The difference between relative humidity and dewpoint
If you stepped outside this afternoon, you probably noticed it almost instantly. The air felt heavy, thick, and distinctly like mid-summer. Naturally, you might have pulled out your phone, checked your favorite weather app, and noticed the relative humidity reading sat at 60-70%. So why does it feel like the air is so much more humid?
The truth is, relative humidity is often not the best reference to how the air actually feels on your skin. If you want to know how miserable it's truly going to be before you head outdoors, you need to ignore the percentage and start looking at a different metric entirely: the dew point.
To understand why your app feels misleading, you have to look at how relative humidity works. The keyword there is relative. It measures how much moisture is currently in the air relative to the maximum amount the air can hold at that temperature.
Think of the atmosphere as a sponge. Warm air is a massive car-wash sponge; cold air is a tiny kitchen sponge. Because warm air expands, it has a much larger capacity to hold water vapor.
When you wake up at 6:00 AM and it's a cool 65 degrees outside, the air can't hold much moisture, so it reaches saturation easily. Your app might read 95% humidity. But by 4:00 PM, as the June sun bakes the ground and heats the air to 85 degrees, that atmospheric "sponge" expands significantly. Even if the same amount of water is hanging out in the air, the percentage drops because the air's total capacity has grown. That is why it can feel incredibly sticky in the afternoon even when the relative humidity numbers look low.
If you want the absolute truth about the summer air, look for the dew point. The dew point is the temperature the air would have to cool down to reach 100% saturation. Because it measures the actual volume of water vapor locked in the sky, it doesn't bounce around just because the afternoon sun warms the thermostat.
When it comes to human comfort, the dew point is the best scale. Below 60 degrees, it will typically feel crisp, dry, and comfortable. Approaching 60 to 65 degrees will feel noticeably more sticky, and you will start to feel some resistance when you walk outside. 65 to 70 degrees is the classic Midwest summer mugginess, and 70+ degrees is tropical or oppressive. This is the "air that you can wear."
Right now, a steady pipeline of moisture traveling straight out of the Gulf has parked dew points right in that sticky mid-to-upper 60s zone. Looking at our upcoming 5-day outlook, that moisture isn't going anywhere anytime soon. Without a strong cold front to sweep the mid-to-late-week air mass away, we can expect these muggy conditions to hold steady, making it feel increasingly tropical as we head deeper into the week.
This means that it is going to be much harder to cool off as the days progress. The human body cools itself through the evaporation of sweat. When the air is already crowded with moisture, your sweat can't evaporate into the atmosphere efficiently. As a result, your natural cooling system stalls, causing your core temperature to rise much faster. So, make sure to pace yourself with any strenuous outdoor activities, watch for the signs of heat exhaustion, and finally, hydrate early, as waiting until you are already thirsty will cause you to be playing catch-up through the entire day.
