Why does this graph for sunlight intensity on land has a steeper slope during sunrise as compared to sunset? I got this image while checking weather data for a city in North India using  Mathematica's Wolfram Alpha query 


I noticed one feature in the graph which i could not explain .
Why does the encircled part 'A' which denotes  sunrise has slightly higher slope as compared to encircled part 'B' which denotes sunset ! 

I also checked the graph for summer month and the pattern was exactly opposite 

Summer Month 


Am i interpreting the data wrong or is it so that sun achieves its highest intensity during the day faster in winter months than in summers ? What could be the reason behind this ?
Too long for a comment, alas:

https://www.mesonet.org/index.php/site/about/other_measurements/#srad notes:


  Solar radiation measurements in the morning and evening are sensitive
  to obstruction (e.g., trees) on the east and west horizons. A delay of
  sunrise or early arrival of sunset at a particular station may be
  explained by examining panoramic site photos available at
  http://www.mesonet.org/index.php/site/sites/mesonet_sites.


Also dig around a bit starting at http://mesowest.utah.edu/cgi-bin/droman/meso_graph_climo_ndb.cgi?stn=SEAM5&amp;unit=0&amp;hours=24&amp;day1=0&amp;month1=&amp;year1=2014&amp;hour1=00&amp;windred=&amp;time=LOCAL&amp;var=SOLR&amp;lastyear=1&amp;vnamev=Solar%20Radiation&amp;stationname=SEAGULL&amp;vlabel=%C2%A0W/m*m and you should find more data.