Difference Between Dew Point and Humidity

Dew Point vs Humidity

Humidity and dew point are two concepts discussed in the vapor systems. Humidity is a very common concept that holds a great significance in our daily lives. Dew point is a concept that is tightly connected with the humidity. A proper understanding in humidity and dew point is required in fields such as meteorology, physics, chemistry and many other fields. A good understanding in these concepts may also be useful in our daily lives too. In this article, we are going to discuss what humidity and dew point are, their definitions, the applications of humidity and dew point, their similarities, and finally the difference between humidity and dew point.

What is Humidity?

The term humidity refers to the amount of water vapor inside a system. Humidity has two different forms. Absolute humidity is a very important factor when it comes to the study of psychrometrics. Psychrometrics is the study of gas – vapor systems. In thermodynamics, the absolute humidity is defined as the mass of water vapor per unit volume of moist air. This can take values ranging from zero to the saturated water vapor density. The saturated water vapor density depends on the pressure of the gas; therefore, the maximum mass of vapor per unit volume also depends on the air pressure. Relative humidity is very important when the real effect of the humidity is concerned. To understand the concept of relative humidity, there are two concepts that need to be addressed first. First one is the partial pressure. Imagine a gaseous system where there are A1 molecules of gas G1 generating pressure P1, and A2 molecules of gas G2 generating pressure P2. The partial pressure of G1 in the mixture is P1/ (P1+P2). For an ideal gas, this is also equal to A1/ (A1+A2). The second concept that has to be understood is the saturated vapor pressure. The vapor pressure is the pressure created by vapor in equilibrium, in a system. Now let us assume that there is still liquid water (however infinitesimal) in a closed system. This means the system is saturated with water vapor. If the temperature of the system is decreased, the system will surely remain saturated, but if it is increased, the result would have to be re calculated. Now let us see the definition of relative humidity. Relative humidity is defined as the percentage of the partial pressure of the vapor divided by the saturated vapor pressure at the given temperature. This is in the form of a percentage.

What is Dew Point?

The dew point of a system is the temperature where the amount of vapor inside a system becomes the saturated vapor. In other words, for a closed system, the dew point is the temperature at which the dew starts to form. At the dew point, the relative humidity becomes 100%. Any temperature above the dew point will have relative humidity lower than 100%, and any point below the dew point will have relative humidity of 100%. The dew point is a temperature and, therefore, it is measured in temperature units.


What is the difference between Dew Point and Humidity?

• Humidity refers to the amount of the water vapor inside a system. The dew point refers to the temperature where that amount of water vapor is the saturated vapor.

• Humidity is measured in either kg/mor as a percentage. The dew point is measured in temperature units such as Kelvin, Celsius degrees or Fahrenheit degrees.


  • relentlesslycaring

    Couldn’t someone explain Dew Point in 12 words or less?