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March-April 2017

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Weather Queries

As a sailor, I have personally sensed the shift in wind direction and a drop in temperature when cold fronts pass. Textbooks show winds following the isobars, which are concentric to the low pressure system. When a cold front arrives and the wind direction changes, what exactly happens? (1) Do air molecules pass through the front? (2) Do the winds push the front forward at the wind speed? (3) How does the wind direction change so predictably and not interfere with the characteristics of each air mass?

Richard Sheridan

Winnipeg, Manitoba, Canada

Do air molecules pass through the front? Air molecules are in a constant state of agitation. At standard temperature (15°C/59°F) and pressure (1013 millibars, 29.92 inches of mercury), their mean free path—the average distance between collisions—is 60–70 nanometers. A nanometer is one billionth of a meter. To put this in perspective, the average width of a human air is roughly 1,000 times greater than the mean free path. The mean velocity of air molecules is the basis for the definition of temperature. The higher the temperature, the faster the molecules move, and they move very fast. At room temperature, air molecules are traveling over 500 meters per second, much faster than a jet airplane. If they are zipping around this fast with such tiny distances between collisions, they are truly in random motion, much like a cloud of gnats on a hot summer day, but sped up almost unimaginably. Thus, clouds of molecules in furious random motion do not move through the front. Instead, they are carried in bulk by the wind.

An intense coastal storm off the Virginia coast. The cold front (blue with barbs) is wrapping around the surface low in a process called occlusion. Ultimately, the low will weaken and become uniformly cold in all quadrants as temperature contrasts disappear.

An intense coastal storm off the Virginia coast. The cold front (blue with barbs) is wrapping around the surface low in a process called occlusion. Ultimately, the low will weaken and become uniformly cold in all quadrants as temperature contrasts disappear.

Do the winds push the front forward at the wind speed? No, unless the wind behind the front is perpendicular to the front. The front moves with the component of the wind that is perpendicular to the front. In other words, a wind of 20 mph from the northeast behind an east-west front will move the front southward at 20 × cos 45° = 14.1 mph. Some vigorous cold fronts, such as the “Blue Norther” of the Texas Panhandle, have strong winds behind them, perpendicular to the front, which pushes the front south amazingly fast. By contrast, fronts that hardly move, called stationary fronts, have perpendicular components of wind, usually small, on either side that are in approximate balance. For example, along an east-west stationary front, the southward component of wind on the north side approximately balances the northward component on the south side.

How does the direction change so predictably and not interfere with the characteristics of each air mass? Isobars are only roughly concentric around low-pressure centers. In particular, cold and warm fronts alike lie in surface pressure troughs, where the contours of constant pressure (isobars) have maximum curvature. To the extent that the surface wind responds to the isobar pattern, there will always be a wind shift when the front passes. That's what makes the direction change predictable.

Fronts are associated with convergence, or a merging of air streams along the front. With warmer air on one side and colder air on the other, the convergence tends to maintain a significant contrast of temperature across the front, which, in turn, maintains the contrast between air masses.

Surface winds are usually stronger close to the surface low than near the periphery. That means that air closer to the surface low, including the front, rotates faster around the low than air farther away. Thus a cold front that begins straight will later become curved, with the maximum curvature near the center of the low. Because of this, over a period of time, cold air will wrap around the center of the low and the contrast between air masses will eventually weaken. This process is called an occlusion. The accompanying surface map for 7:00 a.m. EST, January 23, 2016, shows occlusion in progress in a storm off the Virginia coast. Note the increasing curvature of the cold front (in blue) near the center of the low. Once cold air wraps entirely around a low, it weakens and dies because the strong temperature gradients that gave it birth are no longer in existence.

The text accompanying a weather chart dated January 20, 2010, issued by Freie Universität Berlin, “Berliner Wetterkarte,” mentions a remarkable high pressure system of 1080 millibars (mb) over Mongolia. I know roughly how high and low pressure centers form, but what are the reasons for such extremely high pressure?

Georg Heymann

Erlangen, Germany

I could not find the map to which you refer, but I did find a sea-level pressure analysis for the same high pressure system in the archives at NOAA's National Centers for Environmental Information. At top center in the figure, you will see a center of high pressure (1075 mb) analyzed near 55°N over Mongolia.

Before delving into the causes of extremely high sea level pressure, I pose this thought experiment. Suppose two strictly horizontal pressure surfaces, one of 500 millibars at an altitude of 5,400 meters, and the other of 1000 millibars at sea level. The principle of hydrostatic equilibrium says that the mass of air between these two pressure surfaces is determined by the difference in pressure between the top and bottom of the layer, in this case, 500 millibars. It is also true that the height of the 500-millibar pressure surface is determined by the vertically averaged virtual temperature between 1000 and 500 millibars. Virtual temperature is the same as the actual temperature in perfectly dry air, but moisture in the air can cause the virtual temperature to be slightly higher, usually less than 1°C higher.

Suppose air at the ground (again, elevation is sea level, pressure is 1000 millibars) begins to cool at the center of an air mass with constant 500-millibar heights. This causes the vertically averaged virtual temperature over the cool spot to decrease, which, in turn, lowers the 500-millibar height, which was 5,400 meters. This is a consequence of the ideal gas law, which implies that if a column of air with a unit cross section cools at constant pressure, the height of the column decreases. Now there is a gradient in the height of the 500-millibar pressure surface, and air moves toward the center of low heights, thereby increasing the mass of air over the cold spot and increasing the sea-level pressure to more than 1000 millibars. If the surface cooling is significant and prolonged, one can see how the surface pressure can rise significantly where the cooling occurs.

Very high sea-level pressure is invariably associated with intense cold. In the Northern Hemisphere this occurs most frequently in northeastern Eurasia, from Mongolia north to Siberia.

The stage is set once seasonal snow cover deepens. Albedo is the percentage of solar radiation reflected by the surface back into the sky. When snow cover is deep, particularly north of the tree line or where vegetation is sparse, the albedo easily exceeds 50%. Half or more of the incoming solar radiation is reflected back to the sky and is not available for heating the ground. In midwinter, where extreme high pressure develops, there is little sunlight in the first place at the latitudes in question, 50° to 70°N. Nights are long, and the daytime sun is close to the horizon.

The surface radiation balance can become negative from days to weeks, when the infrared radiation lost to space exceeds the solar radiation received from the sun. Deep snow is a nearly perfect emitter of infrared radiation. How much infrared energy is lost to space depends upon the surface temperature (at lower temperatures, the surface emits less infrared energy toward the sky) and how much infrared radiation is directed downward from the atmosphere. Clouds act as a blanket for infrared radiation. In fact, if clouds lie above a low-level inversion, they will send more infrared radiation earthward than is coming up from below, and the surface temperature will rise. For the most intense cold, the atmosphere must be not only cloudless, but also very dry. That's because water vapor is a greenhouse gas, and water molecules absorb infrared radiation coming up from below and send some of it earthward again.

When winter weather conditions in northeastern Eurasia are relatively quiescent or inactive and the atmosphere is cloudless and dry as noted above, an intense surface inversion slowly builds above the snow-covered ground. Pressure surfaces aloft warp downward over the developing cold air mass, and surface pressures steadily rise. Migratory storms must steer around the cold air mass. Otherwise, they will import warmer air into the area, and their associated low-level winds will destroy the surface inversion. Observations at the center of the cold dome invariably show very low temperature, clear sky, calm wind, a dry atmosphere, and high pressure.

Mean sea-level pressure map for the Northern Hemisphere at 0000 GMT, January 20, 2010. Pressure contours in blue are drawn every 4 millibars. Note the 1075-millibar high at top center, over Mongolia.

Mean sea-level pressure map for the Northern Hemisphere at 0000 GMT, January 20, 2010. Pressure contours in blue are drawn every 4 millibars. Note the 1075-millibar high at top center, over Mongolia.

The commonly accepted world record for high sea-level pressure is 1083.3 millibars (32.01 inches of mercury) at Agata in Russian Siberia. The record was set on December 31, 1968. Agata is at 66° 53'N, 93° 28'E and at an elevation of 261 meters (855 feet). Several measurements of 1080 millibars or greater, even a few above the accepted world record, have come from Mongolia, but these have not been confirmed as a new world record because of the ambiguity in methods for reducing station pressure (the actual pressure as measured by a mercury barometer at the station elevation) to sea level. The ambiguity increases with elevation. The elevation of the Mongolian stations considerably exceeds a somewhat arbitrary cutoff of 750 meters.

Transition

After nearly 37 years of writing the “Weather Queries” column for Weatherwise, I am retiring. Again. Retirement from my full-time government job with NOAA occurred in 2004. After that, I worked 60% time with CIRES, the Cooperative Institute for Research in the Environmental Sciences. I retired from CIRES in early 2010. I quickly realized it would be difficult to continue writing the “Weather Queries” column without a close connection to NOAA and my colleagues there. It was then that I discovered I could keep a desk as a retired volunteer at NOAA's Earth System Research Laboratory. My lifelong fascination with weather and the mental stimulation of writing the “Weather Queries” column left me no choice but to be a retired volunteer. At the lab, I had access to many scientists, print and electronic libraries, and graphics support.

How did it begin? In early 1980, Chester Newton at the National Center for Atmospheric Research, where I worked, asked if I would be interested in taking over a question-and-answer column about weather and climate for Weatherwise magazine. The late Louis J. Batten, widely recognized for his educational contributions to meteorology, had inaugurated the “Weather Queries” column in April 1979. I agreed to write the column for one year on a trial basis. Now, 195 columns and 471 Q&As later, it is time to pass the baton.

I was privileged to work with six managing editors at Weatherwise. Linda Dove was editor when I began. The late Patrick Hughes served from October 1986 through late 1992. Jeff Rosenfeld took over at the end of 1992 and was editor until early 1997. He later became the editor of the Bulletin of the American Meteorological Society and still is. From early 1997 through mid-2003, Doyle Rice was managing editor. In 2004, he became editor of the USA Today weather page. Lynn Elsey was managing editor from July 2003 through August 2006. Margaret Benner Smidt succeeded her, and is still at the helm. I also gratefully acknowledge Kimbra Cutlip, who worked with me as assistant editor from early 1997 through mid-2003, and the current production editor, Andrew Hoffmann, who is flexible when there are problems with galley proofs and meticulous about entering corrections.

Technology has changed dramatically in the past 37 years. Desktop monitors and word processors were becoming popular in the early 1980s but e-mail was not, nor had the Internet made its debut. My early submissions for the column consisted of hard copy text, drafted figures, and photo prints. My sources of information were primarily textbooks, print journals, and scientists I knew.

Today the Internet brings a world of information to a desktop computer. Most of the journals I consult are now in electronic format. Even if our extensive NOAA library doesn't have an article, interlibrary loans deliver PDF copies quickly. Sophisticated word processors handle special symbols and equations effortlessly, and Microsoft PowerPoint and Adobe Photoshop generate custom graphs and images with few limits to editing or choices of presentation.

Atmospheric science has advanced significantly since 1980. The ozone hole was not discovered until the mid-1980s. Chlorofluorocarbons (CFCs), manmade chemicals used in refrigeration, were implicated in the destruction of stratospheric ozone, which, in turn, allowed potentially harmful amounts of ultraviolet radiation from the sun to reach the earth's surface. The Montreal Protocol in 1987 set in motion international efforts to cease production of CFCs. Evidence is mounting that these efforts have been successful in reducing the size and intensity of the ozone hole.

Acid rain had been identified in the 1970s as the culprit in the death of forests in the northeast United States and southeast Canada and the acidification of lakes and streams. Not until 1980 did Congress authorize an 18-year scientific program to determine the causes and effects of acid rain. Sulfur dioxide (SO2) dissolving in raindrops is a principal cause of acid rain. Amendments to the Clean Air Act in 1989 called for a reduction of 10 million tons of sulfur dioxide (SO2) emitted from power plant stacks. This reduction was actually achieved in 2007, and acid rain today is much less a problem than it once was.

Research conducted in the early 1980s at the National Center for Atmospheric Research indicated that wind shear associated with microbursts could pose a serious hazard to landing and departing aircraft in the lowest thousand feet or so. (A microburst causes localized, sudden, strong gusts of wind, which spread out from a center of down-rushing air full of evaporating raindrops.) The Federal Aviation Administration funded development of the Low Level Wind Shear Alert System (LLWAS) that could detect microbursts at airports. Many large airports in the United States now have an LLWAS in place.

Numerical weather prediction has improved markedly since 1980, mainly because of order-of-magnitude increases in computing power; availability of a multitude of new land, ocean, and atmospheric observations, many of them made by satellites; and development of highly complex models of the atmosphere and its response to land and ocean conditions.

Climate change science was embryonic in 1980. By then, the slight cooling trend from 1945 to 1975 had stopped, and a number of scientists recognized that an eventual rise in global temperature was virtually inevitable because of increasing concentration of greenhouse gases in the atmosphere. James Hansen testified to Congress in 1988 that human activity, mainly the burning of fossil fuels, had already warmed the climate. That same year, the World Meteorological Organization created the Intergovernmental Panel on Climate Change, which has subsequently published every few years the most heavily documented and vetted assessment, almost a compendium, of the state of the global climate. Since the late 1980s, climate change has found its way into public consciousness.

What were the toughest questions to answer? Two columns had a very long gestation period, from many months to nearly two years. One explained why the earliest sunset in early December precedes the latest sunrise by nearly four weeks and, similarly, why the earliest sunrise around June 12 precedes the latest sunset by nearly two weeks. It has to do with the earth's elliptical orbit around the sun and the fact that the earth's axis is not perpendicular to its orbital plane around the sun. Once I understood the orbital geometry, I had difficulty conveying the ideas in a non-mathematical way. It took two attempts to answer this question to the satisfaction of then–editor Jeff Rosenfeld (in the June–July 1994 and April–May 1996 issues). The other challenging question involved the relative effects of atmospheric refraction and altitude on the time of sunset as viewed from 40,000 feet. I was a rank neophyte on the subject of atmospheric refraction of sunlight, in particular when the sun is near the horizon. After repeated help from Andrew T. Young, an expert on this subject, I was able to submit this Q&A for the September–October 2016 issue.

And the Q&A that caused the most frustration? A reader asked whether contrails, those thin tracks of white cloud often forming behind jet aircraft, were evidence of a conspiracy, whereby the government was deliberately releasing harmful substances into the atmosphere to sicken the populace. In the November–December 2000 issue, I tried to explain that contrails are the natural result of moisture in hot jet exhaust condensing into an ice-crystal cloud when the atmosphere is sufficiently cold and moist. The explanation did not satisfy and prompted more questions. I used another column (September–October 2002) to describe some of the physical processes that could give contrails a sometimes striking appearance. More letters followed. I finally decided that no arguments I could advance would persuade conspiracy advocates that contrails are essentially harmless. I did not address the conspiracy topic again.

Why do this for 37 years, exactly half of my lifetime? It was fun! The incredible variety of questions from readers never ceased to amaze. Finding answers to questions was often a challenge. When I wasn't sure where to turn, the Web usually provided useful leads. It pointed to refereed publications and to experts, most of whom were generous in explaining their work. I thank colleagues at NOAA's Earth System Research Lab for sharing their subject area expertise. I thank Lorraine Kaimal and Ann M. Reiser for in-house editing, and John Osborn and Will von Dauster for graphics support. I am grateful to Weatherwise readers who submitted questions and often corresponded with me. Sharp-eyed readers gently informed me of errors in my column, which were corrected in later issues.

I'm happy to report that the “Weather Queries” column will continue. My NOAA colleagues, John M. Brown and Paul J. Schultz, will write the column beginning with the May–June 2017 issue. I hope they enjoy the endeavor as much as I have.

The next time you witness something unusual in the sky or read something about weather or climate that sparks a question, send it their way. As I sit in my easy chair watching the swirling snow, gently falling rain, hail pummeling my vegetable garden (heaven forbid), or merely a colorful sunset, I will be happy to read Brown's and Schultz's responses!

Weatherwise Contributing Editor THOMAS W. SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut St., Suite 850, Philadelphia, PA 19106.       

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