Wet or Dry, Rainfall Varies More Than You May Think
Every time I drive across the country I think about the vast differences in water availability that exist from one side of the country to the other. It’s especially noticeable when driving west on Interstates 10, 20 and 40. These routes begin in the well-watered Deep South, cutting through dense forests as they head west to the Mississippi River and into the plains states beyond.
Once Texas, Oklahoma, or Kansas are reached, the forest is quickly left behind. The pine trees change over to treeless plains in little over 50 miles. I-20 shows this the most dramatically in northeastern Texas where it emerges from the hilly Piney Woods into the tallgrass prairies just each of Dallas.
On the other side of the Dallas-Fort Worth area, another dramatic change occurs. The prairie quickly gives way to mesquite country – a landscape dominated by that thorny tree and littered with prickly pear cactus. Creeks become washes, and green becomes brown by the time you get to Abilene.
I created this map to better understand this transition that I have wondered about so many times before. It is clearly shown in the center of the map – rainfall averages plunge from the mid-50s in the lower Mississippi Valley to around 20 inches in central Texas. 20 inches seems to mark the start of the “dry country,” the West.
Through the process, I found that this east-west transition wasn’t actually the steepest change on the map. Both sides of the country have areas with widely varying rainfall rates within a few short miles of one another.
Notice the border area of Georgia and North Carolina. Rainfall increases from 40 inches in the Carolina Piedmont to almost 90 inches in the southern Blue Ridge Mountains, barely 50 miles away. Similar transitions occur in the New England Mountains of Vermont and New Hampshire.
Most dramatically, the West has the steepest variations between really wet and really dry areas to be found in the Lower 48.
Look at Southern California. Home to the driest desert areas in the country with around 3 inches (or less) per year, areas with around 40 inches of rainfall are found just a short 15 miles away. The Southwest has a lot of areas that illustrate the mountain lifting and rain shadow effects well.
The Pacific Northwest has the highest rainfall in the Lower 48 and also some of the most unbelievable transitions.
The Coast Ranges in Oregon, Washington, and California receive upwards of 190 inches in areas. The Cascades are similarly rainy. Move from Cannon Beach, Oregon to Portland to Baker City and the rainfall goes from the high 100s down to 43 at Portland, then up to the lower 100s around Mount Hood, and then it plunges to around 10 as you head east of the Cascades. All of this happens in 125 miles.
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