We often think that the air is weightless but it isn't. The whole weight of the Earth's atmosphere is about 5.5 quadrillion tons (55 followed by 14 zeros). Air is heaviest at sea level because the air molecules are pressed together. It also presses against our bodies but we have pressure inside, so we don't feel it. When you move farther away from the earth's surface the atmosphere gets lighter and lighter because there is more room between air molecules. The weight of the air decreases and so does air pressure.

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