In the real world the air always contains some moisture. The addition of water vapour to a mass of air makes it less dense. Whilst this may appear a bit odd at first this occurs because the molecular mass of water (18) is less than the molecular mass of air (29).
The density of humid air can be calculated as the sum of the densities of the two gases, dry air and water vapour in proportion with their partial pressures.
There is a lot of empty space between gas molecules. The molecules are very far apart, relative to their sizes, and they neither attract nor repel each other much. So distances between molecules aren't affected by the type of molecules in the sample.
That implies that the number of molecules in a given container won't change, when you change only the type of gas filling the container. (This is one way to state Avogadro's Law).
The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified. In the lower regions of the atmosphere (up to altitudes of approximately 12,000 metres (39,000 ft), temperature decreases with altitude at a fairly uniform rate. Because the atmosphere is warmed by convection from Earth's surface, this lapse or reduction in temperature is normal with increasing distance from the conductive source. Although the actual atmospheric lapse rate varies, under normal atmospheric conditions the average atmospheric lapse rate results in a temperature decrease of 6.4C°/km (3.5F°/1,000 ft) of altitude above ground level.
Air expands when heated and contracts when cooled. And while it does, its capacity to hold water changes accordingly. This is why you see dew on the grass in the evening or condensation on a glass of ice water in the summer. In those cases warm, humid air is cooled. As it cools, it loses its capacity to hold as much moisture and the water condenses onto the colder surface. In reverse, you see why the best way to evaporate water is by passing warm, dry air over it. In winter, your furnace heats the air in your house and, by doing so, lowers its relative humidity. That makes the air dry.
In the real world the air always contains some moisture. The addition of water vapour to a mass of air makes it less dense. Whilst this may appear a bit odd at first this occurs because the molecular mass of water (18) is less than the molecular mass of air (29).
The density of humid air can be calculated as the sum of the densities of the two gases, dry air and water vapour in proportion with their partial pressures.
There is a lot of empty space between gas molecules. The molecules are very far apart, relative to their sizes, and they neither attract nor repel each other much. So distances between molecules aren't affected by the type of molecules in the sample.
That implies that the number of molecules in a given container won't change, when you change only the type of gas filling the container. (This is one way to state Avogadro's Law).
The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified. In the lower regions of the atmosphere (up to altitudes of approximately 12,000 metres (39,000 ft), temperature decreases with altitude at a fairly uniform rate. Because the atmosphere is warmed by convection from Earth's surface, this lapse or reduction in temperature is normal with increasing distance from the conductive source. Although the actual atmospheric lapse rate varies, under normal atmospheric conditions the average atmospheric lapse rate results in a temperature decrease of 6.4C°/km (3.5F°/1,000 ft) of altitude above ground level.
Air expands when heated and contracts when cooled. And while it does, its capacity to hold water changes accordingly. This is why you see dew on the grass in the evening or condensation on a glass of ice water in the summer. In those cases warm, humid air is cooled. As it cools, it loses its capacity to hold as much moisture and the water condenses onto the colder surface. In reverse, you see why the best way to evaporate water is by passing warm, dry air over it. In winter, your furnace heats the air in your house and, by doing so, lowers its relative humidity. That makes the air dry.
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