The outstanding phenomenon is called a 'wet microburst'. Part of the cloud over the lake mixed with a strong localised downpour, leading to a stunning rainbow on the surface of the water.
''There were people swimming on the lake at the time. The cloud was spectacular. I have not seen this before,'' filmmaker Alexandra Gregor said, as quoted by the Evening Standard.
Such microbursts can be catastrophic for passing planes and they are initiated by powerful downdrafts from a storm or rain. They are usually visible for anywhere between a few seconds and several minutes.
Spectacular and absolutely textbook wet downburst over Lake Balaton, Hungary on August 17th. Video: Gregor Alexandrának / Met Hir pic.twitter.com/fwpnbx40Ey— severe-weather.EU (@severeweatherEU) August 19, 2019