Rainfall Feast Or Famine: Part 2
In my post late Sunday night about rainfall feasts and famines, I talked about the frequency of excessive rainfall and how it occurs in cycles and as part of a weather pattern. Part 2 talks about the feast or famine cycle taken to a monthly and annual level. Going back to 2001, rainfall in Philadelphia has taken a feast or famine mentality, almost on a monthly basis. Click on the chart below to see the month-by-month rainfall (blue) versus the 1971-2000 average (pink line).
In 87 months since January 2001, Philadelphia has been above average on rainfall 41 times and below average 46, which isn't a big deal when one looks at statistics as it breaks down to 47% of the months being above average and 53% of the months being below average. What was interesting however was how far the disparity from average was. Only 35 of those months have featured rainfall totals within 30% of average. That means that 52 months, or 60% of the time, we have seen rainfall that was generally more than an inch away from average, either above or below. As you can tell in the graph above in the most extreme cases we get rain in bucketfuls (August 2007, October 2005, June 2006) or we go dry such as we did in September 2005 and September 2007.
The wet and dry cycles tend to run together as well. For instance, five of the ten driest months over the last seven years to normal occurred in 2001. Wet months tend to group together as well. 2003, 2004, and 2006 each had two months in the ten wettest months so far this decade, mostly just a couple of months apart. This coincides with the "wet cycle" that I talked about where our area can enter into a rainy pattern with excessive rainfall occurring several times in a period of a few weeks.
Does climate change have something to do with the wet/dry cycles? Even when global cooling was all the rage in the 1970's we saw a three-to-four month period of time of excessive rainfall in late 1977 and early 1978, followed by a very dry period in the fall of 1978. Weather patterns can be influenced by the ever changing climate (global warming induced or not) and those atmospheric and climatic changes can impact the amount of rainfall that one area can receive. However, going back to 1872 we have seen very little change in the year to year trends in rainfall for Philadelphia, with the region generally averaging between 40 and 45 inches of rainfall (see the chart below) despite year to year variations in that rainfall total.
In 87 months since January 2001, Philadelphia has been above average on rainfall 41 times and below average 46, which isn't a big deal when one looks at statistics as it breaks down to 47% of the months being above average and 53% of the months being below average. What was interesting however was how far the disparity from average was. Only 35 of those months have featured rainfall totals within 30% of average. That means that 52 months, or 60% of the time, we have seen rainfall that was generally more than an inch away from average, either above or below. As you can tell in the graph above in the most extreme cases we get rain in bucketfuls (August 2007, October 2005, June 2006) or we go dry such as we did in September 2005 and September 2007.
The wet and dry cycles tend to run together as well. For instance, five of the ten driest months over the last seven years to normal occurred in 2001. Wet months tend to group together as well. 2003, 2004, and 2006 each had two months in the ten wettest months so far this decade, mostly just a couple of months apart. This coincides with the "wet cycle" that I talked about where our area can enter into a rainy pattern with excessive rainfall occurring several times in a period of a few weeks.Does climate change have something to do with the wet/dry cycles? Even when global cooling was all the rage in the 1970's we saw a three-to-four month period of time of excessive rainfall in late 1977 and early 1978, followed by a very dry period in the fall of 1978. Weather patterns can be influenced by the ever changing climate (global warming induced or not) and those atmospheric and climatic changes can impact the amount of rainfall that one area can receive. However, going back to 1872 we have seen very little change in the year to year trends in rainfall for Philadelphia, with the region generally averaging between 40 and 45 inches of rainfall (see the chart below) despite year to year variations in that rainfall total.
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written by Tom @ 4/08/2008 04:12:00 PM
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