Figure 4

Components \(v_{t}\) of the first three normalized principal bases. Unlike the first principal basis showing more or less the same contributions of time windows (red crosses), the second principal basis differentiates night time from the daytime (green squares). Furthermore, removing the first and second variances, one observes high contributions of the evening time (blue circles). Here the time window t (\(=0,1,\ldots , 23\)) is expressed in 24-hour notation; The double-headed arrows designate the night, day, and evening time slots labeled by \(\tau =0,1,2\), respectively