AUSTIN (KXAN) — We haven’t had much sun the last week here in Austin, but even when the sun has been out, visibility has been limited. The haze in the air has made it tough to see distant buildings, distant clouds or the blue in the sky. You can blame three things for this: Humidity, agricultural burning in Mexico, and particulate pollution.
High humidity
The humidity has been tropical with dew points in the 70s making even the early mornings feel uncomfortable. The relative humidity has ranged from 50-100%+ depending on the time of day. This level of humidity in the air isn’t just felt, but you can see the suspended water vapor, which accounts for some of the lowered visibility.
Agricultural burning
According to the National Weather Service, April and May are common months for agricultural burning in Mexico and other Central American Countries. Smoke from those intentional fires can be pushed across the Gulf of Mexico and into Central Texas, especially with a southerly or southeasterly wind.
Late in April our local NWS office shared the following graphic to illustrate the point.
Fine particulate matter pollution
Adding to the lower visibility are a type of pollution known as fine particulates. Specifically this week PM2.5 is the primary pollutant here in Central Texas.
PM2.5 means fine particulate matter pollutants with a diameter at or below 2.5 micrometers in size. A human hair is about 50 to 70 micrometers in diameter.
On Wednesday, PM2.5 levels reached “unhealthy” levels, according to Air Central Texas.
How do these particulates form? The EPA says “most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries and automobiles.” Sulfate aerosols are closer to 0.1 micrometers in diameter.
But if they’re so small how can we see them?
According to NOAA, when the relative humidity is above 60% these fine particulates absorb water in the air and grow in size. The higher the relative humidity, the bigger the droplets.
These bigger, but still tiny droplets, when in high concentrations, can lower visibility and create a haze in the lower levels of the atmosphere. This is caused by the tiny droplets absorbing and scattering the light before it reaches you.
Should I worry about my health?
The level of PM2.5 this week in the Austin Metro is expected to fall back into the “Moderate” category on Thursday according to Airnow.gov. Only people who are unusually sensitive to particulate pollution should ease off the exercise and reduce time outside.
When will the haze clear?
An arriving cold front late Thursday will help to usher out the humidity and bring more clarity to our visibility. You’ll likely find a bluer sky when the clouds break apart on Friday!
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