidi, NOAA
This is in reference to the first question in the Weather Queries section of the September–October issue of Weatherwise, asking whether two hurricane have ever crossed paths.
The answer provided by Tom Schlatter is factually correct concerning the Fujiwhara Effect; however, I wonder whether he, or the COMET Program that he references in the attached figure, realizes that the example is from the Southern Hemisphere? The clockwise motion depicted in the figure can only happen with two merging clockwise-rotating systems. In the Northern Hemisphere, they would have to merge in a “flipped” manner via a counterclockwise rotation and merger. Since the discussion is about “hurricanes” (and not tropical cyclones in general), I think he probably should have stuck to a Northern Hemisphere example.
Roger Edson, NOAA
Weatherwise appreciates our readers' attention to detail and is publishing the corrected image, created by COMET, on page 47.