My brother actually backpacked in the middle of winter, including sleeping out in a tent overnight.
He and some friends once backpacked up Mount Washington in February,
spending the night in a tent, above timberline, with temperatures well below zero.
The next morning they backpacked on to the summit, where the wind was blowing so hard
that they could barely stay on their feet. He said that it actually sounded like a jet engine.
They later found out that at the time they were crossing the summit of Mount Washington,
the average wind speed was about 90mph, gusting to 110, and the temperature near 30 below.
He said that they were dressed more like astronauts than hikers, but it was still a wee bit chilly! 
Warning Sign at Timberline on Mount Washington From the Mount Washington Observatory website: "Home of the World's Worst Weather" Mount Washington presents the most severe combinations of wind, cold, icing and storminess available anywhere in the world where people are on hand to take measurements. The summit lies in the path of the principal storm tracks and air mass routes affecting the northeastern United States, and it is, because of its elevation, biologically and ecologically similar to the subarctic zone.
Mount Washington Facts Elevation: 6,288 feet (1,917 meters) - Latitude: 44° 16' N - Longitude: 71° 18' W Highest wind (world record, April 12, 1934): 231 MPH (372 KPH) Average wind velocity for the year: 35.3 MPH (56.8 KPH) Winds exceed hurricane force (75 MPH) (121 KPH)on an average of 104 days a year. Lowest temperature (state record, January 1934): -47°F (-44°C) Highest temperature (August 1975): 72°F (22°C) Average temperature for the year: 26.5°F (-3°C) The summit is in the clouds about 60% of the time. Average annual snowfall: 256 inches (21.3 feet) Maximum snowfall in a season (1968-69): 566.4 inches (47.2 feet) (14.4 meters). Maximum snowfall in calendar year (1969) 495.2 inches (41.3 feet) (12.6 meters). |

Tuckerman Ravine Trail on Mount Washington at 20 Below Zero 
Tuckerman's Ravine, Mount Washington 
Summit of Mount Jefferson |