The document discusses properties of Earth's atmosphere. It notes that the atmosphere extends over 600 miles from the Earth's surface and is composed of a mixture of gases including nitrogen, oxygen, water, argon and carbon dioxide. It explains that air has mass and takes up space, causing it to exert pressure that decreases with higher altitudes as there is less air pushing down from above. Air pressure at the Earth's surface is greatest due to the weight of overlying air columns.