ICE?

Let's go fly!  ...maybe not.

The current METAR made me think twice:
KSWO 302253Z 03008KT 10SM BKN024 OVC030 M07/M13 A3033

Below freezing. Low clouds.  Should I worry about the aircraft accumulating ice?  I've read [1,2] that most aircraft icing occurs when air temperatures are between 0 °C and -20 °C and I wondered how difficult it would be to verify that claim.  Here's what I found.

First, I downloaded several hundred PIREPS from aviationweather.gov.  Then I filtered the PIREPS for those that reported ice along with the atmospheric temperature. 

For example, here the pilot is reporting rime ice with an atmospheric temperature of -13 °C: 
TYS UA /OV TYS/TM 1331/FL130/TP E145/TA M13/IC MOD RIME ICE/RM ICING STARTING AT 130 AND ENDING AT 115

Next, I constructed a histogram of the atmospheric temperatures with reported ice, and here is the result.  

What is amazing is that this plot is comprised of just 29 pilot reports and it agrees perfectly with the original premise that aircraft structural icing occurs most often between 0 °C and -20 °C.  That's good information to know before poking your head into cold clouds.

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References:
1. http://www.crh.noaa.gov/Image/lmk/Brian%20S/LMK_Icing_Show.pdf
2. https://www.faasafety.gov/gslac/ALC/course_content.aspx?cID=33&sID=152&preview=true
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