An atmospheric phenomenon, resembling a river bore where a gravity wave propagates away from an initial disturbance, such as a line of thunderstorms. They appear to occur most commonly at an inversion, when radiative cooling has created a stable layer near the ground. Typical wavelengths are c.8 km, and speeds of 16–80 km h−1 have been recorded. As the waves travel overhead there may be a reversal of wind directions at the surface. Undular bores are suspected of influencing other atmospheric events (including thunderstorms) and, most notably, one is thought to have intensified a tornado on the outskirts of Birmingham, Alabama in April 1998, causing it to become an F5 tornado (on the Fujita Scale) and create extensive damage. See also morning glory.
Subjects: Meteorology and Climatology.