On August 21, 2017, millions of Americans will be looking skyward as the day turns to night.
PART 1: A PRIMER
On Monday, August 21, North Americans will get an opportunity to observe nature's greatest sky show.
Picture this: The day starts off bright and sunny. Then a bit later you begin to notice that although it is still sunny, the day doesn't seem quite so bright. And still a little while later it seems like some big storm is brewing. Then suddenly, and without any warning, the midsummer day turns strangely dark (Figure 1).
A few stars come out. Birds and animals become confused and quickly head home to sleep. Night insects begin to chirp. All around the horizon there is a strange yellow-orange glow resembling some weird sunset. And meanwhile, up in the sky where the sun should be, there appears instead a jet-black disk surrounded by a softly glowing halo.
Then, just as suddenly, the sky brightens up. The stars disappear, birds and animals awaken, and the sun returns.
What you have just witnessed is the greatest of celestial road shows: a total eclipse of the sun.
The total eclipse that will occur on August 21, 2017, will mark the first time in nearly four decades that such an event will be visible so close to home. “Close,” of course, is a relative term, but for most Americans, this spectacular phenomenon will be visible from their own backyards.
Total solar eclipses are a fortuitous accident of nature. The sun's 864,000-mile diameter is 400 times larger than that of our puny moon, which is 2,160 miles. But the moon also happens to be about 400 times closer to the earth than the sun (the ratio varies a bit, as both orbits are elliptical), and as a result, when the orbital planes intersect and the distances align favorably, the new moon can appear to completely blot out the disk of the sun.
Contrary to popular belief, total solar eclipses are not particularly rare. Astronomers predict sixty-eight to take place during the present century— approximately one every 17.6 months. On such occasions, the moon casts its dark, slender shadow cone (called the umbra) upon the earth's surface. The track traced by the umbra can run for many thousands of miles, but it's also very narrow, at most about 167 miles wide. It has been calculated that, on average, a total eclipse of the sun is visible from the same spot on earth only once in 375 years.
With a silhouette of an acacia tree in the foreground, retired NASA astrophysicist Fred Espenak captured this surreal scene of the eclipsed sun from Chisamba, Zambia, on June 21, 2001. The composite sequence of the partial phases of the eclipse was recorded by triggering the shutter of his camera at five-minute intervals. The sequence goes from upper right to lower left. Note the twilight colors that bathed the entire horizon during the 3.5 minutes of midafternoon darkness.
In recent years, for example, ardent eclipse chasers had to travel to such remote locations such as Novosibirsk, Siberia (2008), Easter Island (2010), or the Norwegian archipelago of Svalbard (2015).
This map utilizes an equidistant conic projection to minimize distortion, and isolates the umbral path through the contiguous United States. Curves denoting the local daylight times of maximum eclipse and constant eclipse magnitude are plotted and labeled at intervals of 15 minutes and 0.1 magnitudes, respectively. The duration of totality (minutes and seconds) is indicated in 10-second increments and correspond to the local circumstances on the central line at each shadow position.
August 21 will mark the first time in this century and the first time since 1979 that a total solar eclipse will cross the contiguous (48) United States (Alaska had its turn in 1990; Hawaii in 1991). And for the very first time ever, the shadow track—better known as the “path of totality”—will sweep only over the United States and no other country, leading some to refer to this upcoming event as “The Great American Eclipse.”
Sight for the Millions
This total eclipse has a potential audience of some 12 million people who fortuitously live within the totality path. However, the amount of people who are within just a one day's drive (~500 miles) of the totality zone number around 220 million!
Not since 1970 has there been an opportunity to see a total solar eclipse in easily accessible and widespread areas of the United States. Admittedly, there were a couple of other opportunities, such as in July 1972 (Quebec and the adjacent Maritime Provinces) and February 1979 (the Pacific Northwest, Northern Plains, and Canadian Prairies), but the areas of visibility were either rather limited or somewhat difficult to reach. And not until April 2024 will there be another opportunity comparable to August 21. In addition, for the first time since 1918, the moon's dark shadow will sweep across the United States from coast to coast.
The Story of the Shadow
At local sunrise on this third Monday in August, the shadow of the moon will first touch the earth at a point in the North Pacific Ocean about 1,500 miles northwest of the Hawaiian Islands. Then, for a period of three hours and 13 minutes, the shadow will race first east-northeast, then east and finally southeast, along the way darkening a narrow strip of the North American continent.
Initially, the shadow will traverse nothing but wide-open ocean for 28 minutes. Finally it will make landfall along the coast of Oregon at Yaquina Head. Traversing the United States (Figure 2), the total eclipse will be visible within a path of darkness stretching from Oregon to South Carolina. The path will average 67 miles in width, but while moving through western Kentucky, about 12 miles northwest of the town of Hopkinsville (population 3,000), the shadow's size will widen to a maximum of 71 miles.
Because the moon's shadow is moving at such a tremendous speed, totality cannot last very long in any one place. The duration of the total phase is always longest along the center of the shadow's path. At the Oregon coastline, totality lasts one minute 59 seconds, as the shadow will be traveling at more than three times the speed of sound (2,400 mph). Heading southeast along the center line, the totality time slowly lengthens, reaching a maximum of two minutes 40.2 seconds at a spot in southern Illinois about a dozen miles southeast of the city of Carbondale. The shadow will slow to 1,450 mph as it moves through the Tennessee Valley. It then begins to increase in speed, and subsequently the duration of totality will begin to diminish. Indeed, when it arrives at the South Carolina coastline, the duration of totality will have dropped to two minutes 34 seconds. The shadow then exits out to sea, finally leaving the earth 75 minutes later at local sunset in the North Atlantic Ocean, 390 miles to the southwest of the island nation of Cape Verde. Table 1 provides local circumstances for 21 cities that are within the path of totality.
Total Eclipse of the Sun, August 21, 2017
|Geographical||First contact||Total eclipse begins||Last contact|
|Yaquina Head, Oregon||9:04 a.m.||42°||10:15:53||1:50||39°||11:36 a.m.||223°|
|Corvallis, Oregon||9:04 a.m.||42°||10:16:56||1:39||40°||11:37 a.m.||224°|
|Salem, Oregon||9:05 a.m.||42°||10:17:21||1:54||40°||11:37 a.m.||224°|
|Madras, Oregon||9:06 a.m.||42°||10:19:36||2:02||42°||11:41 a.m.||226°|
|Idaho Falls, Idaho||10:15 a.m.||44°||11:33:02||1:49||49°||12:58 p.m.||236°|
|Jackson, Wyoming||10:16 a.m.||44°||11:34:56||2:15||50°||1:00 p.m.||237°|
|Casper, Wyoming||10:22 a.m.||45°||11:42:41||2:26||54°||1:09 p.m.||240°|
|Alliance, Nebraska||10:27 a.m.||46°||11:49:15||2:30||57°||1:16 p.m.||244°|
|Grand Island, Nebraska||11:34 a.m.||47°||12:58:36||2:34||60°||2:26 p.m.||267°|
|Lincoln, Nebraska||11:37 a.m.||47°||1:02:37||1:16||60°||2:29 p.m.||270°|
|Kansas City, Missouri*||11:41 a.m.||48°||1:08:54||Edge||63°||2:36 p.m.||276°|
|Columbia, Missouri||11:45 a.m.||48°||1:12:23||2:36||63°||2:40 p.m.||283°|
|Saint Louis, Missouri*||11:50 a.m.||52°||1:18:25||Edge||63°||2:44 p.m.||293°|
|Carbondale, Illinois||11:52 a.m.||57°||1:20:08||2:37||64°||2:47 p.m.||297°|
|Paducah, Kentucky||11:54 a.m.||57°||1:22:17||2:22||64°||2:49 p.m.||298°|
|Hopkinsville, Kentucky||11:56 a.m.||58°||1:24:43||2:40||64°||2:51 p.m.||300°|
|Nashville, Tennessee||11:58 a.m.||59°||1:27:29||1:53||64°||2:54 p.m.||302°|
|Murfreesboro, Tennessee||11:59 a.m.||60°||1:29:09||0:58||64°||2:55 p.m.||303°|
|Greenville, South Carolina||1:09 p.m.||61°||2:38:03||2:10||63°||4:02 p.m.||309°|
|Columbia South Carolina||1:13 p.m.||62°||2:41:51||2:30||62°||4:06 p.m.||311°|
|Charleston, South Carolina||1:16 p.m.||63°||2:46:26||1:32||61°||4:10 p.m.||314°|
* In Kansas City, Missouri, the southern edge of totality lies very near to the Central Avenue Viaduct Bridge in the Central Industrial District. Totality will last longest in the neighborhood of Ferrelview with a duration of about two minutes. In Saint Louis, the northern edge of totality runs close to the Hi-Pointe and Forest Park South East neighborhoods. Totality will last longest in Patch, the southern tip of the historic Carondelet neighborhood, with a duration close to 80 seconds.
As an example of the information obtainable from this table, consider the first line, for Yaquina Head, Oregon. First contact (beginning of eclipse) is 9:04 a.m. PDT. The quantity P.A. is the position angle of the point where the sun's and moon's disks touch. It is measured clockwise around the sun's edge from 0° at the north point of the disk. Hence, the value 42° for Yaquina Head means the first “bite” out of the sun will appear at the upper right part of the solar disk. The table also states that totality for Yaquina Head begins at 10:15:53 and will last 1 minute 50 seconds with the sun at mid-totality standing 39° above the horizon. Finally, the eclipse ends at 11:36 a.m., with the last notch on the sun in position angle 223° (the lower left part of the solar disk).
The geometry of a total solar eclipse.
Notable cities that find themselves inside of the totality path include Idaho Falls, Idaho; Casper, Wyoming; Grand Island and Lincoln, Nebraska; Columbia, Missouri; Nashville, Tennessee; and Columbia and Charleston, South Carolina. The metropolitan areas of Kansas City and Saint Louis find themselves, respectively, straddling the southern and northern edges of the totality path.
Surrounding the dark umbra is the penumbra or partial shadow (Figure 3), also conical but much larger (nearly 6,000 miles) in diameter. The penumbra is simply the half shadow that lies outside every deep shadow, whether it's cast by the moon or a house. Wherever the penumbra falls, a partial eclipse will occur. All of North America will be inside the penumbra, causing a rather large partial eclipse for much of the United States and portions of southwest Canada.
Table 2 provides eclipse conditions for 21 cities that are outside of the path of totality. It is similar to Table 1, except in place of the information given concerning totality, details regarding the maximum phase of the eclipse is given. Magnitude (Mag.) is defined as the fraction of the sun's diameter covered by the moon. The closer a location is to the path of totality, the greater the magnitude of the partial eclipse. Thus, at Seattle, Washington, as much as 93.1% of the sun's diameter will be covered with the sun standing 39° above the horizon. In Honolulu, the greatest eclipse will be 38.7% at an altitude of just 5°.
Partial Eclipse of the Sun, August 21, 2017
|Geographical||First contact||Maximum eclipse||Last contact|
|Anchorage, Alaska||8:21 p.m.||74°||9:16 a.m.||19°||.556||10:13 a.m.||211°|
|Atlanta, Georgia||1:05 p.m.||43°||2:36 p.m.||65°||.971||4:01 p.m.||316°|
|Boise, Idaho||10:10 a.m.||43°||11:27 a.m.||46°||.994||12:50 p.m.||228°|
|Boston, Massachusetts||1:28 p.m.||102°||2:46 p.m.||50°||.702||3:59 p.m.||272°|
|Chicago, Illinous||11:54 a.m.||52°||1:19 p.m.||59°||.889||2:42 p.m.||278°|
|Denver, Colorado||10:23 a.m.||26°||11:47 a.m.||57°||.933||1:14 p.m.||262°|
|Helena, Montana||10:17 a.m.||44°||11:34 a.m.||47°||.933||12:56 p.m.||226°|
|Honolulu, Hawaii*||------------||-----||6:35 a.m.||5°||.387||7:25 a.m.||228°|
|Houston, Texas||11:46 p.m.||01°||1:17 p.m.||72°||.729||2:45 p.m.||309°|
|Knoxville, Tennessee||1:04 p.m.||43°||2:34 p.m.||63°||.997||3:58 p.m.||293°|
|Los Angeles, California||9:05 p.m.||00°||10:21 p.m.||48°||.694||11:44 a.m.||223°|
|Miami, Florida||1:26 p.m.||43°||2:58 p.m.||64°||.822||4:20 p.m.||348°|
|Montreal, Quebec, Canada||1:21 p.m.||94°||2:38 p.m.||50°||.662||3:50 p.m.||262°|
|New Orleans, Louisiana||11:57 p.m.||28°||1:29 p.m.||71°||.799||2:57 p.m.||314°|
|New York, New York||1:23 p.m.||92°||2:44 p.m.||53°||.770||4:00 p.m.||272°|
|Omaha, Nebraska||11:38 p.m.||48°||1:04 p.m.||60°||.984||2:30 p.m.||267°|
|Portland, Oregon||9:06 p.m.||37°||10:19 a.m.||40°||.992||11:38 a.m.||227°|
|San Francisco, California||9:01 a.m.||07°||10:15 a.m.||43°||.801||11:37 a.m.||222°|
|San Juan, Puerto Rico*||2:11 p.m.||136°||3:34 p.m.||44°||.836||4:46 p.m.||331°|
|Seattle, Washington||9:08 a.m.||38°||10:20 a.m.||39°||.931||11:39 a.m.||223°|
|Washington, D.C.||1:17 p.m.||82°||2:42 p.m.||56°||.844||4:01 p.m.||281°|
|Winnipeg, Manitoba, Canada||11:40 a.m.||64°||12:57 p.m.||51°||.763||2:15 p.m.||252°|
* Does not observe daylight savings time. At Honolulu, Hawaii, sunrise (6:12 a.m. Hawaii-Aleutian Standard Time) occurs after the eclipse has begun. Magnitude of the eclipse at sunrise for Honolulu: .270.
A number of cities and towns will lie just outside of the totality zone and will see the sun cut down to an exceedingly thin sliver of light. These include Portland, Oregon; Boise, Idaho; and Knoxville, Tennessee. But a partial solar eclipse pales in comparison with a total one, even when more than 99% of the sun's disc is obscured. When watching the partial phases, precautions must be taken in viewing the blindingly bright sun. Only when it's in total eclipse is the sun is it perfectly safe to look at; only then can you witness the full grandeur of the eclipse.
Phenomena of a Solar Eclipse: The Partial Phases
Scientists as well as millions of amateur astronomers and sightseers from all over the world will be drawn into the path of the moon's shadow.
In those last few minutes before totality, events pile in on each other: An eerie “counterfeit twilight” begins to descend, the distant landscape becomes enveloped in a strange grayish or metallic-looking pallor; the temperature may suddenly dip several degrees or more, allowing the air to contract and pull in gusty winds from all around the approaching shadow. As totality draws nearer, the sky grows still darker.
If you spread out a large white sheet on the ground, you may see shadow bands rippling, flickering, and scurrying about. These stripes of light and shade are believed to be caused by the last of the sun's rays narrowing to a thin filament and becoming distorted by our turbulent atmosphere, just as a star's light is disturbed, making it appear to twinkle.
The Brink of Totality
As the sun narrows to a mere filament of light, it suddenly disintegrates into irregular dots and points of light called “Baily's beads,” an effect caused by the last rays of sunlight streaming through the rugged mountain valleys on the lunar limb.
A prominence of hot gas rises from the sun's surface as observed by the Solar Terrestrial Relations Observatory (STEREO) science team.
Then the moon's dark shadow comes rushing in. Those watching for its approach should look toward the west-northwest sky, where any clouds will be darkening dramatically, as if some great storm were brewing; they may take on a yellowish-tan color, merging to a dark, burnished hue near the shadow edge. The edge itself will be diffuse and of a deep umber color, changing to slate, violet, or gray deeper in the shadow.
The rather clammy light of the sun will then seem to rush out as the moon completely covers the sun.
Totality at Last!
The sun now appears as a jet-black disk rimmed for several seconds by the vivid pastel-pink extension of the sun's atmospheric envelope: the chromosphere. If you use binoculars, you may see tiny flames of pink or magenta in several places around the black disk. Called prominences (Figure 4), these are hot clouds of hydrogen gas pushing up from the sun's surface for tens or even hundreds of thousands of miles into space.
The most spectacular part of the eclipse is the pearly-white corona, which haloes the dark disk of the sun and extends out into space for millions of miles. It can only be seen during totality and differs in size, tints, and patterns from one eclipse to another. It streams outward, ragged at the edge with streaks running through it; sometimes it has a soft continuous look, and at other times long rays may be seen shooting out in three or four directions.
During totality, you will be able to view other celestial objects not normally available in the daytime. The most obvious will be the brilliant planet Venus shining like a dazzling white jewel, positioned well to the right of the sun. If you're stationed in Wyoming and all points east along the eclipse track, you will also be able to see Jupiter, shining less brightly than Venus, far to the left of the sun. A few stars may be visible here and there, and if you have binoculars, you might notice a bluish one that will be plainly visible just to the left of the darkened sun. That will be Regulus, in the constellation Leo and one of the 21 brightest stars in the sky. As for the overall sky illumination, it will be unlike any dusk or dawn you've ever experienced. A weird saffron tint will form a bright border around the horizon, while clouds in the area may take on striking hues of sienna or salmon. Just before the end of totality, the chromosphere will again reappear, followed suddenly by the appearance of a brilliant solitaire of steely-white light set upon a thin, luminous ring—the inner corona.
“Diamond ring” effect. Eclipse of August 11, 1999, taken just after sunrise from a cruise ship off of Nova Scotia.
Then, the streamers vanish, the gem grows, the stars and planets fade away, and the sky fills with light as this “diamond ring” in the sky (Figure 5) soon becomes too dazzling to look at. Everything seen prior to totality now reappears in reverse order as the moon moves off the sun's disc.
Be Careful …
Whenever an eclipse of the sun is due to occur, there are usually many warnings broadcast over the airwaves and in newspapers telling people that there is no safe way to view an eclipse and the best thing to do is watch it on television. Television, however, is a poor substitute for the real thing.
This is not to say, however, that you should not take precautions. Staring at the sun with unprotected eyes or inadequate filters during the partial stages can cause severe retinal damage or blindness.
Here are URLs for two Web pages that will give all the pertinent details you will need concerning the proper ways to safely observe a solar eclipse. The first, http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html, is written by Dr. B. Ralph Chou, who, for 30-years until his retirement in 1992, was affiliated with the Waterloo School of Optometry and Vision Science.
The second URL, http://www.mreclipse.com/Totality2/TotalityCh11.html, is written by Fred Espenak, retired NASA astrophysicist who was that agency's expert on eclipses. Known worldwide as “Mr. Eclipse,” he is the author of numerous books on the subject of eclipses and has participated in many eclipse expeditions.
After 2017, the next total solar eclipse for the United States occurs on April 8, 2024. The path of viewing will stretch from central Texas to northern New England. The duration of totality will average about four minutes. Interestingly, the totality path will again encompass Carbondale, Illinois—that city's second total eclipse in less than 7 years!
Total eclipse paths over the United States for the next 35 years.
Prior to 2017, there were only three total eclipses for the contiguous United States dating back to 1960. In contrast, in the next 35 years, no fewer than five totalities will be visible (Figure 6).
PART 2: ECLIPSE WEATHER AND WHERE TO GO
Unless you have access to an aircraft to fly above the clouds (Figure 7), eclipse observers will have to carefully scrutinize climatological records in and near the path of totality in hopes of choosing the best opportunity for cloudless skies on August 21.
A view of the totally eclipsed sun taken from an Alaska Airlines 737-900 aircraft on March 9, 2016, above the North Pacific Ocean, 700 miles north of Honolulu on March 9, 2016. A thick blanket of clouds would have obscured the eclipse from any ship, but from the aircraft's altitude of 37,000 feet, the cloud cover actually added to the spectacle. This image was taken near the end of a total eclipse that lasted 113 seconds. The clouds in the immediate foreground appear dark because they are still within the moon's shadow, while farther off in the distance the clouds appear brighter because they are being illuminated by a thin sliver of sunlight which has just emerged from behind the dark silhouette of the moon. Note the glint of returning sunlight reflected of the cloud tops. One of the aircraft's winglets – which reduces drag – is visible, extending up from the starboard wing to the left of the eclipsed sun.
Climatological cloud amounts are consulted only because there are no reliable alternatives. Meteorological weather forecasts for eclipse day are not possible more than a week or so ahead of time. In temperate latitudes during the summer, large daily deviations from normal can sometimes occur. Even if eclipse day starts off with exactly normal conditions, it would not be normal at the time of totality because of the approximately 80-minute interval of increasing partial eclipse. During that interval, the reduced solar heating will be attended by decreased cumulus cloudiness but an increase in stratiform cloudiness, as has actually been observed at past eclipses.
Figure 8 depicts the mean percentage of the sky covered by opaque cloud during the daylight hours across the contiguous United States. Based on long-term averages, that part of the totality path generally west of the Mississippi River has the best overall probabilities for clear skies; note the location of the 50% isoneph. The air over the western regions tends to be drier, and because the eclipse will occur during the morning, there is less of a chance for convective clouds to form. This dryness in the western regions is opposed to locations along the totality path east of the Mississippi, however, which are noticeably more humid. Skies also tend to be hazier, and since the eclipse occurs during the afternoon in the East, solar heating of the land produces partly cloudy skies with building cumulus clouds, along with a risk of possible “pop-up” showers and thunderstorms.
Average August opaque cloudiness across the contiguous United States.
Based on Figure 8, the clearest skies along the totality path are in the protected mountain valleys and interior plateau of eastern Oregon, Idaho, and westernmost Wyoming, with opaque cloudiness averaging 30–40%, while August for much of Nebraska is its sunniest month, with an average cloud cover of 45%. The Front Range of western Nebraska, in particular, is another promising location.
The greatest cloudiness on the map is found over three zones: the Pacific coast of Oregon, the western mountains of North Carolina, and near and along the coastal plain of South Carolina. In each of these zones the average percentage of daytime cloudiness is about 60–65%.
Percentage of Sunshine and Cloud Cover
|Percentage of sunshine||Cloud cover (10ths)|
|North Platte, Nebraska*||75||4.7|
|Kansas City, Missouri*||67||4.9|
|Saint Louis, Missouri*||65||5.4|
|Asheville, North Carolina||54||6.1|
|Greenville, South Carolina*||61||6.1|
|Columbia, South Carolina*||66||6.1|
|Charleston, South Carolina*||64||6.9|
* Denotes places within total eclipse track. Data: NOAA, National Environmental Satellite Data and Information Service, National Climatic Data Center, Asheville, North Carolina.
Table 3 summarizes long-term sunshine and cloud cover data based on long-term (30-year) records for 17 stations in and near the path of totality. The apparent inconsistencies between sunshine percentages and cloud cover figures (both are based on the period between sunrise and sunset) arise because thin cirroform clouds are included in the cloud cover data. The sun often shines through these with sufficient intensity to activate a sunshine recorder. The sunshine data probably provides too optimistic an estimate of the probability of seeing the eclipse, while the cloud cover data are too pessimistic. Thus, the true chance of seeing the eclipse to some degree (either in a perfectly clear sky or through a layer of thin cirrus or cirrostratus) lies between those probabilities inferred from the two sets of figures in Table 3.
The clear winner on this list is Boise, Idaho, which lies just outside of the totality path with odds of around 80% of seeing the sun on eclipse day.
On the other end of the viewing spectrum is Asheville, North Carolina, which, like Boise, finds itself just outside of the totality path, but with odds of only around 45% of getting a clear view of the eclipsed sun. This no doubt is due to its location relative to the Blue Ridge Mountains, which can help spawn quite a bit of cumuliform cloudiness during the early-to-mid afternoon. Here, maximum eclipse comes at 2:37 p.m. EDT. The elevation in the Asheville Plateau ranges from 2,000 to 4,400 feet, with North Carolina's tallest peak, 6,684-foot Mount Mitchell, sitting just 20 miles to the northeast of the city.
As noted earlier, the eclipse can serve to help prospective eclipse watchers by causing a very noticeable diminution in cumulus build-up. But, if cloud development is already well-advanced nearly to the point of producing showery rains by eclipse time, any resultant decrease in the cloud cover may only be slight or completely negligible.
Close behind Asheville is Charleston, South Carolina, which we'll touch upon later.
Frequency of Clear Sky or Scattered Clouds
|Idaho Falls, Idaho||68.2|
|Jackson Hole, Wyoming||68.2|
|Kansas City, Missouri||53.2|
|Saint Louis, Missouri||51.0|
|De Soto, Illinois||50.2|
|Saint Joseph, Missouri||46.4|
|Columbia, South Carolina||42.8|
|Greenville, South Carolina||41.9|
|Bowling Green, Kentucky||39.8|
|Charleston, South Carolina||32.0|
Cloud-cover statistics taken from Eclipse Bulletin: Total Solar Eclipse of 2017, August 21, by Fred Espenak and Jay Anderson.
For Table 4, cloudiness was tabulated at 18 weather stations within the path of totality using August cloud-cover statistics at eclipse time, which were derived from surface observations based on data from the NRDC. The results are percentages of observations with clear sky or scattered clouds (cover less than 0.5), and are listed in descending order of merit.
Two locations that score very high on this list are situated in the Columbia Basin of Oregon: Ontario and Redmond. The reason? Moist air from the Pacific flows eastward, reaching the Cascade Mountains that provide a 10,000-foot barrier. On the windward side of the range, orographic lifting occurs. This happens when air is forced to go from a low elevation to a higher elevation as it moves over rising terrain. As it gains altitude, it quickly cools down, and because at lower temperatures less water vapor is needed for saturation and condensation, clouds are created as well as, under the right conditions, precipitation. But when the air passes over the mountains and moves downslope, compression rapidly warms and dries the air, causing any clouds or precipitation to quickly dissipate. Places located within “The Basin” are afforded the benefits of this effect, with much of the residual moisture being effectively scoured out, usually leaving skies mostly clear and sunny. One such community is Madras (pop. ~6,700), the county seat of Jefferson County, Oregon, which is located just south of the central line of totality and considered by many to be perhaps the best for eclipse viewing anywhere in the United States.
Dubious Coastal Climatology
In contrast, along the Pacific and Atlantic coastal plains, the weather might be problematic. Those who plan to greet the moon shadow's landfall near or along the Oregon coastline should be leery of the odds of a significant cloud cover, running anywhere from 50% to nearly 80%.
The Yaquina Head Light was once known in the late 19th century as the “Cape Foulweather Lighthouse,” possibly as a reflection of the local weather. In addition, fog or low stratus clouds form over the ocean and frequently move inland at night, but generally disappear by midday. Unfortunately, totality is during the late morning, so there's no guarantee any residual fog that might be present will have dissipated by eclipse time.
And the eclipse might actually work against those attempting to watch from the Oregon coast. As the eclipse progresses and the morning air cools, it could cause a reformation of stratiform cloudiness. Ground fog, too, could form in the lowlands.
Near and along the coast of South Carolina, in spite of cloud cover values in some cases in excess of 65%, cloudiness usually tends to build over the warmer inland areas, while over the ocean where the air is cooler, cloudiness tends to be less. From historic Charleston (pop. 128,000), it will be mid-afternoon, and the sun will stand 63 degrees above the southwest horizon above a broad expanse of open ocean water, giving prospective viewers along the coastal plain at least a fighting chance of seeing the eclipse.
But late August is also well into the Atlantic hurricane season. In fact, August 21 is less than three weeks from the traditional date (September 10) of peak tropical activity. In the last 60 years, during the August 16–21 timeframe, three tropical cyclones (1976, 1981, and 1991) have approached to within 200 nautical miles of the South Carolina coast. However, the odds that a tropical storm or hurricane might adversely affect eclipse viewing in 2017 admittedly are exceedingly small, probably less than 1%.
In his 1973 novel, Time Enough for Love, science fiction writer Robert A. Heinlein came up with the following aphorism: “Climate is what we expect, weather is what we get!”
How true this is! One should always remember that all of the climatological statistics that have been noted are not absolute. Indeed, it has certainly happened that avid eclipse watchers have in the past ventured to a location that climatology indicated would be a favorable place to watch an eclipse, but in the end came away frustrated when unexpected clouds prevailed instead.
An excellent example of this is the total solar eclipse of July 11, 1991. The Big Island of Hawaii was entirely within the totality path. Climatology dictated that when the northeast trade winds blow, the eastern (windward) side of the island would see heavy clouds and rain, while on the sheltered lee (western side) of the island, the trades would blow downslope and dry out—a pattern not too dissimilar to what is expected in August 2017 for the Columbia Basin. Since the trade winds blow 95% of the time in July, the assumption was that clear skies were a given for the western side of Hawaii. Unfortunately, a tropical upper tropospheric trough (TUTT) moved over the Big Island on eclipse day, bringing more clouds than sun and disappointing the tens of thousands of people who had come to Hawaii to view the eclipse.
In view of such weather uncertainties, plans about where to observe should be kept flexible up to the latest possible time before the eclipse, which occurs on a Monday. Starting the week before, the NWS will provide increasingly reliable forecasts, which will enable people to choose a location where chances of a cloudy sky are low.
On the weekend before, the very latest meteorological data should be used to modify the plans based on climatological data only.
The best probabilities of clear skies appear to be in the region running from western Oregon through Wyoming, where, on average, the odds of seeing the eclipse are on the order of about 70–80%. Going farther east, the odds gradually diminish; across the Piedmont to the Atlantic coast, it appears to be just about a coin toss: 50-50, except perhaps less than 50% for western mountains of North Carolina and the coastal plain of South Carolina. The immediate Pacific coast of Oregon, with its gusty onshore winds and frequent bouts of low cloud and fog, appears to have the lowest odds of seeing the eclipsed sun … probably only around 40%. Regardless of where you plan to be, staying mobile to dodge cloud cover will always enhance your chances.
To those who plan to position themselves in the totality path with hopes of experiencing the phenomena that accompany that magical exclamation “totality!,” we wish one and all good luck and clear skies.
JOE RAO who is based at FIOS1 News in Rye Brook NY is a seven-time Emmy nominated broadcast meteorologist. In 2015, he won top honors in the category “Best Weathercast” by the Associated Press of New York. Joe has traveled to 11 total eclipses and has spent nearly a half hour basking in the shadow of the moon. In March 2016, he made national headlines by convincing Alaska Airlines to delay an Anchorage to Honolulu flight by 25 minutes in order to give passengers and crew an opportunity to witness a total solar eclipse over the Pacific Ocean.