April 20, 2009
These eye catching, banded clouds that formed over the western Gulf of Mexico on April 5, 2009 were caused by the same mechanics as water waves pushed ahead of a boat. The crests of these waves (cloud lines) are signs of air being pushed forward and upward, while the troughs (clear zones) behind them show air that was sinking back down due to the equalizing force of gravity. Thus their name: gravity waves. In this case, the “boat” was an extensive, forward moving arc of air at the surface (a prefrontal trough) intruding into our “water” -- a shallow, stable layer of air called an inversion lying over the cool March Gulf waters. The trough gently pushed the top of the stable inversion upward, and since the air above the inversion was not as stable (had a tendency to rise) and was moist enough to allow condensation, these fascinating clouds formed. It was the stability of the denser inversion and the steady push of the trough into it that allowed the waves to be smooth and undular (too fast of a push would have made turbulent, non-symmetric waves.) Thus, meteorologists refer to the atmospheric event that causes these kinds of gravity waves as an undular bore.