Houston development alters winds increasing smog

New research on paved surfaces

Jay Blazek Crossley, Jun 23, 11.

“Widespread urban development alters weather patterns in a way that can make it easier for pollutants to accumulate during warm summer weather instead of being blown out to sea” according to a new study focused on the Houston region, as reported by the National Science Foundation.

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Study authors believe that this work will be useful for improving understanding and prediction of air pollution events in the future and can be used to guide policies on urban development:

Houston, known for its mix of petrochemical facilities, sprawling suburbs and traffic jams that stretch for miles, has some of the highest levels of ground-level ozone and other air pollutants in the United States.

State and federal officials have long worked to regulate emissions from factories and motor vehicles in efforts to improve air quality.

The new study suggests that focusing on the city’s development patterns and adding to its already extensive park system could provide air quality benefits as well.

“If you made the city greener and created lakes and ponds, then you probably would have less air pollution even if emissions stayed the same,” Chen explains. “The night-time temperature over the city would be lower and winds would become stronger, blowing the pollution out to the Gulf.”

While much work has been done in the Houston region to study and combat the Urban Heat Island Effect, this new study explains how excessive pavement and buildings alter weather patterns in a way that keeps both air pollution and heat hovering around us:

If Houston was covered with cropland instead of pavement, as in one of the computer simulations, inland air would heat up more than marine air during summer days and cause a sea breeze to blow onshore in the afternoon.

Conversely, the computer simulations showed that as the inland air became cooler than marine air overnight, a land breeze would blow offshore, potentially blowing away pollution.

In contrast, the actual paved surfaces of Houston absorb more heat during the day and are warmer overnight.

This results in stagnation for three reasons:

- At night, the city’s temperatures are similar to those offshore. The lack of a sharp temperature gradient has the effect of reducing winds.
- During the day, the hot paved urban areas tend to draw in air from offshore. However, this air is offset by prevailing wind patterns that blow toward the water, resulting in relatively little net movement in the atmosphere over the city.
- Buildings and other structures break up local winds far more than does the relatively smooth surface of croplands or a natural surface like grasslands. This tends to further reduce breezes.
“The very existence of the Houston area favors stagnation,” the article states.

The study also found that drought conditions can worsen air pollution.

This is because dry soil tends to heat up more quickly than wet soil during the day. It releases more of that heat overnight, reducing water-land temperature contrast and therefore reducing nighttime breezes.