NASA MODIS Image of the Day: September 23, 2011 – Fog on Lake Baikal, eastern Russia
The “Blue Eye of Siberia”, Lake Baikal, was shrouded by a large, low cover of fog on a sunny autumn afternoon in late September, 2011.
The fog bank extends along the entire length of the vast lake (about 636 kilometers/395 miles) and spans the entire width (79 kilometers/49 miles).
The fog is so dense that Baikal’s famous deep blue water can only be visualized in a few isolated locations off the eastern bank. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite captured this true-color image on September 22, 2011 at 1:30 p.m. local (Irkutsk) time, long after morning fog would normally be expected to have burned off lake waters. Fogs are frequent over the waters of Lake Baikal, and can appear any time of the year. The lake is known for heavy and persistent fogs in the early summer and in the fall. In the early summer, the land and the air surrounding the lake warms more rapidly than the frigid lake waters. As the warmer summer air blows across the chilly lake surface, condensation occurs and creates fogs. In the autumn, the opposite occurs. The lake water takes all summer to warm, but it is slow to cool. When the chills of the impending winter come, the air temperature often plummets quickly, but the water resists change. As the cold, dry air moves over the warm water, evaporation occurs. Soon the humidity over the lake nears 100%, and fog forms. In more moderate climates the arrival of bright sunshine can warm crisp fall air nearer to the slightly warmer water temperatures. With solar heating, air temperature climbs near to the lake temperature, and evaporation ceases, so the morning fog “burns off” the lake, leaving a clear sky behind. In the case of Lake Baikal, which is the largest lake in the world, the volume of water is so large (it is estimated at over 23,600 km3 or about 5,700 cu mi) that once it finally warms near the end of summer, it will retain the warm for a long time – it takes many weeks of very cold weather to drop the lake temperature the slightest bit. In contrast, air temperatures drop sharply and steeply in a matter of a week or two as the Siberian winter quickly approaches. When the temperature differential between lake and air is both large and persistent, even a bright, sunny day may not allow enough solar heating to close the gap and stop evaporation over the lake. At such times, fog can form even in the middle of a sunny afternoon.
