Air Parcel Expands As It Rises…
• Air pressure decreases with elevation.
• If a helium balloon 1 m in diameter is released at sea level, it expands as it floats upward because of the pressure decrease. The balloon would be 6.7 m in diameter as a height of 40 km.
D:\yu\maker\talk\uci\class\ESS11\image\pressure.vertical.schematic.gif
(from The Blue Planet)