A News 3 viewer recently emailed the First Warning Storm Team, asking us about the difference between relative humidity and the dew point. We use these weather terms in our forecasts to determine how humid or how dry the atmosphere really is. In fact, it comes in handy when the weather starts warming up. However, the terms can be a bit tricky to understand.
Dew point is a measure of the amount of moisture currently in the atmosphere. The National Weather Service Glossary says dew point is “the temperature to which air must be cooled in order to reach saturation.” When the air temperature and dew point are the same, the air is typically saturated. The higher the dew point temperature itself, the more muggy or humid it feels outside. The lower the dew point, the more comfortable it is outdoors. For example, when dew points are in the 50s and low 60s, it is a bit more comfortable because there is less humidity in the air. However, when dew points are in the mid 60s or higher, it starts to feel a bit more sticky and muggy.
The National Weather Service Glossary defines relative humidity as, “a dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature.” So what does all of that mean? Well, relative humidity is presented as a percentage. That percentage is determined relative to the temperature and moisture content. However, relative humidity does not tell you the overall amount of moisture in the atmosphere.
Now, in the mornings, when the air temperature drops to the dew point, we typically have 100 percent relative humidity. However, if the temperature warms up during the day and the moisture content remains the same, the relative humidity value will likely go down. But that does not mean it’s not humid outside. It could likely be quite a muggy day. Hence, relative humidity is relative to the temperature and moisture content. That’s why relative humidity can be a bit tricky. But just as a quick guide, when relative humidity is less than 40 percent, it feels dry outside. When it is greater than 80 percent, it feels moist.
So, overall, dew point is a better measurement of how humid the air feels. Stay with News 3 and the First Warning Storm Team for constant weather updates.
