What is it about?

This paper presents a simple analysis of the relative density of clouds with respect to dry and/or moist air. We present a simpler and general approach to studying air densities by means of comparing the so‐called density temperatures.

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Why is it important?

Although air density comparisons seem to be a well-known subject in atmospheric sciences, it is possible to find contradictory conclusions between authoritative works in the scientific literature. Previous studies usually assume that hydrometeors have effects on density but no effects on pressure. We show that considering the hydrometeors as a colloidal component of a mixture, their contribution to the total pressure comes from the osmotic pressure that resembles the ideal gas law for a gas composed of ‘droplet molecules’. We showed that, for the clouds of the Earth's atmosphere, this osmotic force is several orders of magnitude smaller than the contribution of dry air and water vapor and therefore can be ignored, as is usually done in the literature. This conceptual clarification, although it does not change the practical definition of density temperature, sheds light on the basic concepts of atmospheric thermodynamics. We also pointed out that the validity of this definition holds as long as the cloud droplets may be considered to be in suspension. For cloud droplet sizes greater than 1 μm, the individual bombardment by microscopic gas molecules will have little effect as Brownian forces become insignificant and a new definition of density temperature should come from considerations of hydrodynamics where virtual temperature should be interpreted as the temperature that a parcel of dry air should have in order to experience the same acceleration as a parcel of cloud air.

Perspectives

Our approach can be used to study clouds of particulate matter. It is possible for these clouds to be heavier than surrounding dry air conditions if they have large particulate matter mixing ratio. Besides vertical temperature inversions, this fact may contribute to unhealthy air quality in cities with high emission of particulate matter during dry conditions.

Alejandro Jaramillo
Universidad Nacional Autonoma de Mexico

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This page is a summary of: On the relative density of clouds, Quarterly Journal of the Royal Meteorological Society, July 2017, Wiley, DOI: 10.1002/qj.3099.
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