Information about lake surface temperature can be found on the webpages of the Finnish Environment Institute
The easiest way to convert different temperature units is to use the Unit Converter on FMI website.
In order for you to convert Celsius temperatures to Fahrenheit multiply the wanted temperature by nine and then divide that number with five and then add 32.
To convert Fahrenheit to Celsius deduct 32 from the given temperature, divide that number by nine and then multiply the outcome by five.
The Finnish Meteorological Institute uses Celsius scale instead of Fahrenheit. World Meteorological Organization, WMO for short, has agreed on a standard for temperature measurement which is Celsius. Celsius scale is used globally by other meteorological institutes and organisations which enables world wide exchange of information.
A general overview of the weather in Finland can be found on the Climate in Finland page. At that page, there is information concerning the monthly climate statistics for Helsinki and Sodankylä.
There is also specific information on the distinctive Finnish seasons.