How to detect clear sky instants
How to detect clear sky instants 
... in time series of solar radiation values. The method described here and exploited by the SoDa team is the one of Lefèvre et al. (2013), which has originally been suited for 1 min BSRN stations. For other data sets or other time steps, please take the greatest care in adapting this algorithm.
Recipe of Lefèvre et al. (2013)
Ingredients:
 DHI: Diffuse Horizontal Irradiation
 GHI: Global Horizontal Irradiation
 TOA: Irradiation over a horizontal plane at the Top of Atmosphere
Assumptions
1 min GHI and DHI data, like the one available via BSRN stations
Preparation
 Filter 1: for each 1 min BSRN data, only keep instants when DHI / GHI < 0.3

Filter 2:
 Step 1: take a temporal window of [t  90 min; t + 90 min], and only keep instants when at least 30% of the 1 min data pass Filter 1

Step 2: take a temporal window of [t  90 min; t + 90 min], and compute Kt' as:
KT′ = KT / [1.031 exp( −1.4/(0.9 + 9.4/m)) + 0.1]
with m the air mass computed using the following expression (Kasten and Young (1989)):
m(Θ_{S}) = (p/p0)/ [cos(Θ_{S}) + 0.50572 (Θ_{S} + 6.07995) − 1.6364]
with Θ_{S} the solar zenithal angle expressed in degrees.
and only keep Kt' when standard deviation of Kt' in this temporal window is below 0.02
Extract from Lefèvre et al. 2013, pp 24082409
[...Two filters have been applied on the remaining BSRN data in order to retain reliable clearsky instants: The first one was a constraint on the amount of diffuse irradiance with respect to the global irradiance since the direct irradiance is prominent in the case of clear sky. Only those minutes for which E^{diff} / E^{glo} < 0.3, i.e. when the diffuse component is much less than the direct one, have been retained.
The second filter dealt with the temporal variability of the irradiance. If there is no cloud, the sky should be clear for a long period. Checking this would avoid cases of broken clouds or noticeable spatial heterogeneity around the site if ergodicity is assumed. The first step of this filter was to retain only periods with enough measurements that have passed the first filter. A given instant t , expressed in min, was kept only if at least 30 % of the 1 min observations in both intervals [ t − 90, t ] and [ t , t + 90] have been retained after the first filter. A corrected clearness index, KT ′ (Ineichen and Perez, 1999) is computed for this instant:
KT′ = KT / [1.031 exp (− 1.4/(0.9 + 9.4/m)) + 0.1 ] (10)
... where m is the air mass defined by Kasten and Young (1989):
m(Θ_{S}) = (p/p0)/ [cos(Θ_{S}) + 0.50572 (Θ_{S} + 6.07995) − 1.6364] (11)
... where Θ_{S} is expressed in degree, and p and p0 are respectively the pressure at the site under consideration and that at sea level.
An instant was considered clear if the standard deviation of KT′ in the interval [ t − 90, t + 90] was less than a threshold, set empirically to 0.02. Only these 1 min clearsky instants were retained for the validation. Figure 3 displays a time series of E^{glo} for the selected clearsky instants in the year 2005 in Payerne, in black circles. Payerne experiences a large number of clear skies during the year and many instants are selected. In this graph are also drawn the clearsky instants selected by the algorithm of Long and Ackerman (2000), in lightgrey crosses. One may see that they are differences between the results. The proposed algorithm presents less low values of E^{glo} than that of Long and Ackerman and offers more confidence in the fact that the instant is clear...]
Bibliography about clearsky instant detection
Kasten and Young 1989  Kasten, F. and Young, A. T.: Revised optical air mass tables and approximation formula, Appl. Optics, 28, 4735–4738, 1989  Air mass 
Content: Compute air mass necessary for instance to detect clear sky instants in the method proposed by Lefèvre et al. 2013 about McClear validation
Lefèvre et al. 2013  Lefèvre M., A. Oumbe, P. Blanc, B. Espinar, B. Gschwind, Z. Qu, L. Wald, M. SchroedterHomscheidt, C. HoyerKlick, A. Arola, A. Benedetti, J. W. Kaiser, and J.J. Morcrette, 2013. "McClear: a new model estimating downwelling solar radiation at ground level in clearsky conditions", Atmos. Meas. Tech., 6, 24032418, doi:10.5194/amt624032013.  McClear 
Long and Ackerman 2000  Long, C. N. and Ackerman, T. P., 2000. "Identification of clear skies from broadband pyranometer measurements and calculation of down welling shortwave cloud effects", J. Geophys. Res., 105, 15609, doi: 10.1029/2000JD900077.  Detection of clear sky instants 