photo #4210310

Cumulonimbus storm clouds are just as tumultuous on the inside as they appear on the outside. Highly unstable, they often contain a churning mix of rising air, known as updrafts, and sinking air, known as downdrafts. The updrafts can be powerful enough to suspend large amounts of water, either as raindrops or hail, in the upper parts of the Cumulonimbus. But what goes up tends to come down, and sometimes the evaporation of droplets within the cloud will cool the air enough to weaken the updraft so it can no longer hold up the rain or hail. This causes a column of precipitation to rush down in a deluge, accompanied by high winds. This phenomenon is known colloquially as a cloudburst. It also has the more technical name of a microburst, but if you’re beneath one you’ll wonder why anyone would describe this explosive burst of rain or hail as ‘micro’. Though their width is narrow compared to that of the storm cloud as a whole, microbursts can cause great damage to the ground below, and can even be life-threatening. And for large storm clouds, the column of heavy precipitation can be as much as 2.5 miles (4 km) across. Best viewed from a safe distance, then – and under shelter. Rob Barke observed this microburst over Pescara, Abruzzo, Italy from beneath a bridge. Wise.