Behind the Forecast: Haze

Science Behind the Forecast: Haze

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

LOUISVILLE, KY (WAVE) - A stereotypical summer day can easily be defined as hot, humid and hazy. But what is haze?

Haze reduces visibility and air quality, dulling vibrant sunrises and sunsets and the bright blue of the sky in general as light scatters off of particles in the air.

Research has found that most of the haze droplets in the central and eastern United States are actually sulfuric acid because of the release of sulfur dioxide gas which comes from various manufacturing and refining operations across the country. Sulfur dioxide bonds with oxygen and liquid water eventually forming sulfuric acid.

Haze is more noticeable on hot and humid days because warm air can hold much more water vapor than cold air. This water vapor merges with the various particles in the atmosphere, making them bigger and reducing visibility. Stronger winds push more and bigger particles into the atmosphere helping to further reduce visibility.

Sulfate aerosols are fantastic condensation nuclei, tiny particles on which water vapor condenses. In a situation with moderate humidity, around 60 to 70%, the droplets grow but remain smaller than a typical cloud droplet and are suspended by turbulence. Since sulfate aerosols are small and chemically stable they can remain in the air for days on end and travel far from their source region if winds are strong enough.

Since these haze droplets are larger than normal air molecules, they scatter sunlight more effectively; this is why a hazy sky looks more white than blue. On a hazy day, less direct sunlight reaches the ground.

Precipitation typically washes these chemicals from the atmosphere but may lead to acid rain.

When the Bermuda High expands west during the summer, we're more likely to see hazy days. With not much movement in the atmosphere, plenty of moisture and sunshine, haze easily forms.

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