Relative Humidity vs Dew Point

Weather Blog

Peoria, Ill. (WMBD) – Summer days in Central Illinois are often warm and muggy which can make spending a few hours outside less than desirable and, at times, dangerous. If you’re planning an outdoor event you often turn to the weather forecast to find out which day of the week will be less humid. But what do you look at, the relative humidity or dew point?

The short answer is dew point but let’s explain why.

Both parameters deal with water vapor. The dew point is the temperature at which the air becomes completely saturated (100% relative humidity) and is solely dependent on the amount of moisture in the air while the relative humidity is the percent of saturation at a given temperature. Once the air reaches a relative humidity of 100%, moisture would come out of the atmosphere in the form of fog or precipitation.

The higher the dew point the more moisture there is in the air. This determines how comfortable it will be outside. More often than not the relative humidity is misleading. For instance on a day where the temperature is 40° with a dew point of 40° the relative humidity would be 100%, but if the temperature is 85° and the dew point is 60°, the relative humidity is around 43%. It will feel a lot more humid on an 85° day with 43% relative humidity than it will on 40° day with 100% relative humidity. So if you’re wondering which day of the weekend is going to be more comfortable for your barbecue, look at the dew point. The higher the dew point the muggier it will feel.

General comfort levels for Central Illinois based on dew points

Below 55°- Comfortable
55° to 63°- Comfortable with muggy nights
63° to 70° – Muggy
Above 70° – Becoming oppressive

While the relative humidity may not be the best to look at when it comes to determining comfort, it certainly has other uses. We often monitor relative humidity when we are monitoring fire weather conditions. When the relative humidity drops below 25% for prolonged periods of time the fire danger increases. It’s also a great tool when looking out for fog formation when the relative humidity approaches 100%.

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