Q: Write a phrase, the answer to which is present in the passage. The answer to this question should be completely and unambiguously found in the passage. The query must be about a single or multiple linked entities. 
The increase in the orographic enhancement factor from cloud seeding does not fully compensate for the continuing decrease from air pollution. Cloud seeding for rain enhancement is aimed at accelerating the conversion of cloud droplets into precipitation particles, whereas air pollution has the opposite effect of suppressing the coalescence of cloud droplets into raindrops and the formation of ice hydrometeors. The Asian brown cloud is created by a range of airborne particles and pollutants from combustion (e.g., woodfires, cars, and factories), biomass burning and industrial processes with incomplete burning. The cloud is associated with the winter monsoon (November/December to April) during which there is no rain to wash pollutants from the air. Topographic cross section showing the effects of urban air pollution on precipitation as the clouds move from the California coast east to the Sierra Nevada Mountains. Maritime air (zone 1) is polluted over the coastal and Central Valley urban areas (zones 2, 3): no precipitation decrease occurs. The seeding effect might still persist there, but it is not associated with a decreasing trend of the unseeded conditions. It is apparent that the effects of cloud seeding and air pollution in the low-elevation areas are more limited because of the lack of an orographic enhancement to the rainfall. Air Pollution/Smog: This encompasses many greenhouse gases. Emissions from traffic and manufacturing plants form smog as UV radiation and heat cause the necessary chemical reactions to take place. As a result, the amounts of UV radiation reaching the surface is smaller under these conditions. Scientists call airborne particles of any sort — human-produced or natural — aerosols. The simplest effect of increasing aerosols is to increase clouds. To form clouds, airborne water vapor needs particles on which to condense. With more aerosols, there can be more or thicker clouds. In a warming world, that's good. Sunlight bounces off cloud tops into space without ever reaching Earth's surface, so we stay cooler under cloud cover. But that simplest effect doesn't always happen. If there's no water vapor in the air — the air is dry — aerosols can't make clouds. Different types of aerosols have different effects, and the same aerosol can have different effects depending on how much is in the air and how high it is. Soot particles at certain altitudes can cause cloud droplets to evaporate, leaving nothing but haze. They found that additional pollution over pristine regions of the ocean has a larger effect on the clouds than in already polluted regions. Essentially, adding the same amount of pollution to clean marine air makes clouds brighter and hold more water than when adding the same amount of extra pollution to dirty regions. Scientists at Pacific Northwest National Laboratory and the National Center for Atmospheric Research (NCAR) found that ocean-hugging clouds in clean regions are more affected by pollution than clouds in an already polluted setting. Cloud trails from ships, called ship tracks, are a small-scale example of this effect. Pollution exhaust from ships forms streaks of clouds that are brighter than neighboring clouds. The researchers learned that the amount of pollution already in a region strongly moderates any effect on cloud characteristics from additional pollution. Carbon dioxide is the major greenhouse gas produced by human consumption of fossil fuels, along with methane, nitrous oxide, and other carbon gases. Ship tracks, the long lines of clouds downwind of ships, provided a nice opportunity to study the effect of exhaust particles, called aerosols, on clouds. Short-term air pollution can aggravate the medical conditions of individuals with asthma and emphysema. Long-term health effects can include chronic respiratory disease, lung cancer, heart disease, and even damage to the brain, nerves, liver, or kidneys.
A:
effect of air pollution on clouds