Difference between saturation vapor pressure and partial pressure

Saturation vapor pressure is the maximum pressure exerted by water vapor in the air when the air is fully saturated at a given temperature. It depends only on temperature, not on how much vapor is actually present. At this point, the rate of evaporation equals the rate of condensation, meaning equilibrium is reached.

Partial pressure, on the other hand, refers to the pressure exerted by a specific gas (like water vapor) within a mixture of gases, such as air. It tells you how much of the total atmospheric pressure is due to that particular gas.

If the partial pressure of water vapor is less than the saturation vapor pressure, evaporation occurs because the air can still hold more moisture. If it’s equal, the air is saturated, and no net evaporation happens. When the partial pressure exceeds the saturation value, condensation begins, forming dew or fog.

In short, saturation vapor pressure is the temperature-dependent limit, while partial pressure is the actual water vapor amount in the air. Their comparison helps determine humidity and condensation processes.