A hazy scene(Slava L / Unsplash)Hazy, hot and humid: The classic, alliterative, description of summertime in Louisville. But what makes it so?On the latest edition of Science Behind the Forecast, meteorologist Tawana Andrew discusses some of the causes with LPM’s Bill Burton.This transcript was edited for clarity and length.Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I'm joined by WAVE 3 meteorologist Tawana Andrew. Good morning, Tawana.Tawana Andrew: Good morning. Today's topic can be a little bit more difficult to see. We'll put it that way.BB: Yeah, it can be a little hazy. Oh, I think I just gave it away, didn't I? Yes, we're talking about haze today.TA:  On those high humidity days, that's when you tend to get a little bit more of that haze in the forecast, and especially in the Ohio Valley. With that high relative humidity, water vapor is more likely to merge with various particulates in the atmosphere. Warm air can hold significantly more water vapor than cold air, which is why haze is more prominent on those hot and humid days. When you have these water vapor molecules combining with particulates like pollen, dust, pollutants, they can grow larger and eventually reduce visibility.Research has found that most of the haze droplets in the central and eastern United States are actually sulfuric acid. No need to freak out. It's not enough to actually do anything to impact most humans and even our plants and whatnot.BB: I’m grateful for that news.TA: The reason that we have this is because you have so many manufacturing refining operations across the country that do release sulfur dioxide gas. That sulfur dioxide bonds with oxygen and liquid water, eventually forming sulfuric acid.These sulfate aerosols are really really good condensation nuclei. Water loves to just condense on top of them. When you have a situation like we do in the summer months, where you have moderate humidities, let's say 60 to 70% humidity, the droplets will grow, but they will remain smaller than a typical cloud droplet. That means they're still going to be suspended in the air, a little bit closer to the ground by that ongoing air turbulence that we typically see from the little bit of a breeze and such during the summer months.Since those sulfate aerosols are small and chemically stable, they can remain in the air for days on end. Especially if you have high pressure just sitting over a region, they're not going anywhere. If the wind is strong enough in other situations, those same aerosols can travel far from their source region. Then you can see that haze drifting across the country.Since these droplets are larger than typical air molecules, they will scatter more sunlight. This is why on those hazy days that you see the sky looking more white than blue, because it also helps to limit the amount of sunlight that reaches the ground.That haze is also why you see more of those bright red and orange skies. That's because the haze is scattering more of the blue light, leaving behind the red and the orange light, and you end up with those crazy, cool, vivid sunsets in some cases. It's not just sulfates, it’s also wildfire smoke, which we've seen a lot of this summer.BB: A lot of Canada is still on fire.TA:  Those particulates can also become really good condensation nuclei and create significant haze as they drift through a region. That is why it's not just the smoke that's causing some of that haze for some of us this summer, but it's other things going on in our atmosphere, even just high humidity, that are making the sky seem a little bit more whitish than bluish on those summer days. ...read more read less