# What is the pressure of water-dew resulted from cooling of atmospheric air down to dew point temperature?

Suppose, In a room, Air is at 1 atm presssure. So should be the pressure of any water surface in the room. If the room temperature is cooled downed to dew point temperature, vapor (moisture) will start to condense at Tdp (dew point temp) and Pv (vapor pressure). In this phase changing process, the temperature and pressure of both saturated vapor and saturated liquid will be same and constant. (Tdp, Pv). So, the pressure of dew (saturated liquid) is Pv or 1 atm ? suddenly changes like jump discontinuity? what am I missing?

• You're missing the idea of partial pressures. The 1 atm pressure is the sum of the partial pressures of all the gas species in the atmosphere. The behavior of the water and water vapor is governed by the partial pressure of water vapor. The reason the dew point temperature changes is that the partial pressure of water vapor changes. Oct 22 '20 at 0:35
• Sorry, I couldn't get you. I never said dew point temperature changes. The specific humidity is constant before any formation of dew, (so is the partial pressure of vapor). Partial pressure applies to mixture of ideal gases. Here we assumed vapor is ideal gas.