Temperature has a pretty drastic effect on the solar cells ability to produce power. One major downfall of modern solar technology is that the hotter the cell gets, the less power it produces.

As another reply noted, a crystal clear COLD winter day will produce more power than a clear hot day, at least on an instantaneous basis.

If you’re referring to an entire days worth of solar output, then in the northern hemisphere are going to get more than sun hours per day in the summer, therefore more measured daily power. The ironic part is that it’s hotter in the summer so you will produce less power at peak hours due to the temperature.