The thin blanket of atmosphere that envelops Earth extends several hundred miles into space. From sea level—the very bottom of the ocean of air—to a height of about 60 mi, the air in the atmosphere is made up of the same gases in the same ratio: about 78% nitrogen, 21% oxygen, and the remaining 1% a mixture of argon, carbon dioxide, and tiny amounts of neon, helium, krypton, xenon, and other gases. The atmosphere becomes less dense with increasing altitude: more than three-fourths of Earth's huge envelope is concentrated in the first 5 to 10 mi above the surface. At sea level, a cubic foot of atmosphere weighs about an ounce and a quarter. The entire atmosphere weighs 5,700 trillion tons, and the force with which gravity holds it in place causes it to exert a pressure of nearly 15 psi. Going out from Earth's surface, the atmosphere is divided into five regions. The regions, and the heights to which they extend, are: troposphere, 0 to 7 mi (at middle latitudes); stratosphere, 7 to 30 mi; mesosphere, 30 to 50 mi; thermosphere, 50 to 400 mi; and exosphere, above 400 mi. The boundaries between each of the regions are known respectively as the tropopause, stratopause, mesopause, and thermopause. Alternative terms often used for the layers above the troposphere are ozonosphere (for stratosphere) and ionosphere for the remaining upper layers.