The Difference between Dew Point and Humidity

What is humidity?

When it comes to discussing humidity, people generally talk about the relative humidity (RH), which is related to how close the air is to saturation. Relative humidity, shown as a percentage (%), refers to how much water vapor is in the air at present, compared to how much the air can actually hold. To put it into numbers, 50% RH means the atmosphere contains 50 percent of water vapor that the air can potentially hold under current conditions. In other words, it tells us that the air is fifty percent on the way to condensation. The amount of water vapor the air can hold is often dependent on the temperature of the air at that moment. The higher the temperature is, the greater the amount of water vapor can the air hold before it is saturated. Because water molecules vibrate faster in high temperature, they would be more likely to break free from the neighboring water molecules and evaporate into water vapor. Evaporation and condensation always occur at the same time in our daily lives. When it is at the equilibrium, also known as the saturation point, the condensation rate equals the evaporation rate, which means water molecules are entering and leaving the condensed state at the same rate.
In a closed system, for example, when the temperature goes up, the rate of evaporation increases. With increased evaporation, the amount of water vapor in the space also increases, and more water vapor would condense back to liquid water. The condensation rate increases continuously until it is equal to the evaporation rate and reaches a new dynamic equilibrium. This new equilibrium has greater evaporation rate and condensation rate than the original equilibrium. Therefore, warm air has the capacity to contain more moisture than cool air. As mentioned above, we can also know that RH of 50% at a warm temperature usually has more water vapor than at colder temperature.

What is dew point?

Dew point temperature, usually termed dew point, is the temperature at which air is saturated or achieved 100% relative humidity. A reading of 100% relative humidity means the air temperature has reached the point at which dew can start to form, and the measured temperature is the dew point temperature. At this point, air cannot hold more gaseous state of water because the evaporation and condensation of the water vapor in the air is at the same rate. If the air is cooler than the dew point temperature, the water vapor will begin to condense into the liquid form, such as clouds or fog. Condensation on solid surface, such as window pane, is a result of a combination of surface temperature and moisture in the air.

Conclusion

To sum up, the measurement of relative humidity does not determine how much the actual amount of moisture disperses in the air but the relative amount which is compared to the maximum amount the air is capable of holding. It changes when the temperature increases or decreases. For example, when the temperature goes up, the relative humidity will go down if there is no extra moisture added to the air. In contrast to relative humidity, dew point is an accurate and effective way to measure how much moisture is currently in the air because it is an absolute measurement of humidity, independent of the air temperature.