A grazier in central Queensland has filmed what looks like a huge ball of water falling from a thunder cloud.
Credit: Peter Thompson Uploaded to Higgins Storm Chasing Facebook site
The event took place 80 kilometers (50 miles) north-east of Roma, which in turn is 500 kilometers (311 miles) north-west of Brisbane. Known to meteorologists as microbursts, these phenomena occur when hot, dry air rises to mix with heavy rain-bearing clouds. Some of the water droplets in the clouds evaporate, causing a swift cooling of the air around them, which then starts to sink. The falling air pulls the remaining raindrops with it, creating a sudden downpour.
Credit: PD-USGOV. / Wikimedia Commons
Peter Thompson, who filmed the event on his phone, described the event as like a “water ball falling” and “looked like the bottom of the cloud falling.” Thompson told ABC News,"I reckon from when I first started seeing that ball form to when it hit the ground was about two minutes.”
The Australian Bureau of Meteorology website notes that microbursts can occur without heavy rain as well, bringing a sudden downdraft in air that can prove deadly for pilots. In those cases, most of the raindrops have evaporated before reaching the ground.
“A wet microburst, which can occur with a range of thunderstorm types, is accompanied by significant precipitation at the surface,” The Bureau website reports. “It develops in environments characterized by weak vertical wind shear and deep moisture capped by a dry layer.” For those caught in them, the experience can be like a weak tornado.
Credit: PD-LAYOUT; PD-USGOV / Wikimedia Commons. NASA artist's rendering of a microburst.
Thompson's microburst occurred on January 27, a day when temperatures in his area had reached 39°C. To be defined as a microburst, the events have to take place over an area of less than four kilometers wide. Since there were no rain gauges where the burst took place, we will never know how much rain fell that day. However, the localized nature of the event can be seen from the fact that Roma itself had an unspectacular (by its standards) 13.4 millimeters of rain that day, while Taroom, the nearest town in the opposite direction, had 16.6 millimeters.