Understanding Relative Humidity and Dew Point

Steven Lee
Steven Lee
Technical Director IoT Sensors Group

On hot summer days, we tend to hear people say, “It’s so humid” or “It’s sticky out there.” To the surprise of most people who check the relative humidity level, they find it to be relatively low, for instance 52% RH. In a typical office building, for example where I am sitting at this moment of writing this, the relative humidity is measuring 50.15% RH with ambient temperature of 73.54°F (23.08°C), quite comfortable. So, why is the relative humidity practically the same in both instances? And what makes us tend to say “It feels hotter than it is out there?”

When we express how comfortable or uncomfortable we feel, we are really referring to the dew point. Our body cools us down through evaporation of perspiration and moist air will inhibit that. High precision dew point meters are expensive, bulky and typically not used in the household, such as home weather stations. Instead, relative humidity sensors are used to calculate the dew point in most of these devices. The accuracy of the relative humidity sensor (RH sensor) will determine the accuracy of the calculated dew point. Before we look at the relationship between relative humidity and dew point, let’s look at the basics of each.

Relative humidity (RH) is the ratio between the amount of water vapor (moisture) in the air at a given temperature to the maximum amount of water vapor the air can hold at that temperature. RH is expressed as a percentage (%). A rudimentary way to calculate it is by using a psychrometer to measure the dry and wet bulb temperature. Through the dry bulb temperature and the delta of the wet and dry bulb temperature (depression), the relative humidity can be determined. Now, MEMs-type (micro-electro-mechanical systems) relative humidity sensor ICs can accurately measure the RH level using the principles of an electrical signal change from a moisture absorbing film or structure. The important point to remember is that RH is relative to the temperature of the air at that moment. Warm air temperature holds more moisture than cold temperature. For example, 50% RH at a warm temperature has more water vapor than 50% RH at a colder temperature. This is why we don’t feel “sticky” in an office building with 50% RH.

Dew point is a better representative of the amount of moisture in the air. A dew point temperature below 60°F will feel comfortable for most of us and above 70°F we will feel sticky and makes us feel “hotter than it is out there.” So, what is dew point? We’ve all seen it with a glass filled with our favorite iced beverage on a warm day or when the fog rolls in during our morning commute. Dew point is simply the temperature at which the water vapor condensates to liquid water. The dew point temperature is always less than or equal to the air temperature. At the dew point, the relative humidity is 100% (at constant pressure).

So, remember to check the dew point in your morning weather report. And visit idt.com/humidity to learn how our new relative humidity sensor chips can be used in this and other applications, as well as access technical documentation and get samples.