My family and I went for a swim in Lake Merrasjärvi here in Lahti yesterday evening at about six. The air temperature was about 28-29 C. There was an official announcement at the beach dated on July 17 from the municipal health inspector that had some water quality data in it and also the surface temperature measured on that date. It was unexpectedly high: 25.8 C. There has been nothing but hot weather (occasionally windy though) since then. I suspect the surface temperature was about the same. But there was an important difference. Last week, you could clearly feel there was a colder layer of water a under the surface layer. Yesterday, the water was very warm all the way down to about 1,5 metres.