Why is there snow on mountain tops when it's warm below?

Have you ever stood outside in warm sunshine and spotted a mountain wearing a snowy cap? Why is there snow on mountain tops when it's warm below?

Let's start down below, where sunshine reaches the ground. The ground takes in sunlight and warms up, much like a sunny garden path warming beneath your shoes.

That warm ground passes heat to the air touching it. So the air down low gets warm from the ground, like hands warming around a cosy mug.

All the air above us presses down, like a big pile of soft blankets. That push on the air near the ground is called air pressure.

As air travels upward, it reaches places with less air above it pressing down. That means lower air pressure, a gentler squeeze than the air had down below.

With less air pressure squeezing it, the rising air spreads out and takes up more room. What do you think happens to the air's warmth as it spreads out?

The air cools down. As it spreads out, it pushes against the air around it. That push uses some of its warmth, so rising air becomes colder.

That is why the air gets colder the higher you go. A tall mountain reaches up into that cold air, even when its foot sits in a warm valley.

When snow falls onto a cold mountaintop, the air up there is too cold to melt it. So the snow stays frozen and white.

Down in the warmer valley, snow melts more quickly because more warmth reaches it. The valley can lose its snowy blanket while the colder peak keeps a white cap.

Air pressure. Mountain tops stay snowy because air cools as it rises into lower air pressure, leaving less warmth to melt snow.

Next time you spot a snowy mountain above a green valley, you will know why its top stays white.