# 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 ?

• This is unlikely to be the Sun varying in intensity. Much more likely, the instrument is measuring total incident radiation, which includes direct sunlight and also all the reflected light (off clouds, blue sky, etc) that reaches the detector. I don't know the local weather patterns, but you COULD get a result like this if mornings were cloudier in one season and evenings cloudier in the other season. Big white fluffy clouds reflect additional sunlight, so you can see how this impacts the rate of change when the sun rises or sets. Can you confirm -- incident radiation or solar radiation? Commented Oct 29, 2014 at 16:07
• Is this observational data, or a theoretically calculated curve? Commented Oct 29, 2014 at 16:08
• To my best of knowledge this should NOT be observational data , you can check for yourself at wolframalpha wolframalpha.com/input/?i=weather+new+york search for "weather history and forecast" then click "more" then search for "incident sunlight intensity " as all the rest data it shows there is observational there is a good reason to believe this is too but i suspect the intensity of sunlight would follow such a smooth pattern ! So possibly its a theoretically calculated curve ? Commented Oct 29, 2014 at 16:33
• My guess it that you're seeing hourly data, and the amount of time the sun is up in the "first hour on sunrise" and the "last hour of sunset" is different. In other words, the measurements are non-symmetrical. Do you have (or would you like sources to) timestamped numerical data that might be more helpful (but may be for localities other than India)?
– user21
Commented Oct 29, 2014 at 16:46
• Your hypothesis maybe plausible , yeah i tried searching the web for such(timestamped numerical data) information but failed to find one Commented Oct 29, 2014 at 19:30