The “thermal bar”, well known to Lake Ontario salmon fishermen, is an interesting phenomena found on a hand full of large lakes in temperate climates. Wikipedia says it has been studied in Switzerland, in Russia on Lake Baikal and here on several of the Great Lakes.
Each spring creeks and rivers warm up quickly compared to the mass of deep water in the lake. As their discharges pour in a band of warmer water accumulates along the shore. At the same time the strong sunlight of April and May begins warming the lake's shallows. Soon a strip of relatively 'warm' water of perhaps 50 to 55 degrees is separated from the 39 degree cold dense water of the open lake by a sharp boundary, the “thermal bar” that runs the whole length of the lake shore. As spring moves on the surface of the lake keeps warming and the boundary between warm and cold remains, but the band of warmer near shore water widens. Both sides of the lake warm up and eventually a pool of cold surface water remains in the middle. It shrinks until perhaps around mid June it finally disappears.
I know from a few June cruises, if there are several warm calm days, stratification can occur quite quickly. A week ago you were shivering in three sweaters heading for Canada, while now heading back home a long sleeved shirt is sufficient as you reach along in a nice westerly.
Sea Grant reports that the sharp boundary between warm and cold water actually features a current of sinking water, that further serves as a physical barrier to separate the inshore and offshore waters. This is interesting to science, fish, zooplankton, and water drinkers alike because the stratified near shore water tends to trap and concentrate nutrients and pollutants. For this reason the fishery and public health people and the folks that run water and sewer plants along the south shore like to keep informed as to its extent. It can also influence the distribution of toxic blue green algae blooms by concentrating phosphates from sewage and farm field run off.
One study of Lake Superior states that perhaps 70 % of the total dissolved solids ( and associated pollutants) enter the lake during spring run off. If it's all concentrated inshore the implications for water quality are obvious. Diatoms and zooplankton tend to concentrate along the thermal bar as do bait fish and ultimately all those hungry big silver salmon.
Though I have no science to back it up, it appears that the thermal bar influences the wind. More than once I've seen a sudden shift in wind speed and or direction perhaps from brisk southerly to a lighter northeaster along with the temperature drop after hitting the cold dense inversion offshore while crossing in June. I suspect the cold dense air over the early summer lake might even completely block southerlies. ( A hot air balloonist website states winds are often light and may be from a completely different direction within an inversion.)That cold offshore water would also seem to be a likely fog generator and June can be a foggy month on the lake.
Lake Ontario usually stratifies with a warm layer of surface water from about mid June through September. Then as the sun heads south, the lake loses heat and eventually cools enough to begin mixing again. Ultimately if it's a tough winter, it stratifies in winter with 32 degree water and ice cover. And we head for southern California while the boats hibernate.
in the photos note the crew at the dock in shirt sleeves and on the lake with jackets where the air temp is at least 25 degrees cooler!