Tag Archives: Atmospheric optics

There’s a halo in the sky!

Every so often, southern Arizona is treated to a huge ring surrounding the sun or moon.

Courtesy: Mark Blomquist

So what causes it? Let’s explore the world of optics. (I promise to keep it simple)

Courtesy: Univ. of Illinois
Courtesy: Univ. of Illinois

Hexagonal ice crystals are typically found in high level clouds like cirrus. These ice crystals refract sunlight or moonlight twice, netting a 22° shift from its original path. The net result is the brilliant 22° halo that catches the eye.

Courtesy: Georgia State University
Courtesy: Georgia State University

When the sun is out, the 22° halo can create an added bonus: sun dogs. When the hexagonal ice crystals line up horizontally with the sun, one to two bright beams develop on each side. Known technically as parhelions, these beams point vertically due to the sunlight passing through the flat-falling ice crystals. It’s typical to see a slight rainbow within the sun dogs due to the prism effect from the ice crystals.

Told you I’d keep it simple!


Why the Catalinas glow pink at sunset

Amazing atmospheric optics in action.

Often times during the year, the peaks of the Santa Catalina Mountains glow during dusk and dawn. It’s a phenomenon known as “alpenglow”. Here’s how it works.

Continue reading Why the Catalinas glow pink at sunset

Why the sky looked like this Friday night

Thanks to a unique-looking sunset, social media was buzzing Friday night.

Many News 4 Tucson viewers posted pictures of the eastern sky around sunset, which showed different colored rays of light. What appeared over the Old Pueblo is called anti-crepuscular rays. Here’s how they form.

As the sun was setting, the sunlight was being blocked by cloud cover in the western sky. This cloud cover created shadows, which were cast in the eastern horizon. Because sunlight appears to converges as it gets lower in the horizon, these parallel shafts of light took on the look of the Arizona state flag.

When these shadows appear in the same horizon as the sunrise or sunset, they are labeled crepuscular rays. Because Friday night’s shadows were showing in the opposite horizon as the sunset, they take on the anti-crepuscular labeling.

#WxGeekSpeak: Iridescent clouds

Every week, the blog introduces you to a technical term from the American Meteorology Society’s “Glossary of Meteorology”. Welcome to #WxGeekSpeak!

A recent viewer photo highlighted a rainbow in the clouds. How does this form sans rain? Click the video above to get a lesson in atmospheric optics.

Full definition of iridescent clouds: Ice-crystal clouds that exhibit brilliant spots or borders of colors, usually red and green, observed up to about 30° from the sun.

This irisation results from an optical diffraction phenomenon, usually of several orders. The cloud particles that occasion this phenomenon are very small (a few micrometers), and locally all are of nearly the same size; they result from local near adiabatic lifting and condensation in moist air often in a lenticular (wave) cloud, in pileus above a developing cumulus, or occasionally in irregular patches of uniform color in a region of shallow convection (in this case sometimes called mother-of-pearl cloud).

Wednesday’s Tucson Forecast: Fall perfection continues




Today – Sunny. 88°, west wind 5-10 MPH.

Tonight – Mostly clear. 59°, southeast wind 5-10 MPH.

Tomorrow – Mostly sunny. 87°, northwest wind 5-10 MPH.


Another lovely Autumn afternoon in store, but a few wrinkles await in the forecast.

Highs reach the upper 80s under sunny skies today. Some high cloud cover drifts in late in the day, with mostly clear skies prevailing overnight. Lows bottom out in the upper 50s for much of Metro Tucson.

Clouds continue to increase tomorrow and Friday, ahead of a storm system near Baja California. This feature kicks up gusty winds this weekend and may produce a few showers on Saturday. Highs are expected to hover around the 90° mark through Sunday, while nighttime lows drop to the 60s.

Models hint at a quiet weather pattern unfolding early next week. Afternoon temps peak in the low 90s Monday and Tuesday.



Morning greetings, captured Monday by Doug & Doris Evans. Classic state flag sunrise, created by crepuscular rays.

#WxGeekSpeak: Autumnal Equinox

Every week, the blog introduces you to a technical term from the American Meteorology Society’s “Glossary of Meteorology”. Welcome to #WxGeekSpeak!

What does Autumnal Equinox really mean? Why don’t we have exactly 12 hours of sunlight today? Click the video above for the full details on the first day of Fall.

Full definition of Autumnal Equinox: The equinox (approximately 22 September) at which the sun crosses the celestial equator from the Northern Hemisphere to the Southern Hemisphere.

Tuesday’s Tucson Forecast: Isolated hope for Monsoon thunder




Today – 10% chance of storms. 103°, northeast wind 5-15 MPH.

Tonight – Mostly clear. 78°, southeast wind 10 MPH.

Tomorrow – Mostly sunny. 102°, northeast wind 5-15 MPH.


Slight storm chances return this afternoon.

Metro Tucson stands a 10% chance of storms between 3-7 PM. Favored areas for isolated storm development today include Pinal, Graham and Greenlee Counties. While most of eastern Pima County stays dry, daytime highs reach triple digit territory for the 53rd time this year.

Dry air wins again tomorrow, as isolated mountain storms are expected. A stalled storm system off southern California pulls in better Monsoon moisture later in the week. This returns a daily chance of isolated storms to the Old Pueblo Thursday into early next week.

As moisture increases, daytime highs drop to the upper 90s heading into the weekend.



Rays from the Rincons, captured Monday evening by Jack Challem. These are called anticrepuscular rays. Parallel shafts of sunlight stretch across the sky as the sun sets, with the dark shadows caused be either clouds or mountains.

These rays also appear in the western sky at sunset, known as crepuscular rays.