Undular bore

In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations.

Overview

Undular bores are usually formed when two air masses of different temperatures collide. When a storm (typically a thunderstorm) approaches a layer of cold, stable air, it creates a disturbance in the atmosphere producing a wave like motion. Although the undular bore waves appear as bands of clouds across the sky, they are transverse waves, and are propelled by the transfer of energy from an oncoming storm and are shaped by gravity. The ripple like appearance of this wave is described as the disturbance in the water when a pebble is dropped into a pond or when a moving boat creates waves in the surrounding water. The object displaces the water or medium the wave is travelling through and the medium moves in an upward motion. However, because of gravity, the water or medium is pulled back down and the repetition of this cycle creates the transverse wave motion.

The undular bore's period can measure 5 miles peak to peak and can travel 10 to 50 mph. The medium it travels through is the atmosphere. There are several varying types of ‘‘bores’’ in different layers of the atmosphere, such as the mesospheric bore which occurs in the mesosphere.

Undular bores are believed to be catalysts for thunderstorms. Although a thunderstorm helps create an undular bore, an undular bore can in turn intensify a thunderstorm because it further disturbs the atmosphere.

“Undular bores can have an effect on severe weather conditions,” said Time Coleman, Atmospheric Scientist, National Space and Science Technology (NSST). “A tornado in Birmingham, Alabama in April 1998 that came in contact with an undular bore increased in size and intensity.”

Occurrences

Undular Bores occur as often as once a month.

Related phenomena

The term “bore” is also used to describe positive surges advancing in shallow waters. When the surge's Froude number is between 1.4 to 1.7, the advancing front is followed by a train of well-defined free-surface undulations. The surge is then called an undular surge or undular bore.

A related occurrence of positive surges is the Tidal bore in estuaries.

ee also

[
NEXRAD radar image of undular bore wave]
*Hydraulic jump
*Tidal bore

References and external links

* [http://www.youtube.com/watch?v=aako5siSTgM Video of an undular bore over Iowa]
* [http://www.livescience.com/environment/071015-undular-bore.html LiveScience article]
* [http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2007/04/070411_G12_VIS_ZOOM_10.gifSatellite image of undular bore over Gulf of Mexico]
* [http://www.space.com/imageoftheday/image_of_day_070614.html Possible undular bore over Arabian Sea]
* [http://science.nasa.gov/headlines/y2007/11oct_undularbore.htm?list1043252 One NASA site, of undular bore wave over Iowa]
* [http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17670 Another NASA site, of undular bore wave over Arabian Sea]

* [http://www.livescience.com/environment/071015-undular-bore.html Strange Storm Shaped by Gravity Wave] Live Science, "15 October, 2007"

* [http://espace.library.uq.edu.au/view.php?pid=UQ:120303 Hubert Chanson (2007). Hydraulic Jumps: Bubbles and Bores. In: 16th Australasian Fluid Mechanics Conference (AFMC), Gold Coast, Queensland, Australia, 3-7 Dec. 2007, Plenary lecture, pp. 39-53]