Why are winters cold?

Winters are cold for two main reasons.  Both of these reasons come about because of the fact that the Earth tilts on its axis.  When the Earth tilts, sun comes to hit one of its hemispheres more directly and the other less directly.  The hemisphere which the sun hits less directly gets colder.
The first reason has to do with the length of the day.  As the Earth tilts, the sun “moves” farther towards the equator.  When this happens, days at higher latitudes get shorter and shorter in terms of how long the sun is above the horizon.  Because the sun spends less and less time above the horizon, it has a much shorter time during which it can heat the atmosphere.
When the Earth tilts on its axis, the angle at which the sun’s rays hit Earth also changes.  During the summer, the sun’s rays hit the Earth at an angle that is close to 90 degrees.  This means that the light points (more or less) straight down at the Earth.  The light does not have to pass through much of the atmosphere before it hits Earth and it does not spread out over very much of the Earth.  By contrast, in the winter, the sun’s light hits Earth at a very oblique angle.  It has to go through much of the atmosphere before hitting the surface, robbing it of some of its energy.  In addition, the light spreads out over a larger surface area, bringing less energy to any particular area.
These factors combine to create a situation in which a particular hemisphere of the Earth gets colder when it is winter in that hemisphere.
 
http://www.planet-science.com/categories/over-11s/natural-world/2012/11/why-is-winter-cold.aspx

https://www.loc.gov/everyday-mysteries/item/why-is-it-hot-in-summer-and-cold-in-winter/