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Oxygen and nitrogen make up approximately 98% of the mass of the atmosphere,
and are classified as permanent gases. This means the mixing ratio of the gas
is approximately constant with height and time. Water vapor, however, is a
variable gas. It decreases in concentration with distance from the surface of
the earth. Although water vapor constitutes a small portion of the atmosphere's
mass, it is the most influential variable gas. The molecular weight of nitrogen is 28.01 g/mol, and that of oxygen is 32.00 g/mol. The mean molecular weight of dry air is 28.5 g/mol. However, water vapor has a molecular weight of 18.01 g/mol. When considering the increasing proportion of a volume of air occupied by increasing amounts of water vapor, there must be a reduction in the mean molecular weight of the atmosphere. So, as water vapor content increases, the air becomes less dense. Fronts are defined as boundaries between air masses of different density. Due to differences in density, cold air (more dense) remains closer to the surface and warmer air (less dense) is lifted upward when one air mass advances on another. Although fronts are named for temperature changes associated with their passage and the resulting changes in density, changes in moisture content can affect density as well. A boundary that separates dry from moist air is called a dryline. In the United States, temperatures on either side of a dryline are similar, but the dew point temperatures to the east of the dryline are significantly higher than those west of the dryline. Because temperature is the most important regulator of atmospheric density, a dryline is not considered a front, despite it being a boundary separating air masses of different density. The following may be useful in determining whether or not humidity affects density significantly enough to classify drylines as fronts. |