Tailer's Family - journeying!
Sunset - 15th September, 2007 (e)
No. 5 - 5: Back home to Essex!
A perfect ending to a lovely day.
THE COLOURS OF TWILIGHT AND SUNSET
Stephen F.Corfidi
of NOAA/NWS Storm Prediction Center
Norman, OK 73069
Mar 1996; Updated Dec 2004 and Feb 2009
(A slightly abridged form of this manuscript appeared in Weatherwise magazine, June/July 1996)
Everyone at one time or another has marvelled at the beautiful red and orange colours of a sunrise or sunset. Although colourful sunrises and sunsets can be seen anywhere, certain parts of the world are especially famous for their twilight hues. The deserts and tropics quickly come to mind. Indeed, it is a rare issue of Arizona Highways that does not include at least one sunset view, and one could amass a respectable collection of Caribbean or Hawaiian sunset postcards in just one trip.
Eye-catching sunrises and sunsets also seem to favour certain times of the year. In the middle latitudes and over the eastern half of the United States, for example, fall and winter generally produce the most spectacular low-sun hues.
Why do some parts of the world enjoy more beautiful sunsets than others, and why do they favour certain months? What are the ingredients for truly memorable sunrises and sunsets? These and a few other twilight topics are explored in the following paragraphs.
What dust and pollution don't do
It is often written that natural and manmade dust and pollution cause colourful sunrises and sunsets. Certainly the brilliant twilight afterglows that follow major volcanic eruptions owe their existence to the ejection of dust and other particles high into the stratosphere (More will be said on this a bit later). If, however, it were strictly true that tropospheric dust and haze were responsible for brilliant sunsets, cities such as New York, Los Angeles, London, and Mexico City would be celebrated for their twilight hues. The truth is that tropospheric aerosols --- when present in abundance as they often are over continental areas --- do not enhance sky colours --- they subdue them. Clean air is, in fact, the main ingredient common to brightly coloured sunrises and sunsets.
To understand why this is so, one need only recall how typical sky colours are produced. The familiar blue of the daytime sky is the result of the selective scattering of violet and blue light by air molecules. "Selective scattering," also known as Rayleigh scattering, is used to describe scattering that varies with the wavelength of the incident light. Particles are good Rayleigh scatterers when they are very small compared to the wavelength of the light.
Ordinary sunlight is composed of a spectrum of colours that ranges from violet and blue at one end to orange and red on the other. The wavelength of this light ranges from .47 um for violet to .64 um for red. Air molecules are much smaller than this --- about a thousand times smaller. Thus, air is a good Rayleigh scatterer. In fact, pure air scatters violet light three to four times more effectively than the longer wavelengths. Were it not for the fact that human eyes are more sensitive to blue light than to violet, selective scattering would make a clear sky appear violet instead of blue!
At sunrise or sunset, sunlight takes a much longer path through the atmosphere than during the middle part of the day. Because an increased amount of violet and blue light is scattered out of the beam along the way, the light which reaches an observer early or late in the day is reddened. Thus, it could be said that sunsets are red because the daytime sky is blue. This notion is perhaps best illustrated by example: a beam of sunlight that at a given moment produces a red sunset over the Appalachians is at the same time contributing to the deep blue of a late afternoon sky over the Rockies.
Now what happens when airborne dust and haze enter the view? Typical pollution droplets such as those found in urban smog or summertime haze are on the order of .5 to 1 um in diameter. Particles this large are not good Rayleigh scatterers since they are comparable in size to the wavelength of visible light. If the particles are uniform in size, they might impart a reddish or bluish cast to the sky. But since such aerosols normally exist in a wide range of sizes, the overall scattering produced is not strongly wavelength-dependent. As a result, hazy daytime skies, instead of being bright blue, appear greyish or even white. Similarly, the vibrant oranges and reds of "clean" sunsets give way to pale yellows and pinks when dust and haze fill the air.
But airborne pollutants do more than soften sky colours. They also enhance the attenuation of both direct and scattered light, especially when the sun is low in the sky. This reduces the amount of light which reaches the ground, robbing sunrises and sunsets of brilliance and intensity. Thus, twilight colours at the surface on dusty or hazy days tend to be muted and subdued, even though purer oranges and reds persist in the cleaner air aloft. This is most noticeable when taking off in an aircraft on a hazy evening: a bland sunset near the ground suddenly gives way to vivid colour as the plane ascends beyond the haze. When the pollution layer is shallow, a similar effect is sometimes evident at the surface
Because air circulation is more sluggish during the summer, and because the photochemical reactions which result in the formation of smog and haze proceed most rapidly at that time of the year, late fall and winter are the most favoured times for sunrise- and sunset-viewing over most of the United States. Pollution climatology also largely explains why the deserts and tropics are noted for their twilight hues: air pollution in these regions is, by comparison, minimal. (More information about the origin and behaviour of haze may be found at: www.spc.noaa.gov/publications/corfidi/haze.html)
The role of clouds
Although the twilight sky can certainly inspire awe even when it is devoid of clouds, the most memorable sunsets tend to be those with at least a few clouds. Clouds catch the last red-orange rays of the setting sun and the first light of the dawn. But certain types of clouds are more closely associated with eye-catching sunsets than others. Why?
To produce vivid sunset colours, a cloud must be high enough to intercept "unadulterated" sunlight...i.e., light which has not suffered attenuation and/or colour loss by passing through the atmospheric boundary layer. (The boundary layer is the layer near the surface which contains most of the atmosphere's dust and haze). This explains why spectacular shades of scarlet, orange and red often grace cirrus and altocumulus layers, but only rarely low clouds such as stratus or stratocumulus. When low clouds do take on vivid hues, as they often do over the open ocean in the tropics, it is a clue that the lower atmosphere is very clean and therefore more transparent than usual.
Some of the most beautiful sunrises and sunsets feature solid decks of middle or high clouds that cover the entire sky except for a narrow clear strip near the horizon.
Twilight hues from volcanoes.
Tropospheric clouds are not the only ones that can enhance the beauty of the twilight sky. As already mentioned, particles in the stratosphere also can produce colorful sunrises and sunsets. Stratospheric particles are derived mainly from volcanic eruptions and exist as thin veils of dust or sulfuric acid droplets at altitudes of 12 to 18 miles. Like the stars and planets, these aerosols usually are invisible during the day because they are obscured by the scattered sunlight (blue sky) of the troposphere. About 15 minutes after sunset, however, with the troposhere in shadow and the stratosphere still illuminated by sunlight passing through the lower atmosphere to the west, these high-level clouds come into view. Since their colors achieve greatest intensity after the sun has set at the surface, volcanic twilights are known as "afterglows."
Note that it is only when small volcanic particles have been lofted well into the stratosphere that colorful sunset afterglows appear. Volcanic particles that remain suspended in the troposphere after an eruption are comparatively large in size and number. As a result, they attenuate sunlight and otherwise subdue twilight hues, just like man-made dust and haze. Viewed through a veil of tropospheric volcanic ash, a sunset is dusky and dull.
Mount Pinatubo's sunset afterglows persisted to varying degrees for about 18 months after the initial explosion. In more recent years (especially 1998 and 2003), sunset colours in many areas have been subdued by the introduction of large smoke particles into the lower atmosphere by forest fires across the western United States, Canada, and China.
The preceding paragraphs have provided only an brief introduction to the physics and meteorology of the twilight sky. Further understanding of the nature of sunrise and sunset colours can only increase our appreciation of them.
for the full version, please check out:-
www.spc.noaa.gov/publications/corfid…
and:
www.electro-optics.org/Docs/newsflash/edu…
To see this Large:-
farm5.static.flickr.com/4044/4214964949_df4e8e7a29_b.jpg
Taken on
September 15, 2007 at 18.36 BST
Sunset - 15th September, 2007 (e)
No. 5 - 5: Back home to Essex!
A perfect ending to a lovely day.
THE COLOURS OF TWILIGHT AND SUNSET
Stephen F.Corfidi
of NOAA/NWS Storm Prediction Center
Norman, OK 73069
Mar 1996; Updated Dec 2004 and Feb 2009
(A slightly abridged form of this manuscript appeared in Weatherwise magazine, June/July 1996)
Everyone at one time or another has marvelled at the beautiful red and orange colours of a sunrise or sunset. Although colourful sunrises and sunsets can be seen anywhere, certain parts of the world are especially famous for their twilight hues. The deserts and tropics quickly come to mind. Indeed, it is a rare issue of Arizona Highways that does not include at least one sunset view, and one could amass a respectable collection of Caribbean or Hawaiian sunset postcards in just one trip.
Eye-catching sunrises and sunsets also seem to favour certain times of the year. In the middle latitudes and over the eastern half of the United States, for example, fall and winter generally produce the most spectacular low-sun hues.
Why do some parts of the world enjoy more beautiful sunsets than others, and why do they favour certain months? What are the ingredients for truly memorable sunrises and sunsets? These and a few other twilight topics are explored in the following paragraphs.
What dust and pollution don't do
It is often written that natural and manmade dust and pollution cause colourful sunrises and sunsets. Certainly the brilliant twilight afterglows that follow major volcanic eruptions owe their existence to the ejection of dust and other particles high into the stratosphere (More will be said on this a bit later). If, however, it were strictly true that tropospheric dust and haze were responsible for brilliant sunsets, cities such as New York, Los Angeles, London, and Mexico City would be celebrated for their twilight hues. The truth is that tropospheric aerosols --- when present in abundance as they often are over continental areas --- do not enhance sky colours --- they subdue them. Clean air is, in fact, the main ingredient common to brightly coloured sunrises and sunsets.
To understand why this is so, one need only recall how typical sky colours are produced. The familiar blue of the daytime sky is the result of the selective scattering of violet and blue light by air molecules. "Selective scattering," also known as Rayleigh scattering, is used to describe scattering that varies with the wavelength of the incident light. Particles are good Rayleigh scatterers when they are very small compared to the wavelength of the light.
Ordinary sunlight is composed of a spectrum of colours that ranges from violet and blue at one end to orange and red on the other. The wavelength of this light ranges from .47 um for violet to .64 um for red. Air molecules are much smaller than this --- about a thousand times smaller. Thus, air is a good Rayleigh scatterer. In fact, pure air scatters violet light three to four times more effectively than the longer wavelengths. Were it not for the fact that human eyes are more sensitive to blue light than to violet, selective scattering would make a clear sky appear violet instead of blue!
At sunrise or sunset, sunlight takes a much longer path through the atmosphere than during the middle part of the day. Because an increased amount of violet and blue light is scattered out of the beam along the way, the light which reaches an observer early or late in the day is reddened. Thus, it could be said that sunsets are red because the daytime sky is blue. This notion is perhaps best illustrated by example: a beam of sunlight that at a given moment produces a red sunset over the Appalachians is at the same time contributing to the deep blue of a late afternoon sky over the Rockies.
Now what happens when airborne dust and haze enter the view? Typical pollution droplets such as those found in urban smog or summertime haze are on the order of .5 to 1 um in diameter. Particles this large are not good Rayleigh scatterers since they are comparable in size to the wavelength of visible light. If the particles are uniform in size, they might impart a reddish or bluish cast to the sky. But since such aerosols normally exist in a wide range of sizes, the overall scattering produced is not strongly wavelength-dependent. As a result, hazy daytime skies, instead of being bright blue, appear greyish or even white. Similarly, the vibrant oranges and reds of "clean" sunsets give way to pale yellows and pinks when dust and haze fill the air.
But airborne pollutants do more than soften sky colours. They also enhance the attenuation of both direct and scattered light, especially when the sun is low in the sky. This reduces the amount of light which reaches the ground, robbing sunrises and sunsets of brilliance and intensity. Thus, twilight colours at the surface on dusty or hazy days tend to be muted and subdued, even though purer oranges and reds persist in the cleaner air aloft. This is most noticeable when taking off in an aircraft on a hazy evening: a bland sunset near the ground suddenly gives way to vivid colour as the plane ascends beyond the haze. When the pollution layer is shallow, a similar effect is sometimes evident at the surface
Because air circulation is more sluggish during the summer, and because the photochemical reactions which result in the formation of smog and haze proceed most rapidly at that time of the year, late fall and winter are the most favoured times for sunrise- and sunset-viewing over most of the United States. Pollution climatology also largely explains why the deserts and tropics are noted for their twilight hues: air pollution in these regions is, by comparison, minimal. (More information about the origin and behaviour of haze may be found at: www.spc.noaa.gov/publications/corfidi/haze.html)
The role of clouds
Although the twilight sky can certainly inspire awe even when it is devoid of clouds, the most memorable sunsets tend to be those with at least a few clouds. Clouds catch the last red-orange rays of the setting sun and the first light of the dawn. But certain types of clouds are more closely associated with eye-catching sunsets than others. Why?
To produce vivid sunset colours, a cloud must be high enough to intercept "unadulterated" sunlight...i.e., light which has not suffered attenuation and/or colour loss by passing through the atmospheric boundary layer. (The boundary layer is the layer near the surface which contains most of the atmosphere's dust and haze). This explains why spectacular shades of scarlet, orange and red often grace cirrus and altocumulus layers, but only rarely low clouds such as stratus or stratocumulus. When low clouds do take on vivid hues, as they often do over the open ocean in the tropics, it is a clue that the lower atmosphere is very clean and therefore more transparent than usual.
Some of the most beautiful sunrises and sunsets feature solid decks of middle or high clouds that cover the entire sky except for a narrow clear strip near the horizon.
Twilight hues from volcanoes.
Tropospheric clouds are not the only ones that can enhance the beauty of the twilight sky. As already mentioned, particles in the stratosphere also can produce colorful sunrises and sunsets. Stratospheric particles are derived mainly from volcanic eruptions and exist as thin veils of dust or sulfuric acid droplets at altitudes of 12 to 18 miles. Like the stars and planets, these aerosols usually are invisible during the day because they are obscured by the scattered sunlight (blue sky) of the troposphere. About 15 minutes after sunset, however, with the troposhere in shadow and the stratosphere still illuminated by sunlight passing through the lower atmosphere to the west, these high-level clouds come into view. Since their colors achieve greatest intensity after the sun has set at the surface, volcanic twilights are known as "afterglows."
Note that it is only when small volcanic particles have been lofted well into the stratosphere that colorful sunset afterglows appear. Volcanic particles that remain suspended in the troposphere after an eruption are comparatively large in size and number. As a result, they attenuate sunlight and otherwise subdue twilight hues, just like man-made dust and haze. Viewed through a veil of tropospheric volcanic ash, a sunset is dusky and dull.
Mount Pinatubo's sunset afterglows persisted to varying degrees for about 18 months after the initial explosion. In more recent years (especially 1998 and 2003), sunset colours in many areas have been subdued by the introduction of large smoke particles into the lower atmosphere by forest fires across the western United States, Canada, and China.
The preceding paragraphs have provided only an brief introduction to the physics and meteorology of the twilight sky. Further understanding of the nature of sunrise and sunset colours can only increase our appreciation of them.
for the full version, please check out:-
www.spc.noaa.gov/publications/corfid…
and:
www.electro-optics.org/Docs/newsflash/edu…
To see this Large:-
farm5.static.flickr.com/4044/4214964949_df4e8e7a29_b.jpg
Taken on
September 15, 2007 at 18.36 BST