Thu Mar 22, 2012, 01:35 PM
hatrack (36,024 posts)
Volume Of Inquiries During "Summer In March" Heatwave Enough To Crash Parts Of NOAA Website
One measure of how record-breaking this "Summer in March" heat wave has been is the impact it had on NOAA's National Climatic Data Center web site. The extremes section of the their web site has been down since last Friday, since their software has been unable to handle both the huge number of records being set and the huge demand from people wanting to see these records. The web site came back on-line this morning with software re-engineered to handle the load, but only with data through Sunday.
We can also quantify how rare a meteorological event is by looking at statistics of past years. By averaging together at least 30 years of data to take a representative snapshot of the climate, we can generate a mean and a standard deviation of the data. The standard deviation gives a measure of how much the data fluctuates around the mean.
In comparing deviations from normal across wide regions, it helps to normalize the deviations. A temperature deviation of 3 degrees C may be not that unusual in one region, but may be very significant in another. The solution is to use climatological anomalies (which we often refer to by the Greek letter, sigma.) Calculating the climatological anomaly is a two step process. First, we calculate the difference between a quantity (i.e., temperature) and it's 30-year average value. Then we normalize the difference by dividing it with the 30-year standard deviation. From statistical theory, we know how unusual climatological anomalies are by value:
Odds of a deviation > 1 climatological anomaly=31.7%
Odds of a deviation > 2 climatological anomalies=4.5%
Odds of a deviation > 3 climatological anomalies=0.27%
Odds of a deviation > 4 climatological anomalies=6.34/1000%
Odds of a deviation > 5 climatological anomalies=5.7/100000%
Odds of a deviation > 6 climatological anomalies=1.9/1000000%
So, if we have a 30-year history of high temperatures for a particular date, we'd expect 20 of those years to be 1-sigma years, when the temperature is plus or minus 34% of average (ten colder years, and ten warmer years.) Rare 2-sigma events occur 4.5% of the time, so we should have about 16 of these per year. Even rarer 3-sigma events occur just 0.27% of the time, or just one day per year, on average. Truly extreme 4-sigma events should only occur once every 43 years. Much of Michigan, Wisconsin, Illinois, Minnesota, Northeast Iowa, and the eastern Dakota have experienced multiple 4-sigma days over the past week.
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