One of the biggest questions that people have when it comes to meteorological variables is, what is the difference between dew point and relative humidity? Also, what are their uses for? Simply, the relative humidity is a percentage. This tells you how close the air is to saturation. When the relative humidity is at 100%, then the air is considered saturated and invisible water vapor can condense. The dew point however, is given as a temperature value. For example, if the dew point is 55°, that means that in order for the air to saturate (or the relative humidity to become 100%) the air temperature needs to cool down to that value. In our example, the air would need to cool down to 55°.

Yet, there is still some confusion on when to use these values and how we use them. Typically, we like to only note the dew point in the summertime because it give us a better scale on how humid the air is. Basically, the higher the dew point, the muggier it is and there is more moisture in the atmosphere. Contrast that to the wintertime when the dew points could be 15° or even below zero, now that is really dry air. However, even in the wintertime, the relative humidity can reach 100%. That does not mean that its humid outside, it just means that the air temperature and dew point temperature are the same and the air is saturated.

Relative humidity (RH) can be used differently for the various seasons. During the winter, RH is critical when it comes to snowfall. Since the winter is a very dry period, RH levels need to be quite high in order for snow to fall out of clouds and reach the ground. If RH levels are too low, then the snow can evaporate in the "unsaturated air" and may never reach the ground (that process is called verga). Not only do RH levels need to be high in the atmosphere but near the ground as well for precipitation to fall all the way to the ground. That hold true for rainfall as well.

From one extreme to another, RH levels are important during the summer. Not only for rainfall reaching the ground, but when it comes to wildfires. The lower the RH, the better chance a wildfire can form and spread. In contrast, the higher the RH level, the less likely a fire will form.

Overall, there are many different way we can use the dew point and relative humidity. The both work together in unison and help us out to forecast as well as visualize the state of the atmosphere.

Print this Story