Marilyn is Mistaken about the Clouds

Marilyn is Wrong Copyright © 1996-2005 Herb Weiner. All rights reserved.

Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."

In her Parade Magazine column of July 21, 1996, Marilyn claims that "Clouds are both created and sustained by air currents that are moving upward."

Sorry, Marilyn

Gordon Speer <> spotted Marilyn's error in this answer.

Water Molecules have a molecular weight of 18, based upon their composition of two hydrogen atoms (atomic weight of 1 each) and one oxygen atom (atomic weight of 16). Air is made up primarily of nitrogen molecules (molecular weight of 28, based upon two nitrogen atoms with an atomic weigth of 14 each) and oxygen molecules (molecular weight of 32, based upon two oxygen atoms with an atomic weight of 16 each).

Air currents do not cause water to rise in the air, as Marilyn claims. Since, water molecules are lighter than air molecules, humid air (containing water vapor) is lighter than dry air. Humid air rises because it is lighter than dry air, and is the cause of the updrafts.

As the humid air rises, it cools, causing water droplets to condense. At this point, Marilyn is correct in explaining that the updrafts sustain the clouds, since the water droplets are heavier than air. When the droplets grow large enough to overcome the updrafts, they fall as precipitation.

Humidity and high temperature are the two causes of low pressure systems, since they both lower the density of air. High pressure systems, which are generally clear and free from bad weather, are caused by dry or cold air.

More about Clouds

David Bell <> suggested a link to the Bad Meteorology site.

Other Causes of Updrafts

Rick Aster <> points out that Humidity is not the only or even the primary cause of updrafts. For example, mountains create updrafts; converging surface winds create updrafts; in full sunlight, paved areas and dark buildings create updrafts. These other updrafts may also create clouds if enough humidity is present. Conversely, high humidity may not create an updraft if the air above it also has high humidity.

A Reader Disagrees

Don Weaver <> states that Gordon Speer's explanation is not true on two accounts:

First, water vapor does not rise because of the molecular weight difference between water vapor and N2/O2.¹ A theoretically perfect gas disperses as though no other gasses were present. This is the reason why CFC molecules from ground-level human activity are found as high as the Ozone Layer despite the fact that they have far higher molecular weight than N2/O2. A "real" gas does interact with other gas molecules and may disperse more quickly or more slowly, depending on various factors.

In any case, the dispersion rate is too slow to account for observed rates of cloud formation or any significant migration of moisture.² If this weren't so the atmosphere³ would be perpetually saturated with water vapor as long as a non-trivial surface of liquid water were present.

The only reason why clouds of any significant size form is indeed because of "lifting mechanisms" that bring moist air from low altitudes (where water vapor concentrations are naturally higher due to the low dispersion rate and various surface warming effects) up to altitudes where the adiabatic temperature allows droplets or ice crystals of significant size to persist.

Second, humid air is not less dense than dry air as some opine. Again, gasses disperse as though no other gasses were present. The presence of water vapor does not displace heavier molecules of N2/O2, rather shares the same space and therefore increases the density of the gas mixture. An air mass rises only if it is sufficiently warmed to lower its density below that of air above it.
¹ A plume of water vapor would rise, as a somewhat coherent mass, due to the molecular weight difference. However, such plumes represent a statistically insignificant contribution to atmospheric moisture apart from localized effects of major volcanic ventings (see note 2 below).
² Over short time periods compared to geological epochs.
³ Yes, the "air" is not saturated, rather the "space" of the atmosphere. last updated December 25, 2005 by