Humidity -- we talk about it often in weather. We mostly reference the humidity when the weather gets warm, but there is humidity all the time -- it just depends on whether it's high or low. That then leads to how it "feels" outside. For example, the three H's of summer you're probably familiar with in D.C. - Hazy, Hot, and Humid. It's one thing to be hot in the summer with temperatures in the 80s and 90s, but factor in the humidity and that's when it can feel downright uncomfortable... and don't even get me started on what it does to my hair!
So what exactly is the humidity? And why does it make the air feel so "sticky"? Let's break it down a little. Humidity is the amount of water vapor in the air. It's measured in different ways, but you'll most commonly hear it referred as the relative humidity or the dew point. The image below gives you a visual of the temperature and the amount of water vapor present (think of the beakers as the atmosphere). The lower the temperature, less moisture needs to be present for the air to become saturated. The same thought can be applied for warmer temperatures, but you can see more water vapor can be present.
- FAA - http://www.aviationweather.ws/026_Water_Vapor.php
Let's start off with relative humidity. The relative humidity is the amount of moisture in the atmosphere relative to the amount that would be present if the air was saturated. It sounds confusing, I know. The relative humidity is expressed as a percentage and is based off the temperature. We tend to associate rainy days or humid days with higher humidity, which can be true, but it can also be misleading. Often times the humidity is near 100% in the morning. The reason is the air is typically cooler in the morning and cooler air holds less water vapor - you can go back to the image above to help visualize this. The relative humidity is a good measure of humidity; however, it can be a little tricky to understand. I have always been a fan of the dew point.
The dew point is the temperature the air has to "cool" to, to become saturated. I put the word "cool" in quotations, since the dew point can be a fairly high temperature. Now the dew point temperature will never be greater than the air temperature, but the closer the dew point is to the air temperature, the more water vapor in the air, and the more "sticky" or muggy it feels. Usually when the dew point gets above 65°, that's when it starts to feel uncomfortable.
For example, if the temperature is 86° and the dew point is 70° it will actually feel like 91°! The reason it feels hotter is because it's harder for our bodies to cool us off when there is higher humidity. Our bodies use a process of evaporative cooling, so if there's a lot of water vapor in the atmosphere, it is much harder for our bodies to cool off, as compared to a day when there is less water vapor and lower humidity. The chart below shows the Heat Index which calculates the temperature and relative humidity to determine how hot it feels to our bodies. Hot temperatures combined with high humidity can be very dangerous.
- NOAA Heat Index
Now a lot of the time I hear people refer to the air feeling really heavy on a muggy or humid day. It certainly feels like that, but did you know the air is actually much lighter? Water vapor has a low density, so when you go outside on a hot and humid day, the air is less dense than if it was drier out. You can also relate this theory to baseball, believe it or not. If you're at a game on a humid day, the ball can actually travel farther since the air is less dense. Pretty neat, huh? Gives you something else to think about when you're at the ballgame.
Humidity is present every day -- it's just a matter of how much is there. I think I'll always been a fan of lower humidity days, unless I'm at the pool or beach and can easily cool off!