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Index to Weather Prospects to 2003 Annular Eclipse
The two polar eclipses of 2003 occur in the springtimes of their opposing seasons. One cannot help but be struck by their meteorological parallels - both occur in a marine environment, both have a polar ice cap to consider as an observing venue, both occur in regions of the globe that are plagued by many active low pressure systems, and both are famously cloudy over much of the track. Of the two, the annular eclipse across Greenland, Iceland and northern Scotland offers the poorer weather prospects, though it is easier to reach than November's eclipse over Antarctica. The best possible escape from the gray skies and wet climate is to select a site on the Greenland ice cap where high altitude and a drier air may bring favor on eclipse day. The same is true in Antarctica.
Low pressure systems that develop and cross North America frequently turn to the northeast when they leave the continent, eventually ending their existence over the waters between Greenland and Iceland. This location is the home of the infamous Icelandic low, a semi-permanent depression that dictates not only the weather of its home island but also the meteorology of Britain and northern Europe. The low lies at the boundary of the cold Arctic airmass that lurks to the north and the warmer maritime climate that accompanies the Gulf Stream as it flows past Iceland in its clockwise circuit of the Atlantic.
The clash of warm and cold, both in the water and the atmosphere, generates a never-ending succession of mid-latitude frontal depressions that travel toward the British Isles. In Iceland, overcast skies with rain or snow are the norm, though precipitation amounts are not especially high. With its more southerly latitude, Scotland is able to tap occasionally into the drier air around the large high-pressure system that lingers near the Azores, and so the gloominess of north Atlantic weather is relieved occasionally by southerly breezes.
GreenlandŐs population lives on the coastal margins of the island, especially on the western side where the ice cap lies farther back from the coast. The coasts are surmounted by the Greenland ice cap which rises to over 3000 m in mid-island. In May an east-west oriented high-pressure system lies across the north of the island while an extension of the Iceland low reaches to Cape Farewell at the southern tip. The northeasterly winds between these two semi-permanent systems bring a steady flow of cool and humid Arctic air against the sharply rising east coast. Nearly forty percent of weather observations at eclipse time at Angmagssalik (Table 1.16) have precipitation falling as a result of this onshore flow and the frequency of overcast skies is more than fifty percent. ( see : Key to Table 1.16).
On the west coast winds tend to be lighter and more variable, but Davis Strait is a favorite destination for lows traveling across North America, and so the coast is visited by a steady series of disturbances. Cloud cover from these systems plus a persistent fog and low cloud that arises from the cool waters of Davis Strait combine to make this side of the island as nearly as cloudy as the east. Cloud cover statistics derived from satellite images are subject to a number of complications at GreenlandŐs latitude, but the data do show a tendency to slightly less cloud along the west coast.
Air on the ice cap is much colder and denser than that in the lowlands and there is a steady downhill flow of cold air toward the coast from the interior known as a katabatic wind. Though generally light, the katabatic flow can exceed 100 km/h when channeled by terrain. It is most common on the steep eastern coast where they are known as Piteraq. Downslope winds such as these are warmed slightly and dried by compression and reach the coast as a cold but dry flow that can clear out some of the persistent clouds and bring good eclipse viewing conditions. Though they tend to be most common in the early morning, prediction is difficult, as they are highly variable from place to place.
On the west coast, a southeast Foehn wind often develops bringing warm dry weather as it descends from the mountain peaks. It is the Greenland counterpart of the Chinook of North America. Foehn winds may last for as long as three days and are frequently followed by precipitation. They are more efficient than katabatic winds in clearing out the persistent cloudiness along the coast. A cap cloud on the nearby mountains often heralds these winds.
For the hardiest (and wealthiest) eclipse travelers, the interior ice cap likely offers the best chances for clear eclipse viewing. Observations from the cap are few in number and tend to be more concerned with temperature and wind rather than cloud cover. Automatic satellite measurements of cloudiness are unreliable because of the bright ice background and the cold temperatures of the plateau. A Russian study using ten years of imagery from 1971 to 1980 measured very good sky conditions on the ice cap - 60 to 68% of the observations had clear skies or scattered clouds and only 12 to 18% were overcast. The clouds over the ice cap are also likely to be thinner than that on the coast as the cold air inland is not capable of holding as much moisture as that at lower altitude.
Iceland lies alongside one of the most active weather areas in the Northern Hemisphere. The Icelandic low, about 1000 km to the southwest, is the source of a frequent and endless supply of weather systems that move past the island, each one bringing its own retinue of cloud and precipitation. The Gulf Stream waters that bathe Iceland also warm the air and fill it with moisture. As this warm and humid air encounters the colder water from the Arctic basin west and north, fog and low clouds form and keep gray weather even when an active low is not present.
Southern regions are marginally more promising than those in the north, in large part because of the lower frequency of low cloud and fog. Prevailing winds tend to be from the easterly side, so that there is a modest downslope to the flow at Reykjavik and a consequent drying of the atmosphere. Clear skies are almost unknown, being less than 5% of observations at all stations save one. Because the eclipse occurs very early with the Sun only a few degrees above the northeast horizon, clear or scattered cloudiness is almost mandatory.
Satellite observations show a complex pattern of cloudiness around the island. The largest amounts are in the interior where the higher terrain promotes condensation in the atmosphere. The least cloud is found straddling the south shore, and reaches slightly inland near Keflavik and Eyrarbakki. On the basis of the available data, a location near Reykjavik would be the most promising, though a good eclipse expedition will follow the forecast rather than the climatology.
In spite of its more southerly location, Scotland offers no better prospects for viewing the eclipse than Iceland or Greenland. In spite of this gloomy statement, there is approximately a one-third chance of a sunny morning for the event according to the cloud and sunshine statistics. Aberdeen sees 36% of possible sunshine in May and 34% in June, figures very close to the probability calculated in Table 1.16(which are for the hour of the eclipse).
Once again the main culprit is the series of frequent frontal lows that pass across or north of the British Isles in their eastward migration. Virtually all of these lows will bring precipitation to northern Scotland but spring is the driest season with changeable weather and a chance of a dry spell. Inland areas tend to be cloudier than the coast, in large part because of the higher terrain that promotes the lifting of the moist air masses and their conversion into cloudy weather.
Fog is also common on the Scottish coast, but highly variable from place to place. From the data in Table 1.16, it appears that locations along Moray Firth (Inverness, Lossiemouth) have a relatively low incidence of fog while those exposed to the North Sea (Aberdeen) are much more likely to encounter it. This does not seem to have much of an impact on the probability of seeing the eclipse, for the stations within and near the track are all remarkably alike. On the Scottish mainland the most promising site is at Lossiemouth with a score of 0.34 while the least promising sites have a score of 0.30. These differences are too small to recommend one over another.
While the footprint of the lunar shadow on Earth is very large, the opportunities to view annularity are very limited. In Iceland, the distance from the capital Reykjavik to the opposite end of the island is only about 450 km (though much longer by road). ScotlandŐs annular zone is barely more than 300 km across from the most southerly point to the mainland tip. Both of these distances are short enough to allow ready movement to a clear location on the basis of forecasts on the day before the eclipse. In Scotland, Inverness would seem to be a convenient staging point, and it has a small weather advantage to boot. In Iceland the same benefits can be extended to Reykjavik.
Rapid travel from site to site in Greenland is all but impossible unless by aircraft, and so the site selection must be on the basis of the climatology. The available evidence points to the interior ice cap as the best site by far on the entire track, but cost and opportunity will limit access there. Elsewhere in Greenland, the west coast holds more promise than the east, which dictates a site at Christianshab or Godhavn, the two largest communities within the annular zone. These sites are close to the observation sites of Egdesminde and Jacobshavn in Table 1.16. The Sun does not set on May 31 and so the lucky eclipse observer could be treated to a midnight sun in annular eclipse. A sequence of solar images with an eclipse just 2° above the north horizon would surely be one of the great eclipse photographs - except, perhaps, for those that can be obtained in Antarctica just six months later.
Adapted from:
"Annular and Total Solar Eclipses of 2003" (NASA/TP-2002-211618).Permission is freely granted to reproduce this information and data when accompanied by an acknowledgment of the source:
"From Annular and Total Solar Eclipses of 2003 by Fred Espenak and Jay Anderson, NASA"
WebMaster: Fred Espenak Planetary Systems Branch - Code 693 e-mail: espenak@gsfc.nasa.gov NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771 USA
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