Why is the sky bluer in winter than in summer?
The angle of the sun and humidity, indeed have a major effect on the color of the sky. To start, when there is a higher humidity level there is obviously a greater quantity of water vapor in the atmosphere. This leads to more of what is called Mie scattering. Mie scattering in this scenario is when the water vapor and water droplets direct more blue spectrum of light back into space causing you to see a smaller portion then what is possible with less scattering. Depending on where you live the seasons vary for when there is the least amount of humidity. 

Now secondly, if you live between 30-60 degrees North or South, the suns angle changes enough, that the spectrum of light is affected. When the sun is closer to the horizon, the light is traveling through more atmosphere than it would if the sun were directly above. This leads to Rayleigh scattering, which end and end directs more blue light to your eyes.