Clouds on the Horizon

Convective clouds forming over the Amazon in a blanket smoke. Image: Prof. Ilan Koren Convective clouds forming over the Amazon in a blanket smoke. Image: Prof. Ilan Koren

Horse latitudes, doldrums, calms of Cancer and Capricorn: These are all synonymous names for the forsaken regions of the oceans that sailors of previous eras cursed because the winds that once pushed their sails die out there for weeks at a time. (One suggested explanation for the name “horse latitudes” is that some had to throw the horses overboard there when the supplies ran out.) On land, these subtropical belts – around 30-35° north and south of the equator – help form the world’s deserts.

These regions turn out to be nearly ideal “labs” for studying certain atmospheric physics. Fortunately, today’s researchers do not need to sit on a becalmed ship to study the processes occurring there. Satellite data can tell them exactly what they need to know. And what they want to know, in the case of Prof. Ilan Koren and his group, is whether the human action of polluting the atmosphere – which is usually treated as a local problem – could actually be affecting the climate in such remote locations.

Koren and his team zoomed in on three areas on the southern horse latitudes’ map, and looked at three months of satellite data analyzing clouds and aerosol data for each day.

The experimental question was: Does the aerosol count – whether from dust and sea spray or man-made soot – matter for cloud formation and rain? Some think that there is a saturation limit beyond which added aerosols don’t really affect cloud processes, but Koren and his team have been developing a model which suggests that added aerosols “invigorate” clouds – they create clouds that are larger, higher and rain more aggressively.

Image: Koren et al. Science 6 June 2014: Vol. 344 no. 6188 pp. 1143-1146 Image: Koren et al. Science 6 June 2014:
Vol. 344 no. 6188 pp. 1143-1146

The group chose to look at the horse latitudes data, in part, because the aerosol count there is generally low and the meteorological conditions there are such that so-called convective clouds could form. With no aerosols, clouds should not form at all, because these tiny air-born particles are what provide the nuclei for droplets. According to Koren, the relative humidity needed for a droplet to condense without that little seed would be an incredible 500%! So the model suggests that the clouds there are “aerosol limited,” meaning that the concentration of suspended particles serves as the main factor controlling cloud thickness and coverage. In the paper published by Koren and his group in Science, they demonstrate that convective clouds are aerosol-limited for a wide range of aerosol loading; therefore, any increase in aerosol concentration, whatsoever, will invigorate the clouds.

To really nail their case, they also checked their predictions against CERES data – a different satellite instrument that measures radiation emitted and reflected from Earth to space. That data suggests that back when those unfortunate sailors were waiting out the winds in the horse latitudes – before the advent of fuel-burning engines that let ships chug their way past – cloud formation was quite different from that of today. And in case you were wondering – the preindustrial clouds over their heads were sparser and smaller.

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Do more clouds mean cooling or warming?

The answer is both: Cloud cover returns solar radiation back into space. But the invigorated clouds are higher and longer lasting, so they trap more of the long-wave, heating radiation underneath. That is why modeling the cloud contribution is tricky -- we need to understand how we are changing the effects in both directions.