Mountains have a lot of influence on the weather. They can even create their own climate. Individual mountain ranges form barriers, forcing air to rise up over them. This cools the air and can create wave motion in the atmosphere. In mid latitudes the prevailing winds blow from west to east and mountain ranges that run north to south like the Rockies or the Andes block the smooth flow of air and help shape the weather. These mountains prevent moist Pacific Ocean air from reaching the interiors of the American continents. The Himalayas and the Alps are also important in shaping the weather.

When volcanoes erupt they hurl hot ash and scalding gases into the air. The initial heat generated dissipates quickly but the volcanic dust particles and gases can remain in the atmosphere for two years or more, reducing sunlight and lowering temperatures. The small particles of dust and ash can reach the stratosphere where they spread out to form a thin cloud which reflects sunlight. They are blown into a plume by high-level winds. Sulphur dioxide gas combines with water vapour to form microscopic droplets of sulphuric acid which absorb even more sunlight than dust.

Emissions from the Tambora eruption in 1815 lowered temperatures world wide by about 0.7 degrees C (1.2 degrees F) in 1816, which became widely known as the "year without a summer". But volcanic eruptions often cause spectacular optical effects for a long time afterwards with dramatic and colourful sunsets.