Fireworks smoke, humidity led to super fog and low visibility

Late Sunday night, just as the new year was about to begin, the National Weather Service warned San Antonio residents that the city’s sprawling fireworks celebrations had triggered hazardous weather.

“We are seeing a rapid increase in smoke particulate across the area late this evening associated with fireworks activity,” forecasters posted on their social media page on X. “This may lead to patchy, dense super fog development over the next few hours.”

Visibility at 11 p.m. in some locations had been reduced to less than 2 miles. By 4 a.m., visibility in Bexar County along and east of Interstate 35 was down to a quarter of a mile.

Earlier that night, social media message from the weather service had expressed worry about super fog. Forecasters explained that when the air is especially humid, “smoke particulates can serve as surfaces for water to condense onto, forming fog.” Visibility can easily drop to less than a quarter-mile to near zero.

Relative humidity levels Sunday night had ramped up between 6 p.m. and midnight from 54% to 89%, according weather service data collected at San Antonio International Airport. Humidity then jumped to 96% in the first fours after midnight, as fog and mist intensified.

We’ve seen how fireworks can affect weather, but how about the other way around? For San Antonio on Sunday before midnight, the weather largely cooperated and we enjoyed the city’s fireworks as planned. But here’s a quick rundown of how weather can affect fireworks:

• Rain: The fuses on fireworks must remain dry for them to ignite. Rain can dampen or soak them so it’s impossible for the fuses to go off and the fireworks to explode. San Antonio lacked significant rain chances Sunday night, so residents across the city had a blast shooting New Year’s Eve fireworks.

• Humidity: Humid air can make the colors of the explosions less vibrant. Like rain, moisture from high humidity also poses risks for misfires and wet fuses. Humidity became more of an concern after midnight into Monday morning, leading to a foggy start to New Year’s Day.

• Winds: High winds, particularly in excess of 20 mph, can blow noxious fireworks smoke far from the launch site and into populated areas, triggering asthma and other chronic respiratory health problems. Winds were light, around 5 to 10 mph, in San Antonio on Sunday night.

• Fog/low clouds: Fireworks set against the backdrop of low clouds or fog become difficult to see because of the reflected glare of city lights or simply because of low visibility. This was not much of an issue in San Antonio before midnight Sunday.