Past IEM Features tagged: sigma

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Sigma Swings

21 Jan 2013 05:51 AM
Temperatures warmed nicely on Saturday into the 50s, placing us over a standard deviation from average for the date (called sigma). Temperatures have since crashed and will struggle in the single digits today, which less than negative one sigma from average. The featured chart looks at how often this change of going from at least a positive one sigma high temperature to lower than a negative one within three days. The top chart shows that January is the least frequent month for this event as warm air surges are the least common. The bottom chart presents the yearly total of these events and last year saw the second most at 12 days.

Good: 16
Bad: 5

Tags:   sigma   highs  

Four sigma weather!

22 Mar 2012 05:59 AM
There are many ways that one can express how different our recent weather has been from what we would expect to experience this time of year. Two such methods are shown in the featured graphic for daily high and low temperatures for New Hampton, Iowa. The top graphic is the simple departure in degrees Fahrenheit from the climatological average. High and low temperatures have been around 40 degrees warmer than average, but how exceptional is this? Another method is to compute the standard deviation of the distribution of temperatures for a given date around its long term mean. This value can be used to normalize the raw temperature departure shown in the top chart and is often called "sigma". So how exceptional is a four sigma value? It is roughly a temperature we'd expect to see every 40 years or so. The chart shows that for recent days, the low temperatures have been more exceptional than the highs.

Good: 18
Bad: 3

Tags:   sigma   mar12