At this writing, the temperature is 91F here at the library. Such heat in April is presumably just a statistical anomaly. But apart from these weather fluctuations, which are expected from time to time, has our climate changed?
The traditional USDA zone map, last redrawn in 1990, was calculated using average annual minimum temperatures, based on the lowest temperatures recorded for each of the years 1974 to 1986. In 2003 the map went online with a bit of refinement, including the subdivision of many zones into two parts, one slightly colder and one slightly warmer. So zone 6, where we sat in 1990, was subsequently divided into 6a and 6b.
If you go here and put in the zip 20912, you will be told that you live in zone 6b. If you put in 20012, which is only 4 blocks from the library, you will get 7a. Clearly, according to the USDA map in current use, we are right on the USDA 6/7 border which would mean the average annual low temperature should be almost exactly zero F.
Confusing as that is, perhaps the map itself no longer describes our local climate.
- Try 20912 at most garden company sites and you will get zone USDA 6. example They have good reason to be conservative as they don't want their plants to fail.
- But here? 20912 gives zone 7.
- 20912 input here will give you 7/8. Eight? Don't get excited - their map puts us in the northern part of 7.
Why the variation? Because the Arbor Day Foundation proposes changes to the traditional map, arguing that some areas have warmed a whole zone. Look at their animation. First click play, then differences. Some web sites pull their data from the USDA zone map, others consider the Arbor Day calculations.
Sometime this year the USDA is expected to finally release a new map based on 30 year averages. This is likely to show a northward drift of the zones in many locations, as the Arbor Day maps currently do. (Some background on the Arbor Day map.) Additional explanations here and here.
And is average cold temperature, whatever it may be, even the most appropriate guide? You can also find your heat zone. And for 20912 it is .... 7.
Now 93F. Feels like zone 10.
Posted by library at April 26, 2009 03:57 PM