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Standing downwind from Japan's damaged nuclear reactors, its understandable that some Americans have turned their nuclear radiation panic dials to "11."  Misinformation published on the Internet has only compounded the problem.  Thankfully, The New York Times has published an animated computer model that predicts "extremely minor health consequences."  And now that we have a weather prediction, all we need is a real-time radiation weather map.

Unfortunately, there is no such thing as a "radiation weather map."  However, near-real time reports of current radiation levels at various points in the United States are available from:

• RadNet, at the Environmental Protection Agency's (EPA) Central Data Exchange (CDX)  website
• The private, publicly-driven Radiation Network
• The private, publicly-driven Online Geiger Counter Nuclear Radiation Detector Map at Black Cat Systems

The first requires an elaborate registration process.  The others are immediately available but provide no historical data to suggest natural radiation levels and useful trends.  As such, some visitors are panicked by the sites' reports of innocuous radiation levels.

According to the aforementioned animated computer model, the Sierra Mountains (where I live, thank you) are likely to see the first and strongest detectable traces of radioactive fallout, should there be any.  Unfortunately, none of these radiation tracking services collects data from the Sierra Mountains.  A RadNet installation at Reno, Nevada is presently offline.

With historical data from the CDX, I've charted ambient radiation levels for San Francisco and Sacramento in Microsoft's Excel, presented below.  Data prior to March 11th  offers a baseline of typical radiation levels and daily fluctuations.  If you've gotten yourself into RadNet and would like to roll your own, perhaps for another city, be sure to format the X-axis (date and time), with axis type "Text axis."  By default, Microsoft Excel determines axis types by data but discards time values from date/time strings, making a mess of things.  Fortunately, the CDX data occurs at regular 1-hour intervals and may therefore be safely treated as text values without skewing the time relationship between chart points.

Click on either of these images to reveal a full-size view.  Images last updated 3/19/2011 4:23 PM PST.  Returning visitors, be sure to refresh your browser cache.  No longer updated - please see "UPDATE" and comments, below.

To read the charts, understand that the CDX divides the radiation spectrum into ten contiguous ranges for analysis.  Each has its own natural range and danger level.  Not being a nuclear physicist, I can't detail those with any professionalism and have found no reliable references online.  According to some sources, we should be concerned only with ranges 9 and 10, which both have a natural occurrence of perhaps 50 radiation counts per minute (CPM) and become unhealthy at around 100 CPM.  Again, these are only suggestions gleaned from unreliable sources.  RadNet offers no suggestion of danger levels and recommends that we simply monitor the values for trends against historical data.  I will try to update this chart daily until this situation settles, and more frequently if trends change or if web stats indicate more than a mild interest in this blog post.

I hope you find this information helpful.  In closing, let me offer my sincerest condolences to the people of Japan and to everyone affected by this horrible tragedy.  The Japanese people have demonstrated to the rest of the world an incredible level of honor and selflessness and deserve our greatest respect and support.

UPDATE:  By popular demand, RadNet has published very similar charts for most West Coast stations, with the advantages of a logarithmic scale, automatic updating, and thorough background information to answer most common questions.  Presented below are their current renderings for Sacramento, San Francisco, Los Angeles and Honolulu.  For other cities, visit RadNet Air Monitoring Data from the U.S. West Coast.

What remains unanswered?  It would be nice to find an authoritative explanation of the specific radioactive nuclides relevant to this event, their respective charted ranges, and their accordant danger levels.  It would also be nice to see the RadNet station at Reno, Nevada back online.  While not ideally situated behind the Sierra, it nonetheless stands to receive the earliest and highest radiation readings in the country and could serve as the nationwide barometer, in my untrained opinion.  [Better still would be monitors at altitude in the northwestern Sierra.]  My sincere thanks go to the folks at RadNet for producing these charts.