Most clouds that form on hot sunny days are driven by convection. Thermal changes at the Earth’s surface – such as the change in reflectance between a paved car park and a green field – drive convective motion, causing air to rise over the warmer pocket of land. As the parcel of air rises, it cools and its humidity increases. If the thermal perturbation is great enough, the air parcel may eventually reach a relative humidity of 100%, at which point droplets form and a cloud appears. The clouds observed by Koren, however, occurred below the predicted condensation level and couldn't be explained by the classical thermal perturbation process.

To investigate these anomalous clouds, which are too small to be recorded by weather satellites, Hirsch, Koren and their colleagues simulated detailed air-parcel movements driven by perturbations in either temperature or relative humidity. In addition, they carried out a ground-based field campaign during the summer of 2011 near the Bet-Dagan meteorological station, around 10km east of the Mediterranean coast in central Israel. During June, July and August the atmospheric profile (change in temperature and humidity with altitude) was measured every day at 12:00 UTC (15:00 local time), and cloud bases were measured every ten minutes over a two hour period, using a laser light source instrument known as a ceilometer.

The field data revealed the presence of small short-lived low-level clouds on 27 of the 92 days. These clouds were in a significantly lower position – typically more than 350m – than the estimated condensation level and couldn't be explained by conventional thermal convection processes originating at the Earth's surface.

When the researchers initialized their model with the atmospheric sounding measurements from the Bet-Dagan meteorological station, however, they were able to reproduce these clouds with perturbations in relative humidity originating not at the surface, but in the middle of the boundary layer.

"It's unintuitive but as air becomes more humid it becomes lighter and more buoyant," said Koren, who published the findings in Environmental Research Letters (ERL) . "In warm summer conditions small variations in humidity in the mid boundary layer can cause small pockets of air to rise and in some cases reach 100% humidity and form a small cloud." Koren thinks that these small variations in humidity are especially likely to occur near the coast, where moist ocean air mixes with dry land air.

Koren and his colleagues estimate that these previously ignored clouds could alter the Earth's radiation budget by as much as 4 Watts per square metre – equivalent in magnitude, but opposite in sign, to greenhouse gases and aerosols. If the impact of these clouds is not taken into account, the reflectance could be attributed, incorrectly, to something else.

"In regions where these clouds are common, such as the Mediterranean, we might wrongly attribute their impact to aerosol loading or another atmospheric property," said Koren. They might be small, but these clouds can still pack a punch when it comes to climate impact.

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