The Atmosphere
The atmosphere is the air that surrounds the Earth. It extends upwards for hundreds of kilometres until it reaches space. Earth's atmosphere acts as a blanket. It is able to absorb heat as well as reflect some of the Sun's rays. This keeps the Earth's surface from getting too hot or too cold.
The Layers of the Atmosphere
Atmospheric Pressure - This link demonstrates atmospheric pressure and how water changes its form when heated over 100 degrees celsius.
If Earth did not have an atmosphere then it would look like the moon. The moon has no atmosphere and because of this is not protected from the extreme temperatures of space. On the moon the days are extremely hot, whilst the nights are freezing cold. As there is no atmosphere there is also no moisture, or water vapour, therefore no rain, snow or hail. No living thing can survive on the moon because there is no oxygen.