Eclipse Climatology: Prospects for the 2012 Annular
China, Taiwan, Japan, and the USA Overview May is not a good time to start an eclipse in Asia, but it’s a very good time to end one in North America. In Asia, the cloudy monsoon season is underway, while in Arizona, near the end of the track, the midsummer monsoon is still more than a month away. On the Great Plains, the thunderstorm season is peaking, but the shadow track barely reaches the convective beltway in southern Texas before sunset brings the eclipse to an end. With beginning and middle of the shadow track embedded in regions with a high frequency of cloud, (Figure 1) this eclipse is tailor-made for U.S. observers. Figure 1: Average May cloud amount based on satellite observations from 1982-2008. Source: NOAA. |
China By May 21, the annual southeast monsoon is entrenched over southern China, marked by high humidity, heavy cloudiness, and frequent rainfall. At Guangzhou, the first major city along the shadow track, morning cloudiness averages 78 percent and daytime sunshine is a meagre 31 percent of the maximum possible (Table 1). Fortunately, the eclipse track straddles the coast for the whole of its passage across China and weather prospects along the shores of the South China Sea are markedly better than those inland. Hong Kong, which lies 50 to 80 km from the south limit of the shadow, has an average cloudiness of 65 percent at eclipse time (Figure 2), though only 34 percent of hours in May are sunny (Table 1). Table 1: Climate statistics for selected locations along the eclipse track in China |
Inland China is a hazy place in the morning light and views toward the rising sun are occasionally heavily obscured. The more transparent view across the South China Sea from Hong Kong probably offers a better view of the eclipse if the weather cooperates. Morning thunderstorms are common in the city in May, but even so, the cloud prospects are better than those farther inland. Hong Kong will be a prime site from which to observe the ring of sunlight. The peaks enveloping the city will offer a spectacular vantage point from which to watch the spectacle. At the airport, the annular phase lasts for just under 3½ minutes, about 1 minute less than on the center line near Guangzhou. Japan Japan’s wet weather peaks in June and July, and the approaching rainy season is well underway on the date of the eclipse. Monsoon air is still well to the south however, and so cloud conditions are a little more promising – about 10 percent lower – than those over China. Climatological averages of cloud cover are fairly uniform all along the Japanese portion of the track, between 60 and 70 percent (Table 2), but there are a few locations where the statistics are a little more encouraging. The best of these is near Osaka, at Kansai International Airport, which reports an average cloudiness of 61 percent and a 44 percent frequency of sunny hours. Kansai is well off of the center of the track and so it is a delicate question whether a 6- or 8-percent advantage in climatology is worth the loss of time in annularity. Movement in response to a weather forecast is more than likely the best option rather than obeisance to past measurements of cloudiness. Table 2: Climate statistics for selected locations along the eclipse track in Japan |
The USA As far as cloud cover is concerned, this eclipse belongs to the United States, though viewing locations have to be picked carefully. Where China and Japan can offer no better than a 60 percent average cloud cover, values below 10 percent can be found in several places between California and Texas (Table 3). It’s not a slam dunk however, as eclipse watchers will have to watch for cloud along the coast and over the higher mountain peaks. Satellite-based cloud observations are not so optimistic, but cloud amounts below 45 percent can be found in California, Arizona, and New Mexico. Mountains have two roles to play in cloud production. By forcing air upward on the windward side, mountain barriers lift the air to saturation, manufacturing mid and high-level clouds that then flow eastward to torment observers on the downwind side. Depending on the situation, the topographically induced clouds can extend hundreds of kilometres downwind or dissipate only a short distance east of the high ground as the air descends and dries. In most cases, the thickness and extend of cloudiness in the mountains waxes and wanes with the presence of upper-level disturbances moving in from the Pacific. Figure 2: Average May cloud amount based on observations from the afternoon pass of polar-orbiting satellites beween 1982 and 2008. The nominal time for these data is 2 p.m. The 0.50 contour is enhanced. Data: NOAA/CIMMS/UW-Madison |
The second cloud-promoting process comes from the heating of the mountain slopes by the Sun under clear or mostly clear skies. Mountains in the southwest and west tend to be heavily treed, and so are darker than surrounding light-coloured deserts and plains. The dark areas absorb more solar energy and the ensuing heating builds convective clouds – particularly thunderstorms – over and along the higher terrain. Fortunately, the Arizona monsoon, a season of moist air flows from the Gulf of California, does not arrive until early July and cloud buildups are uncommon, especially on the east side of the Rocky Mountain chain. Most of the cloud problems, if there are any, will come from the passing upper disturbances. For the eager eclipse seeker, the shadow’s centreline make landfall on the Pacific coast of California and Oregon between Crescent City and Eureka, and shortly thereafter crosses the Klamath Mountains, the first of a series of topographic barriers. The exposure to moist Pacific winds brings abundant rainfall to the coast, but even so, the percentage of sunny hours is higher than anywhere on the other side of the Pacific in Japan and China. Crescent City has a well-deserved reputation for fog, a consequence of cold offshore waters, but it’s easily avoided by moving a short distance inland. Cloud cover remains relatively high across the Klamath Mountains, but then drops by almost half as the track drops into the Sacramento Valley and the city of Redding. Figure 3: Graph of average May cloud cover at stations along the track of the eclipse. |
The continuing ups and downs of average cloud cover shown in the cloud cover map (Figure 2) and the graph in Figure 3 reflect the influences of terrain more than any other factor. The general downward trend in cloudiness from California to Arizona and New Mexico is a result of the blocking of Pacific Ocean moisture by the mountains. Within this general trend (Table 3) are features associated with more localized topography: lower cloud in the Sacramento Valley (Redding, Red Bluff and Alturas); east of the Arizona’s Kaibab Plateau (at Page); and over the Llano Estacado (Roswell and Clovis). At Saint George, Utah, average cloudiness is under 10 percent while Cedar City, to the north, it is four times greater. St. George lies on the northern edge of the Mojave Desert, while Cedar City, 80 km (50 miles) distant, lies against and on the wrong side of the 3000-metre (10,000 feet) Markagunt Plateau.
Table 3: Climate statistics for selected locations along the eclipse track in the USA |
By the time that the eclipse shadow reaches Arizona and New Mexico, the Sun is low in the sky and there is a risk that the eclipse will be hidden by high ground before the critical 2nd contact arrives. While it is relatively easy to avoid this fate (it is, after all, a sign of very bad planning), eclipse camps should also be selected to avoid any clouds that form above the mountains. The best solution will be the east side of a flat low-altitude valley or plateau with the mountains low down on the west horizon. There are many choices available. For those eclipsophile who wants to view the solar annulus on the sunset horizon, the only possible destination is the end of the track over Texas between Lubbock and Abilene. This is thunderstorm country, particularly in May, as the region is exposed to the Gulf of Mexico moisture that flows northward onto the Great Plains. The dry desert air from New Mexico ebbs and flows through this part of Texas depending on the daily winds and weather, but the overall cloudiness, at 30 to 40 percent, is still relatively low. The spectacle of a sunset annular will make some chasers willing to gamble on the storms, which can provide an alternative chase opportunity if the eclipse is clouded out. Ultimately, there is no excuse for missing this eclipse anywhere across the western US for an observer who is willing to plan and travel. Reliable weather forecasts are readily available for a week or more ahead of the eclipse, the transportation network is excellent, and in some parts, the climatological portents are among the best the world can offer. After being short-changed on eclipse tracks for the past 20 years, the U.S. finally has an easy chance to see one without crossing an ocean. |
,当Kp指数达到6或以上时，我国境内有可能观察到极光；当前极光活动水平
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级，当地磁暴级别达到G2或以上时，我国境内有可能见极光
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