Why in news?
A rare roll cloud was observed off Portugal’s coast during a heatwave when cool Atlantic air collided with hot, dry air.
Understanding roll clouds
- Phenomenon: Roll clouds are long, low‑level, tube‑shaped clouds that appear to roll along their length. They are detached from other cloud systems and are not associated with tornadoes.
- Formation: Arise when contrasting air masses meet—typically cool, moist maritime air and warm, dry continental air—creating a temperature inversion. This triggers atmospheric gravity waves: as the cool air undercuts warm air, the rising limb cools and condenses into a cloud while the descending limb remains dry.
- Typical locations: Coastal transition zones, such as the U.S. Great Plains, Australia’s Gulf of Carpentaria and Atlantic coasts of Europe, are conducive to roll clouds. They often appear in the morning (also called “morning glory” clouds).
- Implications: Roll clouds are primarily a visual and meteorological spectacle, signalling instability and temperature gradients in the lower atmosphere. They may precede storm development.