Atmosphere

The mesosphere is the coldest atmospheric layer surrounding the earth. It becomes cold enough to freeze water vapor in its atmosphere into ice clouds. These ice clouds are blue-white and are called noctilucent clouds or polar mesospheric clouds. These clouds are more visible at sunset from the earth's poles.

The exosphere is the highest and top layer of the Earth's atmosphere. It marks the edge of space. There are very few molecules in this layer. The lightest atmospheric gases such as hydrogen and helium exist throughout the exosphere.