The end of summer and start of autumn brings out a colorful change in scenery across the Northern Hemisphere. Here, two women ride their horses across a field near Gleidlingen in the Hanover region of Germany on Sept. 2, 2013.
Blueberry and dwarf birch leaves turned to autumn colors, resting among green bearberry leaves and white lichens in Denali National Park, Alaska.
Dew drops on a tangled spider web create a lens reflecting the American Flag.
Georgia Bulldogs quarterback Aaron Murray (11) and the rest of the team gather around Georgia Bulldogs head coach Mark Richt prior to the Georgia Bulldogs 41-30 victory over the South Carolina Gamecocks at Sanford Stadium in Athens, Ga., on Sept. 7.
A pug wears a jockey figure on the back in the Pug and Bulldog race in Wernau, Germany on Sept. 1, 2013. Around 180 pugs and bulldogs took part in the race.
Dahlias bloom at the Hoehenpark Killesberg in Stuttgart, Germany, on Aug. 31, 2013.
Three leafcutter bees (Megachile spp.), probably alfalfa leafcutter bees (Megachile rotundata), snack on a squash blossom.
A 15-meter-long dinosaur made from 2,000 pumpkins stands at Karls Erlebnis Dorf in Roevershagen, Germany on Sept. 6, 2013.
A hay stack turns gold at sunset in a country field near Lucinere, Joue sur Edre, France.
Aspen trees (Populus tremuloides) look like autumn flames at Fishlake National Forest, Utah.
A grove of red Aspen trees in Fish Lake Basin, Fishlake National Forest, Utah reflects a rough heart shape.
Leaves on the campus at St. John's University in Collegeville show that autumn has started in Minnesota.
People participate in the Tomato Royal during the inaugural Bull Run at Virginia Motorsports Park in Petersburg, Va., on Aug. 24, 2013.
A reveler plays in tomato pulp after the annual Tomatina (tomato fight) in the Mediterranean village of Bunol, near Valencia Aug. 28, 2013.
David Ramos/Getty Images
Revelers celebrate covered by tomato pulp while participating the annual Tomatina festival on Aug. 28, 2013 in Bunol, Spain.
Heat waves kill more Americans every year than hurricanes, tornadoes, floods, earthquakes and lightning combined, according to the Centers for Disease Control. A recently identified atmospheric pattern may give people a two- to three-week warning that a deadly heat wave threatens to engulf the United States.
This map of air flow a few miles above ground level in the Northern Hemisphere shows the type of wavenumber-5 pattern associated with US drought. This pattern includes alternating troughs (blue contours) and ridges (red contours), with an “H” symbol (for high pressure) shown at the center of each of the five ridges. High pressure tends to cause sinking air and suppress precipitation, which can allow a heat wave to develop and intensify over land areas. (Haiyan Teng, National Center for Atmospheric Research)
National Center for Atmospheric Research scientists observed a particular sequence of five alternating low and high-pressure systems in the atmosphere far above the North Atlantic. When that pattern moved over the continental United States, it increased the chances of a heat wave.
The five high-pressure systems ringed the mid-latitudes of the north like pearls on a necklace moving slowly westward. When that pattern emerged, the United States faced double to quadruple the chance of a serious heat wave after approximately 20 days. The pattern blocked movement in the atmosphere and allowed hot, dry conditions to take hold. For example, the pattern preceded the heat waves and droughts of 1952-54 and 1988.
To discern the pattern, the atmospheric scientists used weather records and atmospheric measurements since 1948. A computer simulation of 12,000 years of U.S. weather supplemented the historical observations.
The results of the weather analysis and simulation will be published in Nature Geoscience.
Top Image: Sequence of the sun setting, silhouetting a lone windmill on plains in Colorado. (Ed Darack/Corbis)