Big question
Why do mountains have snow?
Mountains have snow because the air gets colder the higher up you go, roughly 6 to 7 degrees Celsius colder for every 1,000 metres of altitude. Even on a hot day at sea level, a tall mountain's peak can sit high enough in this cold air to stay below freezing and hold snow.
Air Gets Thinner and Colder
Air near the ground is squeezed under the weight of all the air above it, which keeps it relatively dense and warm. Climb higher and there is less air pressing down, so the air spreads out, or expands, and expanding air cools down, a process scientists call adiabatic cooling. This is why the environmental lapse rate, the average rate at which temperature drops with height, works out to roughly 6.5 degrees Celsius for every 1,000 metres you climb. A mountain 4,000 metres tall can have a summit around 26 degrees colder than its base, easily enough to turn a warm valley into a freezing, snow-covered peak.
The Snow Line
Scientists talk about a snow line, the height above which temperatures usually stay cold enough for snow to survive rather than melt. Below the snow line, any snow that falls tends to melt fairly quickly; above it, snow can pile up season after season, sometimes packing down into solid ice over many years. Where exactly the snow line sits depends on more than just height: it is lower near the poles, where the whole climate is colder, and higher near the equator, where you need to climb much further before the air turns cold enough, which is why some equatorial mountains still carry snow caps despite sitting in the tropics.
Why Snow Can Last Through Summer
High mountain peaks often keep snow even in the middle of summer, because at that altitude the air rarely warms enough to melt it completely. Fresh white snow also reflects a large share of the sunlight that hits it back into space instead of absorbing it as heat, which helps it survive even on bright, sunny days. Over many years, in the coldest, highest spots, layers of snow can compress into thick sheets of ice called glaciers, which flow slowly downhill under their own weight, carving valleys as they creep along, sometimes over centuries.
Try it yourself
Ask an adult to help you check the temperature at ground level and then somewhere higher, like the top floor of a tall building or a nearby hill, using a thermometer or a weather app. Notice how it drops the higher you go.
Questions children ask
Why is it colder on top of a mountain if it is closer to the Sun?
Being a few kilometres closer to the Sun makes no real difference, since the Sun is about 150 million kilometres away. Mountains are cold because air thins and cools as it rises, not because of distance from the Sun.
What is the snow line?
The snow line is the height on a mountain above which it usually stays cold enough for snow to remain instead of melting away. It sits lower near the poles and higher near the equator.
Do mountains near the equator ever have snow?
Yes, tall equatorial mountains like Mount Kilimanjaro and parts of the Andes can carry snow or glaciers, because their summits reach high enough into the cold upper air. The equator being hot only applies near sea level.