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Results and Discussion

Temporal Variations in TCC

Figure 2 shows temporal variations in the all India averaged mean annual and seasonal TCC for the period 1951-2010. Mean annual TCC for India is 43.5 %, which is showing a significantly decreasing trend at the rate of -0.44 % per decade. Figure 2a suggests that annual mean TCC has been decreasing steadily since the late 1970s. Time series of annual mean TCC indicates the highest TCC (48.9 %) in 1961 and the lowest (39.2 %) in 1992 as shown in Fig. 2a. Seasonal mean TCC values are 25.8, 32.8, 69.1 and 34.5 % for winter, summer, monsoon and postmonsoon seasons, respectively. The highest cloudy winter season in India was in 1998 with mean TCC 35.4 % and lowest was in 1951 having mean TCC 18.7 %. Summer season total cloud cover was the highest in 1951 (38.8 %) and lowest was 28.1 % in 1973. In monsoon season, the highest obtained TCC was in 1961 (76.2 %), while the lowest cloud cover was in 2009 (61.3 %). It is evident from Fig. 2d that mean TCC for monsoon has been steadily declining since the mid-1980s. For postmonsoon season, TCC was highest in 2010 (43.5 %) and lowest in 1988 (26.3 %). Seasonal TCC trends are decreasing for all seasons, but it is significant for the monsoon season only, where rate of decline is -0.93 % per decade as shown in Fig. 2d. Annual and seasonal temporal variations as shown in Figs. 2a-e indicate consistent lower TCC during last two decades of the data period.

A.K. Jaswal

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Temporal variations in all India total cloud cover

Fig. 2 Temporal variations in all India total cloud cover (TCC) for a annual, b winter, c summer, d monsoon and e postmonsoon during 1951-2010. Dashed line in blue indicates long-term mean, while linear trend line is shown in red. TCC trends are significant at the 95 % confidence level for annual and monsoon season only (color figure online)

Spatial pattern of mean total cloud cover (TCC) for a annual, b winter, c summer, d monsoon and e postmonsoon. Contours are coefficient of variations CV (%), while long-term mean TCC is shaded

Fig. 3 Spatial pattern of mean total cloud cover (TCC) for a annual, b winter, c summer, d monsoon and e postmonsoon. Contours are coefficient of variations CV (%), while long-term mean TCC is shaded

Spatial pattern of trends in mean total cloud cover

Fig. 4 Spatial pattern of trends in mean total cloud cover (TCC) for a annual, b winter, c summer, d monsoon and e postmonsoon. Stations having trends significant at the 95 % confidence level are marked by an outer circle in black

Scatter plots of all India mean total cloud cover

Fig. 5 Scatter plots of all India mean total cloud cover (TCC) and diurnal temperature range (DTR) anomalies for a annual, b winter, c summer, d monsoon and e postmonsoon. Data series are anomalies from the 1961-1990 period

Spatial pattern of correlation coefficients between total cloud cover (TCC) and diurnal temperature range (DTR) for a annual, b winter, c summer, d monsoon and e postmonsoon

Fig. 6 Spatial pattern of correlation coefficients between total cloud cover (TCC) and diurnal temperature range (DTR) for a annual, b winter, c summer, d monsoon and e postmonsoon

Scatter plots of all India mean total cloud cover

Fig. 7 Scatter plots of all India mean total cloud cover (TCC) and number of rainy days (NRD) anomalies for a annual, b winter, c summer, d monsoon and e postmonsoon. Data series are anomalies from the 1961-1990 base period

Spatial pattern of correlation coefficients between total cloud cover (TCC) and number of rainy days (NRD) for a annual, b winter, c summer, d monsoon and e postmonsoon

Fig. 8 Spatial pattern of correlation coefficients between total cloud cover (TCC) and number of rainy days (NRD) for a annual, b winter, c summer, d monsoon and e postmonsoon

 
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