Air Pollution and Atmospheric Visibility
Have you ever spent time in a large city? If so, the odds are you’ve seen the sky engulfed in a brownish-yellow or grayish-white haze due to air pollution. Such haze can reduce visibility from miles (kilometers) to yards (meters). Mountains or buildings once in plain sight can suddenly be blocked from view.
Air pollution that reduces visibility is often called haze or smog. The term smog originally meant a mixture of smoke and fog in the air, but today it refers to any mixture of air pollutants that can be seen. Smog typically starts in cities or areas with many people, but because it travels with the wind, it can appear in rural areas as well.
One consequence of smog over any given area is that it can change the area’s climate. Smog reduces the amount of the Sun’s energy reaching the Earth’s surface. In some cities, this reduction has been as high as 35 percent on particularly smoggy days. The reduction is greatest when the sun is low on the horizon because the sunlight has to travel through a greater amount of polluted air as its angle drops.
A reduction in solar radiation may not be the only thing air pollution inhibits. It may be altering rainfall patterns as well. Particulates in the air often form condensation nuclei that attract water vapor. When enough moisture accumulates around natural dust particles for example, droplets of rain typically fall. But certain specks of air pollution, such as black carbon, can be too small to produce raindrops big enough to hit the ground. Since rain flushes dust, soot, and chemicals from polluted skies, atmospheric visibility could also be negatively impacted as a consequence.
Scientific field campaigns such as MILAGRO are one way scientists can research atmospheric processes to prove or disprove such ideas. For now, if rain isn't in the forecast when atmospheric visibility is low and smog is high, than wind is likely your best hope for a return to clear skies.