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Cloud Appreciation SocietyIn the Negev desert region of Israel, the massive geological feature called Makhtesh Ramon is a wide box canyon created over millions of years by water erosion as harder rock at its edges was eroded less quickly than softer rock within. Cold ‘katabatic’ winds can howl down its steep cliffs from the open plains above. These are winds that blow whenever cold, dense air sitting at higher elevation drains down slopes under the force of gravity. On the exposed, high-altitude plains of the Negev, air at ground level can be cooled overnight as the ground rapidly radiates away its heat into the starry night sky. Sometimes, the air cools enough to form ground-level cloud, or radiation fog. Thanks to the katabatic winds, the fog in the cold, dense air floods down into the Makhtesh Ramon. Whenever air sinks in our atmosphere, it compresses and so warms, and this can be enough to cause the fog droplets to evaporate away again as they descend. The result looks like the ghosts of waterfalls returning to haunt the channels they created many millennia ago.Radiation fog spotted by ‘ZeevStein’ descending the cliff walls of Makhtesh Ramon, Negev desert, Israel