Summer Stratification - Thermal stratification
Thermal stratification is fairly pronounced during the summer seasons in most lakes of the temperate (cold) regions but is rare in lakes of tropical (hot) and subtropical regions where it occurs only in very deep lakes. This is so, because the rate of mixing of layers is very fast in case of tropical lakes whereas, the temperate lakes retain well defined layers showing different temperature. These layers do not mix rapidly. Therefore the temperate lakes exhibit clear stratification with respect to temperature. Let us understand how thermal stratification develops in water bodies and why it is maximum during the summer seasons.
In lakes the top one metre of the water surface directly absorbs around 90 per cent of the total solar radiation falling on it and is considerably more heated in the process. Consequently, the lower sub-surface layers receive progressively less radiation and remain relatively cool. Thus, the lake becomes thermally stratified, with its water forming layers due to temperature differences or thermal gradients. Thermal stratification is maximum during the summer season, primarily due to two reasons. Firstly, due to the fact that solar intensity increases during this period and it heats the surface layer greatly while the lower layers remain comparatively cool. Secondly, the thermally stratified layers offer resistance to mixing by wind. The fairly pronounced stratification of lakes developed in summer is called summer stratification or stagnation.