It’s that time of year when the lexicon of the meteorologist regularly includes the phrase dew point. This concept is often confused with relative humidity which doesn’t tell us anything about how you’re going to feel. It’s the dew point that is the key to the experience you have when you step outside, especially in the summer.
For Boston, dew points have been trending higher over the past two decades, and are likely to continue as our atmosphere warms.
As a definition, the dew point represents the temperature at which the air will be full of moisture. If the actual temperature and the dew point are the same, then the air is full of moisture and we have a relative humidity of 100 percent — but this isn’t often the case. You could have a dew point of 30 degrees and a temperature of 30 degrees and a relative humidity of 100 percent or a temperature of 70 degrees and a dew point of 70 degrees and a relative humidity of 100 percent. See how the relative humidity doesn’t mean anything to how you feel?
What’s more useful is looking at just the dew point to see how much moisture is in the air right now. Meteorologists use the dew point as a way to understand the amount of grams per kilogram of moisture floating around in the air. The more moisture that’s in the air, the harder it is for your skin to lose moisture — your sweat mechanism is less effective and you feel hotter. A lower dew point gives a more effective evaporation rate so even on a hot day when you sweat that sweat evaporates and you feel cooler. This is why the heat in the desert doesn’t feel as bad as the heat around here with our higher humidity.
If the dew point is under 55 degrees most of us are simply comfortable, even as it gets quite hot. Between about a 55 and 60-degree dew point, the air starts to feel a little more like summer, the crisp air is gone, but it’s not necessarily uncomfortable. Especially in the middle of summer dew points under 60 degrees tend to be a little more refreshing.
At the beginning of summer, we’re not used to the humidity so it might be a little less comfortable. Once the dew points get into the lower 60s it’s noticeable and many of you would say “Hey, it feels a little sticky outside today”. Dew points in the mid-60s make it uncomfortable for a greater number of people and that number continues to rise with the dew point as they get into the upper 60s. This is especially true if the temperature is above 80 degrees.
From there as the dew point goes up the number of people who enjoy the heat and humidity starts to decrease. Once dew points are above 70 degrees, again especially with high temperatures, air conditioners are on and the amount of time folks spend outside comfortably tends to go down. Complaining about the humidity goes up even higher. Every summer we end up with a few days of dew points in the mid-70s and even getting close to 80. This is pretty miserable stuff and tough to be outside.
As the climate continues to warm our days with higher dew points are also going up. The chart below shows that on average dew points climb rapidly during June and by the end of the month 75 percent of the time we will experience dew points above 60 degrees during the core of the humidity between mid-July and mid-August nearly every day will have a dew point at least 60 if not higher.
We are now entering the season of higher dew points and over the next 3 months, it’s the air conditioning that’s going to bring about the comfort more than a change in air mass.
Welcome to Summer.