Factors in Ozone Formation

Earth's climate has experienced big changes in the past. For example, during the Mesozoic era (the era of the dinosaurs) the mean global temperature may have been as much as 6º - 8ºC (11º - 14ºF) warmer than it is today. Fossils of large trees, warm-water mollusks and cold-blooded reptiles have been found in what today are arctic climates.

The end of the dinosaurs and the beginning of the Cenozoic era 65 million years ago brought a cooling trend. At the height of the ensuing ice age 15,000-23,000 years ago, the world was about 4ºC (7ºF) cooler than it is now. Sea level dropped as polar icecaps grew, and early humans were able to migrate across the Bering Strait from Asia to North America.

Rain also affects ozone formation. Rain often improves air quality by both "raining out" pollutants and cooling air temperatures. Similarly, cloudy days lower ozone levels by reducing temperatures and sunlight. Ozone alerts usually occur when forecasts call for clear skies or scattered cloud coverage.

Geographical conditions--topography--also affect ozone formation. Mountains, valleys, and bodies of water affect ozone's ability to dissipate or settle. For example, mountains often block air movement, so that stagnant air collects in the valley between two mountains. In coastal areas, land and sea breezes may transport pollutants offshore during the afternoon, when the land is warmer than the sea, and onshore during the evening, when the sea is warmer than the land.