Here in Vermont, when we think about significant rain events, Hurricane Irene comes to mind. Compared to Houston’s Harvey, Irene’s statistics are modest. Only about 11 inches of rain in a day or so. But here the water is concentrated in the streams between the ancient hills and mountains. That’s where the roads were built and most of the towns and villages were established. So water velocity becomes impressive and a lot of earth is moved down stream. If we had the kind of rain Houston is having, with the exception of those of us who live on hills, Vermont would have to start from scratch.
On average Houston and Vermont get about the same amount of rain a year, 45 to 47 inches. But when the water hits the ground in Houston, it behaves differently. There are no rocky streams or deep valleys for the water to follow. In Harris County, where Houston has been built, the land is flat. Houston’s 667 square miles were originally swampland laced with bayous. Houston now has 22 square miles of water and 86 of parks. The rest is paved roads (including 739.3 miles of freeways and expressways in the metropolitan area on which 71.7% of its residents drive to work alone in their cars), parking lots and air-conditioned buildings, laced with bayous and man-made water courses. So there is no easy way for the water to get out.
In 2001 parts of Houston received up to 40 inches of rain, causing what was then the worst flooding ever known in that city. In August 2005 2.5 million people, the largest evacuation in U.S. history, left Houston in anticipation of Hurricane Rita, though little damage occurred. Reports are that parts of Houston will receive a total of up to 50 Harvey inches. Perhaps because of the embarrassingly unnecessary evacuation in 2005, there was no mass evacuation this time.
For those interested in statistics about rain on Houston: Large raindrops are roughly 5 mm (a little less than ¼ inch) in diameter. They can get as large as 9 mm, but above 5 mm they tend to break up into smaller drops due to air resistance. The larger they are, the faster they fall. The terminal velocity of a large raindrop is about 9 meters per second (30 ft./second – 20 miles per hour.) Based on estimates of an average of 30 inches of rain falling on Houston over the period of Harvey’s visit there I have worked it out that approximately 1,982,190,000,000,000 (nearly 2 quadrillion) large raindrops will have fallen on Houston’s 667 square miles. This amounts to 3,473,000,000 gallons or 14.5 million tons of water.
As to the effect of climate change, increased temperatures result in increased evaporation and therefore more rain. Of course the effects vary. Most of the increase is between 30 and 50 degrees north latitude. (Houston is nearly at the 30 degree line.) Higher temperatures in dry areas simply make them dryer. In the United States annual precipitation has increased by about 6% since 1900. In the American south the increase has been 11%. Heavy downpours have increased overall, with parts of Texas experiencing an increase of 700% compared to the decade of the 1950s.
A few years ago Fieldstone Publishing created The National Audubon Society Field Guide to Weather. There are now more than 500,000 copies in print. That doesn’t make me an expert, but it gives Fieldstone a foothold in this increasingly wet subject. (You can purchase a copy here.)
We at Fieldstone Publishing extend our thoughts and support to the people and communities of coastal Texas and Louisiana at this time of unprecedented crisis.
Yours From The Field,