A vocabulary of landscape — a landscape of words
1068 entries
undular bore
An undular bore is a train of waves traveling through the atmosphere, produced when a disturbance runs into a stable layer that cannot easily rise and the layer rings like a struck surface. Where the air is moist enough, cloud forms on each crest and clears in each trough, so the wave train becomes visible as a set of long parallel bands marching across the sky.
Etymology
The name is borrowed directly from hydraulics, where an undular bore is the smooth wave train that travels up a river ahead of a tidal bore, which is on this site. Meteorologists took the term because the physics is the same: a disturbance moving into a medium that supports waves, producing a regular procession rather than a single front.
Notes
The Morning Glory of the Gulf of Carpentaria, morning glory, is the most spectacular example, arriving as a set of roll clouds hundreds of miles long. Smaller ones are common and are often photographed at dawn as a ripple of parallel bands, and they can produce a sudden wind shift at the ground as each crest passes, which is a hazard for aircraft on approach.