Humid air always contains dry air and steam. Once the saturation is exceeded water droplets and ice occur. The calculations presented are limited to relative saturation up to 100%. In good approximation dry air and steam can be regarded as ideal gases. The calculations are valid for humid air at ambient pressure and temperatures between 5 - 70 [°C].
The dew point of a humid air is the temperature at which its realive humidity reaches 100%. Relative humidity equals 100%, if the partial pressure of steam equals its saturation pressure.