Fig. 2

a Scatter plot of individual principal component coefficients PC1 and PC2 obtained for daily frequency–time spectrograms. The colors of points distinguish individual Kannuslehto campaigns—blue points correspond to the 2016/2017 campaign, brown denote the 2017/2018 campaign, green indicate the 2018/2019 campaign, and red points were measured during the 2019/2020 campaign. b–e Frequency–time spectrograms obtained using the first two principal component coefficients. Their coordinates are plotted in a by orange crosses and correspond to b [-200, 130], c [350, 110], d [-200,-90], e [200,-130]