Answer:
The Northern hemisphere is tilted away from the sun in Winter. But it isn't how close we are to the sun that makes it colder. Because the sunlight doesn't shine as directly onto our part of the earth, Winter air is cold. It doesn't have as much or as strong sunlight to warm it up. The temperature is also determined by the movement of warm and cold air. When both warm air and cold air enter an area, they can cause a Winter storm, bringing more snow, sleet, and wind. You also sometimes hear people talk about "wind chill." The wind chill is how cold it feels because wind makes it seem colder than the actual temperature.

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Ask Jack: Seasons, Solstices, and Equinoxes
http://www.usatoday.com/weather/askjack/wfaqsson.htm
Among the questions answered by USA Today's Ask Jack column are these seasonal questions. Find out why the Winter solstice in December is not the coldest day even though it's one of the shortest and more!

New Science: Change of Seasons
http://liftoff.msfc.nasa.gov/news/2001/news-autumnalequinox.asp
This site by NASA explains how how the sun shines on the earth at different times of year and how the earth tilting affects temperatures.

How Winter Storms Form
http://www.weather.com/encyclopedia/winter/form.html
Find out about types of Winter storms, what air pressure and movements are need to create Winter storms, and definitions of weather terms at this site.

Activity: Wind Chill
http://coolspace.gsfc.nasa.gov/northpole/wind_chill_lesson.pdf
This activity for middle school classrooms helps students understand the wind chill factor and how it affects people.