Weather Words - J

-- J --

January: The first month of the year and the second, and middle, month of meteorological winter. January is Chicago's coldest, snowiest and second driest month. Only February has less precipitation than January.

January 24: In 1916, Browning, Mont., experienced a 24-hour temperature drop of 100 degrees (+44 to -56); in 1982, chinook winds gusted to 140 m.p.h. at Wondervu, Colo.; in 1967, 7 died in an outbreak of 32 tornadoes across Iowa, Missouri, Illinois and Wisconsin.

January in Chicago: In temperature records dating from 1871, January is the only month during which Chicago has never recorded a temperature of 70 degrees. The highest recorded January temperature is 67 degrees (Jan. 25, 1950).

January thaw: A period of mild weather popularly supposed to recur each year in late January in New England and in other parts of the northeastern United States, and even in the Midwest.

Jaws: A reef located one-half mile off the north shore of Maui, Hawaii, that, because of its peculiar underwater structure and its exposure to the stormy North Pacific Ocean, regularly generates the world's highest breaking waves. A "small" wave at Jaws has a 20-foot face; a large wave, 50 feet or more.

Jet streak: A region of maximum winds that moves through the axis of a jet stream. It is the zone of the strongest winds within a jet stream.

Jet stream: (1) A belt of very strong horizontal winds found within a narrow depth in the atmosphere, usually occurring 6 to 9 miles above the ground. These winds often steer the movement of surface air masses and weather systems.

Jet stream: (2) A belt of very strong winds found within a narrow depth in the atmosphere, usually occurring 6 to 9 miles aloft. These winds often steer the movement of surface air masses and weather systems.

Jet stream winds: Atmospheric currents of swiftly flowing winds thousands of miles long and hundreds of miles wide, but only a few miles thick. Jet-stream core winds often exceed 100 m.p.h. and occasionally exceed 280 m.p.h.

Jetlet: A "small" jet stream; a relatively small region of maximum winds aloft, especially a small area of high winds whose maximum strength falls within the arbitrary range of jet-stream wind speeds.

Johnstown, Penn.: Location of this nation's worst flash flood. Following heavy rain, a poorly maintained dam above Johnstown collapsed on May 31, 1889. The resulting flash flood killed 2,209.

Johnstown, Penn., flash flood: It is the worst flood disaster in U.S. history: After 6-8 inches of rain, the poorly maintained South Fork Dam collapsed at 3:10 p.m. on May 31, 1889, sending a 40-foot wall of water and debris down the Little Conemaugh River, claiming 2,209 lives at Johnstown and other riverfront communities.

July: The seventh month of the year and the middle month of meteorological summer. It is Chicago's warmest and fourth wettest month.

July 31: On this date in 1976, the Big Thompson flash flood in northeast Colorado kills 156 following 10 inches of rain; in 1987, Canada's second-worst tornado kills 27 in Edmonton, Alberta.

Jumping the mountains: Meteorological jargon for the process whereby low pressure centers, unable to cross high mountain ranges, dissipate upon approach to high elevation, then reform in the lee.

June 1: It marks the beginning of meteorological summer (the warmest three months of the year) and it is also the beginning of the six-month period referred to as the North Atlantic hurricane season.

Jupiter at opposition Oct. 23: When Jupiter appears exactly opposite the direction from the sun in the sky it reaches opposition. At this point Jupiter is at its brightest and closest to Earth for the year.

Jupiter's Great Red Spot: Located in Jupiter's Southern Hemisphere, it is the Solar System's largest known weather system -- a cold high pressure system with counterclockwise circulation.

Juvenile water: Water brought to the surface or added to underground supplies from magma.