Actually seems like these are scandinavian things, I thought they would be more commonly used...
Thermal seasons, copy from
http://en.wikipedia.org/wiki/Season
"
In Sweden and Finland, meteorologists use a definition for the seasons based on the temperature. Spring begins when the daily averaged temperature permanently rises above 0°C, summer begins when the temperature permanently rises above +10°C, summer ends when the temperature permanently falls below +10°C and winter begins when the temperature permanently falls below 0°C. "Permanently" here means that the daily averaged temperature has remained above or below the limit for seven consecutive days. This implies two things: first, the seasons do not begin at fixed dates but must be determined by observation and are known only after the fact; and second, a new season begins at different dates in different parts of the country.
"
Thermal sum, I did not even find anything in English wikipedia:
http://fi.wikipedia.org/wiki/Tehoisa_l% ... C3%B6summa
But in short it is calculated by summing daily average temperature minus 5 degrees, from all days where average temperature is over 5 degrees (the formula is shown in the Finnish wiki page. Different crops/plants require different heat sums before they can be harvested, and that is why some institutes in Finland calculate those values.
And after more searches there seems to be an english term Growing-degree day:
http://en.wikipedia.org/wiki/Growing_degree_day which describes the same thing. Looks like in generic form this formula has also other 'base values' which are subtracted from the average temperature than the 5 degrees which is most normal in Finland (most likely due to most normal farming crops, the length of the season where average temperature is over 10 degrees is just too short).
I'll check the tracker, and try to add my wishes there!
Jani