Q: I live in Burnley, which is a large town in northeast Lancashire, England. Burnley is in the foothills of the Pennines, the upland area running down the center of northern England. Because we are fairly high up and in the west of the country, we get more than our fair share of wet weather. In relatively recent years, Burnley was one of the biggest cotton spinning and weaving places in the world because, for one thing, the industry needed a humid climate.
I hope that has set the scene. What puzzles me is the frequency with which we experience the following weather phenomenon. All day, winter or summer, the cloud base is low, with or without rain. Then, toward late afternoon, the clouds begin to lighten and by, say, 6:00 p.m., there isn't a cloud in the sky. I thought the cause might be the temperature drop as the sun sinks toward the horizon, but I would like to know.
A: I consulted John M. Brown, a meteorologist colleague whose opinions I respect. Together we developed a hypothesis that might explain the clearing skies that Burnley frequently experiences in the early evening. We believe there are two causes of this phenomenon: a sea breeze, related to Burnley's proximity to the Irish Sea, just 30 miles west, and a mountain-valley circulation, related to the topography around Burnley.
I'll describe the sea breeze first. For the purpose of discussion, assume that the weather is benign, skies are mostly clear, and winds in the lower troposphere are light. A few hours after sunrise, the sun heats the land surface so that its temperature rises to match that of the adjacent sea. Assume that the pressure surfaces at any given altitude in the lower troposphere are horizontal when land and sea surface temperatures become equal. As the land warms further, air in contact with the land warms as well. With time, a column of air over land becomes warmer than a similar column over the sea because the sea warms much more slowly than the land. When a given mass of air warms at constant pressure, its volume increases. In a warming column of air, this expansion causes the distance between two pressure surfaces to increase. If the sea-level pressure is the same on land and sea, then constant pressure surfaces above the land will rise with respect to those over the sea. At a given altitude, say 4,000 feet, the pressure over land will become higher than that over the sea. Air tends to flow toward lower pressure, so that, aloft, a breeze will begin to blow from land to sea. This removal of mass from a column of air over land reduces the pressure at the surface, thereby creating a pressure gradient at sea level. In response, a sea breeze flows toward the lower pressure on land.
Figure 1 illustrates this process. The sea-breeze circulation develops and strengthens during the daytime heating cycle, whereby cooler air over water comes onshore with the sea breeze. There is low-level convergence along the leading edge of the sea breeze, and clouds may form above that location. Aloft, say at 4,000 feet, the flow is from land to sea, completing the circulation.
Caption: A diagram of the sea breeze circulation. During the daytime heating cycle, a shallow sea breeze develops, transporting cool marine air inland toward lower pressure. The compensating return flow from land to sea occurs several thousand feet above the surface. The leading edge of the sea breeze acts as a mini cold front. Low-level convergence occurs where the cooler marine air meets the warmer air over land. Clouds often form above this boundary.
The sea breeze is most active during the spring and summer, when the temperature contrast between the chilly Irish Sea and the warmer ground to the east is greatest. The sea breeze can easily penetrate 30 or more miles inland. How does the topography around Burnley affect the sea breeze?
The map in Figure 2, produced by the United Kingdom's Ordnance Survey, shows Burnley in the upper right quadrant, adjacent to several other large towns. Burnley lies at an elevation of about 390 feet at the confluence of the River Brun and the River Calder. The River Calder flows basically east from Burnley for about 10 miles until it joins the River Ribble as a major tributary. The River Ribble continues east another 20 miles in a broadening valley until it empties into the Irish Sea. Elevations are color-coded on this map. A white background indicates land near sea level. Higher elevations (to above 1,500 feet) are colored in shades ranging from light yellow to tan. Note Pendle Hill, 557 meters (1,827 feet), NNW of Burnley, Boulsworth Hill, 518 meters (1,699 feet), to the ENE, and Hameldon Hill, 409 meters (1,342 feet), to the SW.
Caption: A section from the Ordnance Survey map of the area around Burnley, Lancashire, England, and points west to the Irish Sea. Burnley is in the upper right-hand quadrant of the map.
The sea breeze brings moist air from the Irish Sea up the Ribble and Calder Valleys, which rise gently from the Irish Sea to Burnley. Burnley is surrounded on three sides by high hills, and, as moist, sea-breeze air from the Irish Sea is forced to rise in the vicinity of Burnley, the air cools to the point at which condensation of vapor occurs, and low clouds form, perhaps first at the head of the valley, but they may then spread laterally over Burnley. Drizzle may fall from these clouds. As the land cools during the evening, the sea breeze dies out. Eventually, a land breeze may develop during the night, the reverse of the circulation shown in Figure 1. This brings a gentle downslope flow to Burnley, which acts to dissipate the clouds.
The mountain-valley circulation can be active in all seasons. The high ground surrounding Burnley heats up in the morning sun, causing hilltops and ridges to be warmer than air at the same altitude overlying the valleys. This heating causes pressure surfaces over the hills to arch gently upward, resulting in higher pressure a few thousand feet above the hills than at the same altitude over the valleys. Aloft, air flows away from the hills and ridge tops, thereby reducing the surface pressure over high ground. In response, surface air tends to flow up the slopes toward this lower pressure. Low-level convergence of air at the ridges and hilltops is the reason why the first clouds of the day usually form over high ground in quiescent weather situations. A mountain-valley circulation is usually established by late morning or early afternoon, and it can reinforce the sea-breeze circulation if and when the latter reaches Burnley.
In the evening, the highest ground, which may lie above the clouds, begins to cool first. As a result of this cooling, the air near the hilltop becomes denser than surrounding air and begins to sink toward the valleys. This has the effect of clearing the low clouds over Burnley, which may experience a down-valley breeze after sunset. In spring and summer, development of a land breeze at night could amplify the down-valley breeze.
The above is our hypothesis to explain why there is frequent clearing late in the day over Burnley. It is applicable only when there are no active fronts or low-pressure systems traversing the area. In any active weather pattern, the sea-breeze and mountain-valley circulations would be suppressed.
Q: I enjoyed the article in the March/April 2013 issue of Weatherwise about mirages as related to the Titanic. The set of photographs on p. 26 of the setting sun caught my eye and triggered a question: What effect would a mirage have on the green flash? Could the green flash even be observed during a mirage?
A: If you go to Google.com, type in "green flash," and ask for images, you'll see an assortment of photos of the green flash, some legitimate, others not. Among these photos, you'll find examples in which the upper tip of the sun appears detached from the rest of it, similar to some of the photos on p. 26 of the March/April 2013 issue of Weatherwise. In the Google images, the upper fragment is distinctly green, whereas in the photos on p. 26, it is yellow-orange. Clearly, distortion of the setting sun's shape is not a sufficient condition for a green flash.
When the sun is on the horizon, its light rays take a very long path through the atmosphere. The atmosphere bends these light rays ever so slightly, and preferentially by wavelength. Light at longer wavelengths (red and orange) is bent less than light at shorter wavelengths (blue and purple). This means that the purple image of the sun is displaced slightly higher in the sky than the red image, with the result that the setting or rising sun should have a purple rim along the top and a red rim along the bottom. Under standard atmospheric conditions, this effect is too small to be seen by the naked eye. The naked eye has a resolution of about one arc minute, but the width of the colored upper rim is only about 1/16 of an arc minute. Sometimes, however, the atmospheric temperature stratification acts to greatly magnify the color transition at the upper rim of the sun.
When such magnification occurs, one might ask why don't we see a purple flash (purple is at the long-wavelength end of the visible spectrum) instead of a green flash. The reason is that atmospheric molecules scatter short-wavelength light much more effectively than longer-wavelength light, by the time light from the setting sun reaches the eye of the observer, very little purple or blue light is left in the spectrum, but enough green is left to be seen, as in the figure, where the last tiny remnant of the setting sun has a distinctly green hue.
It is rare to see the green flash for three reasons: (1) It is very small. The sun subtends only 0.5 degree of arc in the sky, and the green flash occupies much less than 1/10 of that. (2) It is short-lived, lasting perhaps a second or so. The exception to this would be near the earth's poles, where sunset is very gradual. (3) The temperature stratification along the path of the rays of the setting sun has to be just right for magnifying the image of the upper rim of the setting sun. I have often looked for the green flash, but have never been lucky enough to observe it. A clear view of the horizon, for example, a hilltop overlooking the ocean, is preferred for observing the green flash. The sun sets behind mountains where I live.
Mirages are multiple images of the same object caused by atmospheric refraction (bending) of light rays. Mirages of one kind or another are necessary to magnify the image of the green upper rim of the setting sun so that it becomes visible to the eye. An inferior mirage, in which the inverted image of an object appears below the erect one, is a common cause of the green flash. This requires an atmospheric layer, usually not more than a few meters thick, in which density increases with height. This can happen over strongly heated ground, when the temperature decreases very rapidly with altitude, at a rate of more than 1.5°F per hundred feet of altitude.
Superior mirages are so named because the inverted image appears above the erect one. They are caused by inversions, or increases of temperature with altitude. These are common throughout the troposphere. They can cause a green flash, but, as is true for the inferior mirage, the observer must be close to, or even within, the thermal structure causing the mirage. Very sharp inversions can cause ducting, in which light rays are trapped within the inversion. Viewers positioned at the lower edge of a duct may see a green flash.
A word of caution: Don't look directly at the sun, even on the horizon, unless its brightness has been substantially dimmed by atmospheric aerosols.
Serious aficionados of the green flash should consult the Web site of Andrew T. Young at San Diego State University: http://mintaka.sdsu.edu/GF/.
Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at email@example.com, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St., Suite 800, Philadelphia, PA 19106.