Why doesn't the snow and ice on the mountain melt all the year round?

Some peaks in Qilian Mountain, Tianshan Mountain, Kunlun Mountain and Himalayan Mountain in western China are often covered with snow and ice, like wearing a "white hat", which will not disappear even in summer. In the tropics, some peaks are covered with snow all year round. This is because the temperature on the mountain is very low and the weather is very cold. Why is it so cold on the mountain? Because the higher the mountain is, the thinner the air is, and the heat from the sun is easily lost. Roughly every rise 100 meter, the temperature will drop by about 0.6℃, so at a certain height, the temperature will drop below 0℃, and the ice and snow will not melt all year round. The dividing line of this height is called the snow line. The closer you get to the poles, the lower the snow line is, because the temperature there is already very cold; In the tropics, the snow line is higher. After the snow and ice piled up on the top of the mountain, the sun shone here. Because the surface of ice and snow reflects sunlight strongly, it can generally reflect 50%? 90% of the light and heat, most of the heat is reflected, which makes the temperature here low and the ice and snow are not easy to melt. So at the top of those mountains whose height exceeds the snow line, there is snow all year round. In the strong sunshine, there will be some melting, and in summer, there will be more melting. But it didn't take long for snow to come down and replenish it, so that it could always exist and form a glacier to move down.

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