![]() ![]() It looks like the outdoor air is more humid. Let’s look at the example of when I opened the windows last week. To decide whether it’s OK to open the windows or not, the dew point is a much better metric because it tells you which air has a higher concentration of water vapor. Dew point map of the United States, 4 Oct. But there’s an even better number to look at, and fortunately, weather apps show this number. You also need to know what the indoor and outdoor temperatures are. They see the outdoor relative humidity at 70% and say, “Oh, I should keep the house closed up now because that’s too humid.” There’s a better metricĪnd that’s wrong! Well, it might happen to be right accidentally, but you can’t look at the relative humidity by itself. In a humid climate, you don’t want to do that when the outdoor air is more humid than indoor air, right? But the trap many people fall into is looking only at the relative humidity. Opening the windows means letting outdoor air into your house. And the outdoor relative humidity at the time was about 90%. Gorgeous fall weather! About a week ago I opened one of my basement windows and put a fan in it to blow outdoor air into my house. Low temperatures in the low to mid 50s Fahrenheit (10-13° C), highs in the 70s Fahrenheit (25ish Celsius), and sunny. It’s humid and warm here now, but a week ago we had some nice cool weather. We still have humid days, though, so many people have the question: When is the humidity low enough to open the windows? I opened the windows to 90% relative humidity ![]() ![]() The weather is cooling off here in Georgia, so we don’t have to run our air conditioners all the time now. (See the related articles below for more.) But here’s an angle that I haven’t covered before. I’ve been hammering on this point for a decade in this blog, covering it in several different ways. The second map better comports to our ideas of areas that are more “humid.Relative humidity is such a confusing way to talk about moisture in the air. The desert Southwest looks like most people would expect, but not so much everywhere else. If you only saw the first map, you would be forgiven for scratching your head in confusion. The two maps below show the 1) average annual relative humidity and 2) the average annual dew point. This is a truer reflection of the moisture regime. The average dew point in Alaska is the lowest of all 50 states. A cold airmass simply cannot hold a lot of moisture. This is a function of the low temperatures. Dew points under 30☏ feel notably dry.Īs noted earlier, when looking at relative humidity, Alaska is the most humid state. A dew point over 60☏ is where it starts to “feel humid.” Dew points under 60☏ generally feel comfortable. ![]() in the summer months, this is a common value. It’s the amount of moisture that makes you sweat even at night without any physical exertion. A dew point over 75ׄ☏ is very oppressive. There are some magical dew point numbers that represent handy guides for determining how much moisture is in the air. There are other measures of moisture not discussed here, they include: specific humidity, mixing ratio, and vapor pressure. Looking at the sponge diagram, the dew point represents the temperature if the dry part of the sponges (yellow areas) were removed. In short, the dew point is a temperature value that represents the minimum temperature an airmass can achieve given the amount of moisture in the air. While not technically a direct measure of moisture, dew point is a relatable measurement to most people. The most popular is something called the dew point. There are a number of ways to measure the moisture in the air that do not have this issue of relativity. This leads to a situation where the same city has a very high relative humidity at one time of day and a very low humidity at another time of the day – even with no change in the amount of moisture in the air. If the amount of water vapor in the air is constant throughout the day, the relative humidity changes dramatically as the temperature rises and falls. In most instances, the air is coolest in the morning and warmest in the late afternoon. In the previous example, the warmer airmass actually contains 2.7 times as much water vapor as the cooler airmass – even though they both are reporting 50% relative humidity.Īnother aspect to this is the (diurnal) trajectory of temperatures throughout the day. This demonstrates why using relative humidity is a terrible metric for surface moisture. In the graphic above, an 80☏ airmass that is 50% full of water vapor is shown as a sponge that is significantly larger than a 50☏ airmass that is also 50% saturated. ![]()
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