Before and during sunset last night, there were some amazing halos visible around the region. Here are two views from Ellensburg, the first by Richard Wilson and the second by Tom and Barbara Jones, both taken near sunset.
Courtesy of Richard Wilson
Courtesy Tom and Barbara Jones
You start by noticing the semi-circular halo, located 22 degrees from the sun. But there is more….a sun pillar extending vertically from the setting sun. And perhaps most impressively of all, there is the upper tangent arc at the top of the halo, right about the sun (looks like an upside-down triangle or perhaps some wings on the halo).
Halos, sun pillars, and upper tangent arcs are all atmospheric optical effects associated with sunlight interacting with ice crystals in the atmosphere. Most halos are the result of cirrostratus clouds… high-level clouds made up of small ice crystals.
We can view the cirrostratus clouds from above, using a high-resolution satellite image at 7:51 PM (see below). The wispy ice clouds are clearly visible and if you very closely you can see lots of lines. These are contrails from jet aircraft, with contrails being enhanced when the atmosphere is near or at saturation (and to get clouds you need saturation–100% relative humidity).
Ice crystals cause a halo…a ring around the sun…by preferentially bending light by 22 degrees as the sun’s rays pass through the crystals (see schematics below from the University of Illinois),
The sun pillar is the result of falling ice crystals acting as little mirrors, something illustrated in the figure below. When larger ice crystals fall from the clouds, they tend to orient horizontally, reflecting the solar rays to your eyes in a way that produces a vertical column of light.
The origin of the tangent arcs at the top of the halo is more complex, they occur when sunlight passes through column-shaped crystals that are oriented horizontally. Light can pass through the columns in various ways, producing a fan-like pattern of light (see schematic below). A more detailed explanation is found here.
firstname.lastname@example.org (Cliff Mass Weather Blog)
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