Tuesday, February 3, 2009
A dry winter?
Several of you have suggested that this winter has seemed unusually dry. Is this true? It turns out that the answer is a bit complicated.
If you look at the precipitation since October 1 (known as the beginning of the water year), we have been only a little bit drier than normal. For example at Seattle we have "enjoyed" 18.19 inches since that date, while normal is 20.32 inches. Thus, we have been 2.13 inches below normal, which is really no big deal...and well within the range of normal variation. So is this an usually dry year overall? Not really by this measure.
But there is something else. Have we had an unusual number of dry days? The answer to this question is different. Look at the attached plot of precipitation this fall and winter--blue is normal and red is this year. At the end (now), the blue is above the red by about two inches..consistent with the numbers I give you above. But the really interesting part is how we got the wet stuff. Most of it occurred in relatively short, very intense periods...one in early November and the other in early January. Much of the fall and winter was characterized by extended dry periods... like the one we are in now. The only period with longer term continuous precipitation was during the snow of mid to late December...and snow just doesn't seem as wet as rain does it? I suspect that if we compared the number of dry days this winter with the normal situation, we would find that this year so far was highly unusual regarding the large number of dry days. Perhaps one of you would like to track those numbers down. Perhaps it will make you feel better that you missed out that trip to Hawaii.
Posted by Cliff Mass at 5:34 PM