Mean cloud cover, derived from satellite imagery, is close to 50% for north central Mongolia with a little less cloud to the west over the Altai Mountains and a little more along the Russian border and the area east of Lake Baikal (Figure 7). Statistics from the U. S. Air Force Worldwide Airfield Summaries (Table 14) show that the Ulaanbaatar area has an average of 14 days in March with sunny skies (less than 3/10ths cloud) and good visibilities. Depending on the type of cloud, eclipse observations might be possible with up to half the sky covered. Sunshine statistics for stations along the eclipse track typically show 55% to 65% of the maximum possible hours. All of these factors suggest that the probability of seeing the eclipse in Mongolia is a little above 60%.
Thanks to the very strong anticyclonic center guarding the westerly approaches to Mongolia and the Himalayas to the south, low pressure systems are infrequent over the eclipse track. Instead, cloud cover which does reach the area generally arrives with upper level fronts attached to lows passing well to the north. These warm air disturbances are lifted off the ground, unable to dislodge the cold air masses at the surface. Cloud cover tends to be higher based altos and cirrus types rather than low level stratus and fog. The warmer temperatures which accompany this cloudiness may raise the mercury to the -20° C mark in midwinter, a phenomenon known as a Tuva thaw for that part of Russia at the north limit of the eclipse!
Stratus and stratocumulus clouds form over the Ulaanbaatar area infrequently during the spring months, with only 9% of the hourly reports indicating this type of cloud. It is found more frequently in the valleys of the Altai Mountains where 22% to 26% of reports mention it. However, when stratus types are reported, they tend to be relatively heavy, covering half to two-thirds of the sky.
Mid-level altostratus and altocumulus clouds are reported in 38% of the observations from the capital and up to 50% over the Altai Mountains. They also tend to bring relatively heavy cover, similar to the lower clouds. Cirrus level clouds have similar frequencies and amounts, as they usually accompany the mid level clouds when weather systems are passing overhead. Watch for cloud approaching from the north and northwest on eclipse day, as this is the prevailing wind direction aloft. If a rare low pressure system is moving toward the eclipse track, keep a weather eye on the west.
Completely clear skies are reported about 20% of the time at Ulaanbaatar and about 5% less over the Altai Mountains. With spring sunshine, mountain valleys warm very quickly and chinook winds often develop which can fill the air with sand and dust. This season is just beginning at eclipse time.
Table of Contents