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.

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Tags:   sigma   mar12