Question

Why is water vapor pressure lower at altitude?

a.Cold air cannot hold very much water.
b.Rain doesn't fall at high elevations.
c.Mountains are only found in dry regions.
d.Humidity, not water vapor pressure, is the low variable.

Answer 1

Water vapor pressure is lower at higher altitudes due to lower atmospheric pressure and cooler temperatures, which reduce the vapor pressure of water.

Step-by-step explanation:

Water vapor pressure is lower at altitude because at higher altitudes, the atmospheric pressure is lower. With less pressure pushing down on the water, the vapor pressure decreases. As air rises over a mountain range, it cools, causing water vapor to condense and precipitation to fall on the windward side of the mountain. The air that reaches the leeward side is dry, with less water vapor, leading to a rain shadow effect where little precipitation falls.

Furthermore, vapor pressure is also temperature dependent. As the temperature increases, the vapor pressure increases because molecular speeds are higher and more molecules can escape into the gas phase. Since high altitudes tend to have lower temperatures, this results in a lower saturation vapor density, and thus, a lower vapor pressure of water.