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CARBON DIOXIDE VS. WATER VAPOR
AS GREENHOUSE GASES

METEOROLOGIST JEFF HABY

By quantity, there is much more water vapor than carbon dioxide in the atmosphere. Water vapor varies from a trace in extremely cold and dry air to about 4% in extremely warm and humid air. The average amount of water vapor in the atmosphere averaged for all locations is between 2 and 3%. Carbon dioxide levels are near 0.04%. That means there is more than 60 times as much water vapor in the atmosphere than carbon dioxide in average conditions. Both water vapor and Carbon dioxide are greenhouse gases. They both trap outgoing longwave radiation between the earth and the atmosphere. This has an effect of keeping temperatures warmer than they otherwise would be. Carbon dioxide is a more efficient greenhouse gas than water vapor when both are in equal quantities. However, they are not in equal quantities. There is much more water vapor than carbon dioxide in the atmosphere. In day to day weather forecasting, the greenhouse effect from water vapor is important while carbon dioxide is not. The atmospheric greenhouse effect from clouds and water vapor causes cloudy nights to be warmer than clear nights, all else being equal.