BBC: Weather at Altitude: Looks at how the weather is different at high altitudes.
Met Office: Mountain Safety: Highlights several dangers about the mountains, whether going for a long expedition or short hike.
Mountain Weather: Contains information on weather and pays specific attention to weather that occurs in the mountains.
Mountain Weather: Contains soundings for several locations which show how the temperature and dew point change with altitude.
Mountain Weather Information Service: Contains forecasts for areas of the United Kingdom. Available as PDF files.
Mountain Weather Unpredictable: Explains how it's difficult to predict weather in the mountains.
MountainWeather.com: Contains weather information for Jackson Hole, Wyoming, USA and the Teton Mountains. Radar images and forecasts.
NOVA: Volcano Above the Clouds: Find out how mountains create their own distinct weather patterns, often causing wet weather on one side and a dry, clear climate on the other.
Rocky Mountain National Park Weather: Explains the weather you're likely to see at different times of the year.
USAToday.com: Understanding Mountain Weather: Talks about avalanches and winter weather see on the mountains of the United States.
Weather Factors in Mountainous Areas: Explains the dangers a pilot could experience because of weather in the mountains.