|
|---|
|
|
|---|
Index to Weather Prospects to 2003 Total Eclipse
- 2.10 - Introduction to Weather Prospects for November 23
- 2.11 - Weather Patterns
- 2.12 - Coast of Queen Mary Land - Mirny
- 2.13 - Interior of Antarctica
- 2.14 - Coast of Queen Maud Land - Maitri, Neumayer, and Novolazarevskaja
- 2.15 - Selecting a Site
- 2.16 - Weather Web Sites for November 23 Total Eclipse
- Table 2.11 - Antarctic Station Climatology for the Total Solar Eclipse of 2003 Nov 23
- Key to Table 2.11
This will probably be the first eclipse to be systematically observed from Antarctica - the coldest, windiest, highest and driest place on the Earth. The meteorology is simple because it is dominated by one factor: cold. Cold air forms in the center of the Antarctic Plateau, close to the south pole, and flows outward to the margins of the continent. The flow is primarily katabatic, driven by gravity as the heavy frigid air moves downhill toward the sea. Where the terrain funnels and confines this steady flow, wind speeds reach exceptional values, among the strongest ever recorded.
Weather stations are isolated and mostly confined to the shores. Inland, automatic stations measure a small selection of the possible variables: wind, temperature, pressure but not cloud cover, humidity, visibility or precipitation. It doesn´t matter much - in the cold air absolute humidities are always low and precipitation is sparse. Data are often missing and frequently suspect. The length of the climatological record is short so that the statistics represent more “what is possible or typical” rather than give a serious account of the average conditions.
On the coast and at a very few inland locations, humans supplement the machine measurements, and the full suite of observations is available. For the eclipse observer, cloud cover is usually paramount, but in the extremes of the White Continent, temperature, wind and wind chill demand the attention of eclipse planners far more than in any other location in the world. While Mongolia and Siberia offered the possibility of extreme cold in the 1997 eclipse, Antarctica in 2003 virtually guarantees it.
The eclipse track is not forgiving, and there are only four venues from which it can be realistically observed. Two of these are at coastal locations, one near the Russian base at Mirny and the other near the western limit of the eclipse where a collection of German, Indian, Japanese and Russian bases congregate. The third site is “inland”, at whatever point adventure and finances can take an eclipse expedition. The fourth possibility is aboard an icebreaker at sea or near the coast along the sunrise portion of the path.
Antarctic winds flow outward from a cold high pressure system overlying the continent, turning gradually toward the west as they approach the coast. Descending from the 3000 meter mid-continent plateau, the air is dried even farther, bringing bright sunny skies to the continental margins. In some regions the winds are exceptionally strong - over 100 km/h - as the local topography squeezes and redirects them. Blizzards and blowing snow are common in such circumstances.
As the cold outflow moves over the relatively warm waters surrounding the continent, the air is rapidly saturated with moisture. Clouds form quickly bringing snow showers to the coastal regions and offshore ice fields. The wind shear along the edges of the cold air outbreaks and the strong temperature contrast between air and water helps to create a continuous supply of small scale lows along the coast.
Farther offshore, the southeast winds flowing off of the continent gradually abate and the westerly global flow asserts itself. This change in wind direction creates a zone of intense low-pressure development. The lows that form tend to travel east and southward, gradually moving toward the Antarctic coast where they weaken and die. Some stronger lows can penetrate into the continent, but these are rare, though the lack of observation and measurement makes the exact frequency of such systems uncertain. Even though the low itself may be confined to coastal waters, the higher clouds from such systems can penetrate well inland.
Lows moving along the coast tend to bring onshore winds as they approach and offshore southerlies as they depart. The first pushes moist air against the land and brings extensive stratiform cloudiness and continuous precipitation - either rain or snow - while the latter reinforces the normal cold outflow with its sunny skies from the continent margins.
The interior of the continent offers a considerably more relaxed climatology, notwithstanding the low temperatures. Water vapor is in short supply and weather systems from the north have difficulty scaling the steep slopes of the coast. Unfortunately observations of cloud cover are almost non-existent in central Antarctica, and satellite observations are limited by the lack of contrast between the icy surface and the clouds and the long polar nights. The Russian base at Vostok is the only one in the vicinity of the eclipse track that can provide cloud data but their statistics are probably representative of the conditions over much of the mid-continent portion of the track.
Temperatures along the coast are relatively warm and have the potential to rise above zero, but those inland are much lower. The coldest temperatures are a result of the long Antarctic nights, low humidity, high albedo, low cloud cover and high altitude of the interior that allows the surface to radiate energy into space with little interruption. The warmest temperatures that are found in the interior -18° C at Vostok) are a result of advection from coastal regions. On the eclipse track the coldest temperatures tend to be found near the Australian automatic weather stations LGB20 and LGB35. Fortunately such temperatures come with much better prospects for clear skies. Since warm temperatures on the coast are associated with cloudy weather, falling temperatures will be preferred on eclipse day as these bring in the drier air from the interior.
Though winds inland are about half the strength of those on the coast, the lower temperatures in the interior dominate the calculation of wind chill to bring values which can reach into the minus sixties. Frostbite can occur in less than four minutes in such weather. While the actual temperatures are considerably warmer, observers will probably want to limit their outdoor exposure while waiting for the lunar shadow, or at least make sure they have some shelter from the wind.
Mirny is the main Russian base in the Antarctic, situated on the coast of Queen Mary Land on a small peninsula that juts northward into the Davis Sea. Established in 1956, the station has the distinction of being best placed within the eclipse track, for the central line lies only a few tens of kilometers to the east. The station buildings, a collection of two-story rectangular blocks topped by various instrument domes, lies on four rocky outcrops about 20 meters above sea level. South of the station, the land rises to a height of 1500 meters within 100 km. A few small rocky islands lie offshore, but land-fast ice is observable much of the year. The 24-hour polar day begins on December 10 at Mirny, and so eclipse day will be one with long daylight and a short twilight.
Mirny´s weather is governed by strong downslope winds and a high frequency of offshore low pressure systems. While the blizzard season is receding in November, winds at eclipse time average a brisk 41 km/h (Table 2.11) with the highest reported value in the past 15 years reaching over 110 km/h. Temperatures are modest by Antarctic standards, with an average of -7° C and a range from minus 20 to the freezing point at eclipse time. The high winds bring a significant wind chill to the station however, and eclipse observers will have to dress accordingly. By Canadian and north European winter standards, the average eclipse-day conditions at Mirny appear to be relatively tolerable though the wind speeds are higher than is typical of the EarthÕs northern hemisphere.
Cloud cover at Mirny averages 6.4 tenths, a large value when compared with opportunities at past eclipses but comparable to the other coastal stations on and near the track. The distribution of cloudiness is a typical U-shape ( Figure 2.5), with a high frequencies of low and high cloudiness and smaller frequencies of intervening values. If we make the assumption that the probability of seeing the eclipse is proportional to the frequency of the various levels of cloud cover, then Mirny offers a 36% chance of a successful expedition. This is slightly optimistic as no account has been taken of the low 13° altitude of the Sun.
Low-pressure disturbances moving toward the station can bring heavy clouds and precipitation for several days in a row but November is approaching the driest months of the year and these are relatively uncommon. Eclipse observers will be looking for southerly katabatic winds blowing from the high plateau, for such winds bring dry cold air and clear skies. Such winds are more frequent in the evening and overnight, which favors the sunset portion of the eclipse track rather than the morning portion at Mirny. The wind speed is critical however, for even under clear skies, blizzard conditions can develop if winds are too strong - anything above 35 km/h brings poor visibility and blowing snow. Heavy snow is rare during the summer months from November to February. Fog and ice crystal haze are rare at any time.
While human observations are in limited supply, several automatic weather stations lie within the track of the eclipse and can provide temperature and wind information. These stations, LGB20, LGB35 and LGB59 (Table 2.11) show a considerably colder climate than the coastal regions. Though winds are frequently lighter on the interior plateau, wind chill values are colder because of the lower initial temperature. Observing from distant inland sites along the eclipse track will require special care for equipment and people.
The interior stations have a short period of record - from 4 to 10 years - and the climatology of the interior is only poorly revealed. For the three Australian automatic stations, average temperatures at eclipse time ranged from -28°C to -33°C. Extreme temperatures dropped as low as -41.5°C at LGB20. Wind chill values are 15 to 20 degrees colder on average, reaching below -60°C at LGB20. At LGB20 and LGB35, wind speeds are relatively low, about half of those at LGB59. The explanation likely lies in the configuration of the surrounding terrain. The prevailing wind direction at LGB59 is also at odds with most of the other interior stations, being northerly rather than the more typical southerly outflow.
For cloud cover statistics we are forced to rely on the data collected at Vostok, which lies considerably farther inland and at a higher altitude. Here we see a complete reversal of the cloud patterns of the coast, with clear skies being the most frequent report. The mean cloudiness of 3.4 tenths is nearly half that of Mirny, and the probability of seeing the eclipse rises to 66%. While inland sites on the track are probably not quite this favorable, the implications cannot be ignored - for this eclipse, inland areas are most likely to see the event by a considerable margin.
The impact of frontal cloudiness and weather systems is muted by distance from the oceans and the high altitude of the interior. Most of the clouds are at mid and high levels where, being composed entirely of ice crystals, they are frequently semi-transparent. This implies that the probability of seeing the eclipse is greater than the formal calculation (66%) would suggest.
The atmospheric transparency of inland Antarctica is legendary. The air is unpolluted except for occasional water vapor and ice crystals. Snowstorms are rare and tend to be at a minimum in the spring. During calm and clear weather, ice crystals are frequent, bringing a hazy sky and occasional ice fog. Both of these phenomena are more likely in winter than summer but their main effect on the eclipse will be to produce a halo around the sun. Drifting snow is common, occurring on nearly one third of the days at Vostok.
The eclipse track comes to an end just beyond Antarctica's coast and the solar altitude is very low for observation sites in this area. The Russian base Novolazarevskaja and the near-by Indian base at Maitri are the two stations within the path, and von Neumayer (Germany) lies just to the west. Maitri and Novolazarevskaja, four kilometers apart, are situated on the rocky ground of the Schirmacher oasis, a group of low-lying hills interspersed with many glacial lakes. Neumayer lies on a flat ice shelf in Atka Bay, buried beneath the snow and ice. These stations are 80 to 100 kilometers from open water because of the presence of the ice shelf at the foot of the continent.
Temperature, wind chill and wind statistics for the sunset stations are similar to those at Mirny but cloud statistics are more variable. Neumayer is one of the grayest locations on the track with an average cloudiness of 7.6 tenths. Novolazarevskaja averages 5.7 tenths and Maitri only 3.9 tenths, a rather discordant amount given their proximity to the others. The reason for the much lower cloud cover at Maitri may be due to local conditions that favor downslope winds or to the short period of record that just happens to lie in a relatively sunny time. A comparison of hourly reports from Maitri with those at Novolazarevskaja shows a few large differences but a near-zero bias, so the second explanation would seem to be the most reasonable. The cloud climatology of the Russian base with its longer period of record seems to be more reliable for planning purposes.
With this in mind, cloudiness patterns along the eclipse sunset coast are similar to those at Mirny ( Figure 2.5). Most of the cloud is at mid and high levels, a factor that does not augur well for such a low altitude eclipse. The formal probability of seeing the eclipse is calculated to be between 27 and 52 percent, with the 44% at Novolazarevskaja being the most representative figure. These must be regarded as too large in view of the very low angle toward the eclipse.
Antarctic eclipse adventurers are likely to have little choice in their selection of an observing site, as most travel will be to existing bases. Most sites offer some alternative locations relatively nearby: at Novolazarevskaja the airport is 15 km inland from the base, and most other sites have inland and upland scientific bases. Where movement is possible, it is best to look for a spot where the southeast katabatic winds are most favored, in the northwest lee of a plateau for instance.
In interior regions, site selection will be more versatile, as almost all access will be by plane. Flat vistas where the winds are lightest should be easy to locate - a high spot might even be best, as winds will tend to flow downhill and away. Satellite imagery (most big bases have it) can be used to predict cloud motion and help select a site with a high prospect of clear skies.
The interior offers the best chances and the biggest challenges. Prospects for clear skies are twice those of the coast, depending on the distance and altitude at the site. Coastal regions offer more comfort and easier access, but at the expense of a high cloud frequency. Mirny seems to be the best lowland site, in large part because of the higher Sun. Observers there should attempt to move as far inland as possible to take advantage of the better weather away from the coast.
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" Special thanks to National Space Club summer intern Lauren Williams for her valuable assistance in preparing this web page. (July 2003)
Eclipse Predictions & WebMaster: Fred Espenak Planetary Systems Branch - Code 693 e-mail: espenak@gsfc.nasa.gov NASA's Goddard Space Flight Center, Greenbelt, Maryland 20771 USA
|
|---|
