Weather SOL 4.6 atmosphere the layer of air that surrounds our planet air pressure particles of air pressing down on the Earth’s surface troposphere the layer of the Earth’s atmosphere closest to Earth stratosphere the layer of the Earth’s atmosphere containing most of the atmosphere’s ozone greenhouse effect the warming of earth caused by the atmosphere trapping thermal energy from the sun air mass a huge body of air which all has similar temperature and moisture front the border where two air masses meet cirrus wispy, high altitude clouds that are made up of ice crystals cumulus puffy cotton-ball clouds that begin to form when water droplets condense at the middle altitudes cumulonimbus towering, dark rain clouds with nimbus, or halo, of gray-white stratus dark gray clouds that form a low layer and sometimes bring light rain or snow shower barometer measures air pressure humidity amount of water vapor in the air hygrometer Instrument that measures humidity