The sky was 'smiling over North Idaho' on May 24
While Sharon and I and our two poodles, 'Sholeh' and 'Genny' were enjoying the beaches at Newport, Ore., this past week, a 'rare' meteorological phenomenon was occurring in the skies above North Idaho.
According to Mel and Beverly Knutson of Post Falls, at 6 p.m. on May 24, there appeared an 'upside-down' rainbow in the sky above them. They came by the house on Wednesday to show me the unusual photo that Beverly took on her cell phone of the 'circumzenithal arc.'
Mel said that this so-called CZA event "almost looked like a comet blazing through the skies." The 'red' at the bottom of the rainbow was "overwhelming." This rainbow faded a bit to a 'pinkish tinge' that made the sky look as if it were "on fire," according to Mel.
Just what is a 'circumzenithal arc' (CZA)?
According to the definition from Creative Commons, the circumzenithal arc or circumzenith arc (CZA), also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow; but it arises from refraction of sunlight through horizontally-oriented ice crystals, generally in cirrus clouds, rather than from raindrops. It forms no more than one-quarter of a circle centered on the zenith and on the same side as the sun. Its colors are from blue on the inside to red on the outside of the arc. It is one of the brightest and most colorful halos. Its colors are purer than those of the rainbow because there is much less color overlap in its formation. The first impression is that of an upside-down rainbow.
The circumzenithal arc has been called "a smile in the sky." It is rarely noticed because it occurs so far overhead. Cirrus clouds which cause a sun dog can cause a circumzenithal arc when they reach the zenith, if the sun is low in the sky.
Sharon and I saw a huge SUN DOG in the skies over eastern Washington on our way to the Newport beaches on May 18.
The light that forms the CZA enters an ice crystal through its flat top face, and exits through a side prism face. The refraction of almost parallel sunlight through what is essentially a 90-degree prism accounts for the wide color separation and the purity of color. The CZA can only form when the sun is at an altitude lower than 32.2 degrees. The CZA is brightest when the sun is at 22 degrees above the horizon, which causes sunlight to enter and exit the crystals at the minimum deviation angle; then it is also about 22 degrees in radius, 3 degrees in width. The CZA radius varies between 32.2 degrees and 0 degrees depending on the solar altitude. Toward either of the extremes it is vanishingly faint. When the sun is above 32.2 degrees, light exits the crystals through the bottom face instead, to contribute to the almost colorless parhelic circle.
Next week in 'Gems,' I'll have a California drought update.
NORTH IDAHO WEATHER REVIEW AND LONG-RANGE OUTLOOKS:
May proved to be a delightful month of 'sun and showers' and fairly warm temperatures with only a couple of frosty mornings that required the overnight covering of sensitive plants.
As of this Thursday morning writing, May 29, we had measured just 1.31 inches of precipitation during the month, compared to the 119-year normal since 1895 of 2.37 inches and last year's May rainfall total of 1.91 inches.
May's last couple of afternoons on May 30 and 31 were expected to be sunny and warm with highs in the upper 70s and lower 80s. This nice weather was expected to last until the second week of June when more showers and thunderstorms may occur across North Idaho and the Inland Empire.
Following these showers in early to mid June, we should see much drier and warmer conditions across the Inland Northwest. Afternoon high temperatures may reach the lower 90s by the end of this month.
As far as the weather conditions for the upcoming summer of 2014 are concerned, thanks to a stationary ridge of high pressure camped over the region, we should see mostly dry and warm to hot weather between early July and mid September, almost 'perfect' for outdoor activities, provided one wears a hat and lots of protective sunscreen.
Following nice weather for this year's edition of the North Idaho Far and Rodeo in late August, the upcoming fall of 2014 could turn wetter than usual, if the new El Nino gets rolling in the warming waters of the eastern Pacific Ocean. Stay tuned for further long-range weather updates.
Cliff Harris is a climatologist who writes a weekly column for The Press. His opinions are his own. Email sfharris@roadrunner.com
Weekly Weather Almanac
• Week's warmest temperature: 78 degrees on May 30
• Week's coldest temperature: 39 degrees on May 30
• Weekly precipitation: 0.30 inches n May Total: 1.31
• Precipitation month to date: 0.00 inches
• Normal precipitation month to date: 0.06 inches
• Precipitation month to date last year: 0.00 inches
• Precipitation year to date: 16.45 inches
• Normal precipitation year to date: 12.08 inches
• Precipitation last year to date: 11.25 inches
• Normal annual precipitation: 26.77 inches
• Total precipitation last year: 25.93 inches
• Precipitation predicted this year: 33.70 inches
• Wettest month on record (since 1895): 9.91 inches in December 1933
• Wettest year on record (since 1895): 43.27 inches in 2012
• Driest month on record (since 1895): 0.00 inches (14 times)
• Driest year on record (since 1895): 15.18 inches in 1929
Readings taken week ending 3 p.m. Sunday, June 1