Great Rainbow Pictures
May 15, 2006, 9:46 pm
Thanks to Randy Toffel of Hoover for these great pictures of a double rainbow over the Birmingham metro this evening:
by James Spann
in Pictures
Ever Heard Of A Red Sprite?
May 15, 2006, 9:27 pm
We got on the subject of sprites and jets in the weather office yesterday... thought you would enjoy this material on the very interesting subject from the University of Alaska in Fairbanks:
Red sprites and blue jets are upper atmospheric optical phenomena associated with thunderstorms that have only recently been documented using low light level television technology.
The first images of a sprite were accidently obtained in 1989 (Franz et al., 1990). Beginning in 1990, about twenty images have been obtained from the space shuttle (Vaughan et al., 1992; Boeck et al., 1994).
Since then, video sequences of well over a thousand sprites have been captured. These include measurements from the ground ( Lyons, 1994; Winckler, 1995) and from aircraft (Sentman and Wescott, 1993; Sentman et al., 1995).
Numerous images have also been obtained from aircraft of blue jets ( Wescott et al., 1995), also a previously unrecorded form of optical activity above thunderstorms. Blue jets appear to emerge directly from the tops of clouds and shoot upward in narrow cones through the stratosphere. Their upward speed has been measured to be about 100 km per second.
Anecdotal reports of "rocket-like" and other optical emissions above thunderstorms go back more than a century (Lyons, 1994), and there have been several pilot reports of similar phenomena (Vaughan and Vonnegut, 1989). Possibly associated gamma ray bursts and TIPPS have also recently reported. Together, these phenomena suggest that thunderstorms exert a much greater influence on the middle and upper atmospheres than was previously suspected.
Sprites are massive but weak luminous flashes that appear directly above an active thunderstorm system and are coincident with cloud-to-ground or intracloud lightning strokes. Their spatial structures range from small single or multiple vertically elongated spots, to spots with faint extrusions above and below, to bright groupings which extend from the cloud tops to altitudes up to about 95 km. Sprites are predominantly red. The brightest region lies in the altitude range 65-75 km, above which there is often a faint red glow or wispy structure that extends to about 90 km. Below the bright red region, blue tendril-like filamentary structures often extend downward to as low as 40 km. Sprites rarely appear singly, usually occurring in clusters of two, three or more. Some of the very large events, such as shown in Figure 1, seem to be tightly packed clusters of many individual sprites. Other events are more loosely packed and may extend across horizontal distances of 50 km or more and occupy atmospheric volumes in excess of 10,000 cubic km.
Blue jets are a second high altitude optical phenomenon, distinct from sprites, observed above thunderstorms using low light television systems. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (Mach 300), fanning out and disappearing at heights of about 40-50 km.
Why Haven't Sprites and Jets Been Reported Before?
Sprites appear to be elusive for several reasons.
(1) Sprites only occur above active thunderstorm systems. To see them requires visual access to the region above the storm, unobstructed by intervening clouds, and viewing against a dark stellar background. In most locations these conditions occur only rarely.
(2) Sprites are dim and can only been seen with the dark adapted eye. On average, their brightness compares to moderately bright aurorae, 10-50 kiloRayleighs. In the human eye, this corresponds approximately to the crossover threshold intensities of cones of the retina, which respond to color, and the somewhat more sensitive but achromatic parfoveal rods, which permit night vision. The dark adapted eye most readily sees sprites in parfoveal vision, when not directly looking at them. Thus, they may quite literally appear only as flashes out of the corner of the eye. Because of their dimness, sprites cannot be viewed in the presence of nearby bright lights, as would be found in a city.
(3) Cloud illumination from sprite-producing cloud-to-ground or intracloud lightning activity is often orders of magnitude brighter than sprites. This lightning activity can easily distract the casual observer from noticing the fleeting and delicate dance of red sprites high in the sky above the storm raging below.
(4) Sprites appear to have a duration of only a few (3-10) milliseconds. This is too brief to permit shifting one's gaze to obtain a visual fix.
(5) Sprites occur randomly with only about one percent of lightning strokes. The mere occurrence of lightning therefore cannot be used as an event marker to indicate that a sprite has occurred above a thunderstorm.
When all of these factors are taken together it is not surprising that sprites have been so elusive. However, they can be seen with the unaided human eye.
How to Look for Sprites and Jets
*A clear view above a thunderstorm is required. This generally means the thunderstorm activity must be on the horizon. Additionally, there must be very little intervening cloud cover.
*Best viewing distance from storm is 100-200 miles (200-300 km). At these distances sprites will subtend a vertical angular distance of 10-20 degrees. This is 2-4 times the separation of the pointer stars in the Big Dipper.
*For observing sprites, it must be completely dark. (i. e. no longer twilight)
*Eyes must be completely dark adapted. Use same criteria for this as for astronomical observing. If you can see the Milky Way, then it is probably dark enough and the eyes have adapted enough to see sprites.
*Fix your gaze on the space above an active thunderstorm. Do not be distracted by underlying lightning activity in the storm. Block out the lightning if necessary using a piece of dark paper in such a way as to still being able to view what is going on above the cloud.
*Sprites will be very brief flashes just on the edge of perceptability. They occur too quickly to follow with the eyes, but their strange vertically striated structure and dull red color may be perceived.
*Patience will be rewarded. If the right kind of storm is present and one's viewing geometry is favorable, then there is a greater likelihood of seeing a sprite than of seeing a shooting star or comet.
Red sprites and blue jets are upper atmospheric optical phenomena associated with thunderstorms that have only recently been documented using low light level television technology.
The first images of a sprite were accidently obtained in 1989 (Franz et al., 1990). Beginning in 1990, about twenty images have been obtained from the space shuttle (Vaughan et al., 1992; Boeck et al., 1994).
Since then, video sequences of well over a thousand sprites have been captured. These include measurements from the ground ( Lyons, 1994; Winckler, 1995) and from aircraft (Sentman and Wescott, 1993; Sentman et al., 1995).
Numerous images have also been obtained from aircraft of blue jets ( Wescott et al., 1995), also a previously unrecorded form of optical activity above thunderstorms. Blue jets appear to emerge directly from the tops of clouds and shoot upward in narrow cones through the stratosphere. Their upward speed has been measured to be about 100 km per second.
Anecdotal reports of "rocket-like" and other optical emissions above thunderstorms go back more than a century (Lyons, 1994), and there have been several pilot reports of similar phenomena (Vaughan and Vonnegut, 1989). Possibly associated gamma ray bursts and TIPPS have also recently reported. Together, these phenomena suggest that thunderstorms exert a much greater influence on the middle and upper atmospheres than was previously suspected.
Sprites are massive but weak luminous flashes that appear directly above an active thunderstorm system and are coincident with cloud-to-ground or intracloud lightning strokes. Their spatial structures range from small single or multiple vertically elongated spots, to spots with faint extrusions above and below, to bright groupings which extend from the cloud tops to altitudes up to about 95 km. Sprites are predominantly red. The brightest region lies in the altitude range 65-75 km, above which there is often a faint red glow or wispy structure that extends to about 90 km. Below the bright red region, blue tendril-like filamentary structures often extend downward to as low as 40 km. Sprites rarely appear singly, usually occurring in clusters of two, three or more. Some of the very large events, such as shown in Figure 1, seem to be tightly packed clusters of many individual sprites. Other events are more loosely packed and may extend across horizontal distances of 50 km or more and occupy atmospheric volumes in excess of 10,000 cubic km.
Blue jets are a second high altitude optical phenomenon, distinct from sprites, observed above thunderstorms using low light television systems. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (Mach 300), fanning out and disappearing at heights of about 40-50 km.
Why Haven't Sprites and Jets Been Reported Before?
Sprites appear to be elusive for several reasons.
(1) Sprites only occur above active thunderstorm systems. To see them requires visual access to the region above the storm, unobstructed by intervening clouds, and viewing against a dark stellar background. In most locations these conditions occur only rarely.
(2) Sprites are dim and can only been seen with the dark adapted eye. On average, their brightness compares to moderately bright aurorae, 10-50 kiloRayleighs. In the human eye, this corresponds approximately to the crossover threshold intensities of cones of the retina, which respond to color, and the somewhat more sensitive but achromatic parfoveal rods, which permit night vision. The dark adapted eye most readily sees sprites in parfoveal vision, when not directly looking at them. Thus, they may quite literally appear only as flashes out of the corner of the eye. Because of their dimness, sprites cannot be viewed in the presence of nearby bright lights, as would be found in a city.
(3) Cloud illumination from sprite-producing cloud-to-ground or intracloud lightning activity is often orders of magnitude brighter than sprites. This lightning activity can easily distract the casual observer from noticing the fleeting and delicate dance of red sprites high in the sky above the storm raging below.
(4) Sprites appear to have a duration of only a few (3-10) milliseconds. This is too brief to permit shifting one's gaze to obtain a visual fix.
(5) Sprites occur randomly with only about one percent of lightning strokes. The mere occurrence of lightning therefore cannot be used as an event marker to indicate that a sprite has occurred above a thunderstorm.
When all of these factors are taken together it is not surprising that sprites have been so elusive. However, they can be seen with the unaided human eye.
How to Look for Sprites and Jets
*A clear view above a thunderstorm is required. This generally means the thunderstorm activity must be on the horizon. Additionally, there must be very little intervening cloud cover.
*Best viewing distance from storm is 100-200 miles (200-300 km). At these distances sprites will subtend a vertical angular distance of 10-20 degrees. This is 2-4 times the separation of the pointer stars in the Big Dipper.
*For observing sprites, it must be completely dark. (i. e. no longer twilight)
*Eyes must be completely dark adapted. Use same criteria for this as for astronomical observing. If you can see the Milky Way, then it is probably dark enough and the eyes have adapted enough to see sprites.
*Fix your gaze on the space above an active thunderstorm. Do not be distracted by underlying lightning activity in the storm. Block out the lightning if necessary using a piece of dark paper in such a way as to still being able to view what is going on above the cloud.
*Sprites will be very brief flashes just on the edge of perceptability. They occur too quickly to follow with the eyes, but their strange vertically striated structure and dull red color may be perceived.
*Patience will be rewarded. If the right kind of storm is present and one's viewing geometry is favorable, then there is a greater likelihood of seeing a sprite than of seeing a shooting star or comet.
Showers Popping Up
May 15, 2006, 3:27 pm
The Monday afternoon map discussion video is on the server:
http://www.jamesspann.com/
Don't forget all of our video and audio products are avalable now on iTunes; just search for ABC 33/40 in the podcast section.
Showers have developed this afternoon over the northern part of Alabama, generally north of I-59. With such a huge upper low to the north, this is to be expected. And, a few widely scattered showers are possible tomorrow and Wednesday. But, the chance of any one neighborhood getting wet remains relatively small, and temperatures will remain unusually pleasant for mid-May in Alabama.
LATE IN THE WEEK: Looks like Thursday will be dry, but at the end of the week a cold front will approach from the north, and that might trigger a few showers late Friday or Friday night. Moisture will be very limited, and like the activity today the showers should be widely spaced.
WEEKEND PEEK: The 12Z GFS looks pretty dry for the weekend. Drier air will move into Alabama following the Friday night front, and for the moment the weather looks mostly dry and warm Saturday and Sunday with only isolated showers possible. We will probably take out the mention of isolated showers if the models continue to trend dry. Afternoon temperatures should reach the low 80s on Saturday and Sunday, which is near normal for this time of the year in Alabama.
LONG RANGE: The maps are showing a shift in the upper pattern over North America toward late May, as the primary jet stream shifts to the north. This is to be expected, and it should mean warmer and more humid weather during the last week in May. Highs in the 80s, and scattered afternoon showers and storms showing up in the forecast more frequently. Another words, our summer pattern seems to be forming right on time.
I enjoyed speaking to the third graders today at Shades Mountain Christian... also had some of the high school students in there that are studying meteorology. Look for them on the KIDCAM on ABC 33/40 News at 5:00 this afternoon!
The next map discussion video will be posted by 7:00 a.m. tomorrow!
http://www.jamesspann.com/
Don't forget all of our video and audio products are avalable now on iTunes; just search for ABC 33/40 in the podcast section.
Showers have developed this afternoon over the northern part of Alabama, generally north of I-59. With such a huge upper low to the north, this is to be expected. And, a few widely scattered showers are possible tomorrow and Wednesday. But, the chance of any one neighborhood getting wet remains relatively small, and temperatures will remain unusually pleasant for mid-May in Alabama.
LATE IN THE WEEK: Looks like Thursday will be dry, but at the end of the week a cold front will approach from the north, and that might trigger a few showers late Friday or Friday night. Moisture will be very limited, and like the activity today the showers should be widely spaced.
WEEKEND PEEK: The 12Z GFS looks pretty dry for the weekend. Drier air will move into Alabama following the Friday night front, and for the moment the weather looks mostly dry and warm Saturday and Sunday with only isolated showers possible. We will probably take out the mention of isolated showers if the models continue to trend dry. Afternoon temperatures should reach the low 80s on Saturday and Sunday, which is near normal for this time of the year in Alabama.
LONG RANGE: The maps are showing a shift in the upper pattern over North America toward late May, as the primary jet stream shifts to the north. This is to be expected, and it should mean warmer and more humid weather during the last week in May. Highs in the 80s, and scattered afternoon showers and storms showing up in the forecast more frequently. Another words, our summer pattern seems to be forming right on time.
I enjoyed speaking to the third graders today at Shades Mountain Christian... also had some of the high school students in there that are studying meteorology. Look for them on the KIDCAM on ABC 33/40 News at 5:00 this afternoon!
The next map discussion video will be posted by 7:00 a.m. tomorrow!
The Ten Lowest
May 15, 2006, 10:33 am
Just went for another walk on our community walking track and the weather is perfection. Here are the ten coolest temperatures I could find around the Alabama countryside this morning:
39 in DeSoto State Park
41 in Hamilton and at Cullman Airport (Vinemont)
42 in Crossville
43 in Gaylesville
45 in Addison and Heflin
46 in Muscle Shoals, Decatur, Ft. Payne, Meridianville, Jasper, Wedowee
47 in Pinson, Huntsville, Albertville, Ashville
48 in Blue Pond (Cherokee County)
50 at Birmingham Airport
52 at Anniston Airport
USA EXTREMES
The coldest in the lower 48 states this morning was 25 at Limon, Colorado.
The hottest Sunday was 114 in Death Valley.
A WORLD EXTREME
...and this final note, which you probably don't care about, the projected low for Vostok, Antarctica tomorrow night is 106 below zero with sustained winds of 15 mph. This means the wind chill will be a bitter 174 below zero. Will need at least a light coat and to have socks on.
39 in DeSoto State Park
41 in Hamilton and at Cullman Airport (Vinemont)
42 in Crossville
43 in Gaylesville
45 in Addison and Heflin
46 in Muscle Shoals, Decatur, Ft. Payne, Meridianville, Jasper, Wedowee
47 in Pinson, Huntsville, Albertville, Ashville
48 in Blue Pond (Cherokee County)
50 at Birmingham Airport
52 at Anniston Airport
USA EXTREMES
The coldest in the lower 48 states this morning was 25 at Limon, Colorado.
The hottest Sunday was 114 in Death Valley.
A WORLD EXTREME
...and this final note, which you probably don't care about, the projected low for Vostok, Antarctica tomorrow night is 106 below zero with sustained winds of 15 mph. This means the wind chill will be a bitter 174 below zero. Will need at least a light coat and to have socks on.
A Cool Breeze
May 15, 2006, 5:52 am
The Monday morning map discussion video is on the server:
http://www.jamesspann.com/
I loved reading Bill's posts about "Granny vs the Weather Bureau"... funny stuff.
TODAY-WEDNESDAY: The massive upper low continues to cover the eastern U.S. The center of the vortex is north of us, but it is close enough for a few cloudy periods through Wednesday. And, while we don't expect any widespread rain, you sure can't rule out a few isolated showers or pathes of light rain in a pattern like this. The chance of any one spot getting wet at any one time will be fairly small, but understand a few raindrops are not out of the question.
Temperatures will also remain below normal; we should be in the low 80s here in mid-May; but most places will have a hard time reaching 70 degrees today with a cool breeze. No complaints here!
LATE THIS WEEK: We should warm into the upper 70s on Thursday and low 80s by Friday. Another eastern U.S. upper trough will form this weekend, not not as deep as the one we have now. For now the chance of any significant rain on Friday looks pretty small, but a few widely scattered showers might show up over the northeast corner of the state Friday afternoon.
WEEKEND PREVIEW: With an upper trough overhead, we can't rule out a few isolated showers Saturday or Sunday, but with little moisture available it looks like the weekend will be mostly dry. High temperatures should be close to 80 degrees with a mix of sun and clouds.
LONG RANGE: The pattern finally begins to look more like early summer toward the end of May, as the primary jet stream shifts to the north, and the Bermuda high sets up. The GFS is advertising warmer and more humid weather at the end of the month. Highs in the 80s, and scattered showers and storms.
SKYCAM NETWORK: Don't forget you can see images and current weather from all of our SKYCAM network sites here:
http://www.abc3340.com/static/skycams/skycamsmain.html
I will be doing a weather program today at Shades Mountain Christian School... I should have the afternoon video produced on time by 3:30 this afternoon!
http://www.jamesspann.com/
I loved reading Bill's posts about "Granny vs the Weather Bureau"... funny stuff.
TODAY-WEDNESDAY: The massive upper low continues to cover the eastern U.S. The center of the vortex is north of us, but it is close enough for a few cloudy periods through Wednesday. And, while we don't expect any widespread rain, you sure can't rule out a few isolated showers or pathes of light rain in a pattern like this. The chance of any one spot getting wet at any one time will be fairly small, but understand a few raindrops are not out of the question.
Temperatures will also remain below normal; we should be in the low 80s here in mid-May; but most places will have a hard time reaching 70 degrees today with a cool breeze. No complaints here!
LATE THIS WEEK: We should warm into the upper 70s on Thursday and low 80s by Friday. Another eastern U.S. upper trough will form this weekend, not not as deep as the one we have now. For now the chance of any significant rain on Friday looks pretty small, but a few widely scattered showers might show up over the northeast corner of the state Friday afternoon.
WEEKEND PREVIEW: With an upper trough overhead, we can't rule out a few isolated showers Saturday or Sunday, but with little moisture available it looks like the weekend will be mostly dry. High temperatures should be close to 80 degrees with a mix of sun and clouds.
LONG RANGE: The pattern finally begins to look more like early summer toward the end of May, as the primary jet stream shifts to the north, and the Bermuda high sets up. The GFS is advertising warmer and more humid weather at the end of the month. Highs in the 80s, and scattered showers and storms.
SKYCAM NETWORK: Don't forget you can see images and current weather from all of our SKYCAM network sites here:
http://www.abc3340.com/static/skycams/skycamsmain.html
I will be doing a weather program today at Shades Mountain Christian School... I should have the afternoon video produced on time by 3:30 this afternoon!
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