We follow the Vaakapesäseuranta site which tracks the weight gain of a few selected hives around the country. We keep our eyes on the one in Lohja which is not so far from our apiary. The graph shows kg weight gain (set at zero at the beginning of the season) and the date. Readings are taken once or twice a week.
The comments below the graph are all in Finnish but if anything interesting happens I'll post a translation.
As you can see from the graph, the hive was losing weight during the first two weeks of May, due to the cold spell (night frosts) we were having. We were actually feeding our own hives during that cold spell. It is not unusual to lose a whole hive through starvation if there is a late cold spell, especially if that delays the flowering of something like dandelions (which was the case this year).
Today was warm and nice, well over 20C. So, the next time they weigh the hive, we should see the line go up again.