The spectacle of a double rainbow

Dear Tom,
On March 11, I saw a large rainbow with another one that was very dim surrounding it. Were my eyes playing tricks on me?

–Terry Sandhill
Dear Terry,
The spectacle of a double rainbow — so-called primary and secondary (or supernumerary) — is a grand sight and several were seen across northern Illinois and Indiana on March 11.
Rainbows form when sunlight passes into raindrops and, after reflection within the drops and refraction, leaves the raindrops and returns to our eyes separated into the colors of the spectrum. The color sequence in the primary rainbow, from the outside inward, is red, yellow, green and blue. A secondary, and fainter, rainbow forms above the primary when some of the light is reflected twice within each raindrop. The supernumerary color sequence is reversed because of the double reflection.

4PM Storm Update

Snow continues to fall across much of the Chicago area this afternoon.  The snow is not as intense, or as heavy as it was earlier today, and is mixing with rain over some southern suburbs. 

While some accumulation has been noted this afternoon, it appears that a good portion of the snow is melting, especially on road surfaces and sidewalks.

This Evening:   The snow or snow/rain mix is expected to decrease in intensity.  Any additional accumulation should be 1″ or less. 

Overnight:  The snow should taper to flurries or sprinkles.   Surface temperatures will fall below 32F away from Lake Michigan, leading to some re-freeze.   Be on the lookout for icy patches on sidewalks, secondary roads and bridges if you are going to be out and about tonight. 

A few recent snowfall totals:

3.0″  Cary

2.5″  De Kalb

1.5″  Batavia

1.1″  Chicago-O-Hare

1.1″  Chicago- Midway


12:30PM Storm Update

The steady snow that has encompaseed much of the Chicago area this morning will become more intermittent this afternoon as the atmospheric forcing that is driving the precipitation begins to weaken. 

North of I-80:   Total snow accumulation of 1″ to 3″ is expected on grassy areas/rooftops.   Pavement (sidewalks and roads) should remain generally wet with some slush.

South of I-80:  Little or no accumulation is expected (under 1″). 

Tonight:   All of the snow and rain should taper or end with little or no additional accumulation expected. 


Below:  The short term snowfall forecast from NCEP’s (National Center for Environmental Prediction) RUC model.  Note that the additional snowfall accumulation this afternoon across northern Illinois should be around 1″ or less, while the heaviest snows (perhaps in excess to 8″!) are expected to fall from Kansas City to Tulsa.  
snwtot_nc_12.pngGraphic courtesy of NOAA/NCEP 


Snow expected to continue through early to mid-afternoon

The latest radar trends coupled with new computer model guidance suggests the snow will continue across much of the Chicago area through early to mid-afternoon before the precipitation begins to taper off.

Snowfall totals across some northern and western suburbs may appraoch 4 inches before the snow ends. 

Most highways and roads remain wet to slushy, so travel has not been greatly affected thus far.

Here are a couple of recent snowfall totals as of 10:30AM

2.4″  Northbrook

2.0″  Arlington Heights

0.7″  Chicago-Midway

Below:  St. Michael’s School in south suburban Orland Park where little or no snow accumulation has occured through 11AM

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Below: Snow covers most of the ground at 11:00AM this morning at Hinsdale Middle School in Hinsdale, IL

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