small scale cumulus clouds near Saanich, BC, Canada
Credit: E. Wiebe

Beautiful small cumulus clouds appeared during the morning of 27 June, 2016. Overnight cooling of the lower atmosphere under still conditions leaves a very stable lower atmosphere. As the sun rises in the morning energy is supplied to the base of the atmosphere (the Earth's surface) and the lower atmosphere begins to warm. This changes the vertical stability profile and causes initially small parcels of air to become more buoyant than their surroundings so that they rise. The water vapour in these blobs of air stays relatively fixed since mixing is a slower process than the rising motion. When the blobs rise they also expand and cool adiabatically (without much exchange of energy with their surroundings). This cooling causes the relative humidity in the parcels of air to increase. When the relative humidity reaches saturation (near 100%) condensation onto tiny, microscopic condensation nucleii can occur and a cloud can form. Condensation releases latent heat during water's transition from water vapour to liquid (or solid) water. This energy slows the cooling of the air parcel as it rises but isn't itself the reason that the air in the clouds doesn't rise forever. Air parcels stop rising when there is no longer a buoyancy force lifting them against gravity. They stop when they reach a level where they are neutrally buoyant.

Cumulus clouds are the visible evidence of relative strong vertical motion. That motion can be initiated from any level in the atmosphere. It need only occur that an air parcel becomes more (or less for sinking air) buoyant than its surroundings. The relatively flat bottom of cumulus clouds reveals the level in the atmosphere where condensation in a rising air parcel commences, not where the vertical motion starts. The sharp billowy top boundary of active cumulus clouds is evidence of the strength of the vertical motion, pushing air masses with different properties out of the way, and of the relative weakness of the diffusive properties that act to make the properties of different air masses the same across boundaries.

Video from 2016-06-27.

Gordon Head Middle School Weather Station.

UVic / SEOS / Climate Group / About Front Page Picture / Small Cumulus Clouds Last updated: Monday, 04-Jul-2016 09:53:02 PDT
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