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Posted at 10:30 AM ET, 09/15/2009

An Inch of Rain - What Does it Mean?

By Don Lipman
A rain gauge. Image courtesy NASA.

Although the definition of "an inch of rain" sounds simple enough--rainfall totaling one inch in some sort of measuring cup--what an inch of rain really stands for is sometimes not always clear. When I ask people if they think an inch of rain is "a lot of rain" or only "a little rain," I usually get both answers. And actually, both are right.

If it falls in, say, just 15 minutes, an inch of rain is a real gulley-washer or, as southerners say, a "frog strangler." Basements flood, streams overflow, and roads can sometimes become impassable in such a "flash flood." This type of downpour is of little value to vegetation. In fact, it does more harm than good since it washes a lot of good topsoil away. In this case, most people would probably agree that a lot of rain has fallen.

On the other hand, if the same amount of rain falls over a 24-hour period, that would be a "good soaker," greatly aiding vegetation, as ideally, growing things need about an inch of rain per week during the growing season. In this instance, many would probably say that a decent, beneficial rainstorm had occurred.

Looking at it in still other ways, if that same inch of rain were to fall: (1) evenly over Washington, D.C., about one billion gallons would have fallen or (2) evenly over a 10-acre farm, about 270,000 gallons would have fallen, obviously a considerable amount of water for farmers and everyone else.

But let's say that the inch of rain took one month--or even a year--to fall. (In parts of South America's Atacama Desert, one inch of rain doesn't fall in ten years.) With normal evaporation and intervals of sunshine, an inch of rain under these circumstances would be of little value here except maybe for the hardiest of plants. Elsewhere, such as the desert southwest where there's only 10-15 inches of rain annually, it's a different story. Plants have adapted to scoop up and store every drop that falls. And the human population depends largely on underground aquifers (rapidly diminishing), Colorado River diversion, etc. So, at least for now, the large cities like Tucson are surviving, but with global warming, the deserts are certainly under attack by Mother Nature.

To complicate matters even further, during winter in the snow belts, it's even more important than usual for forecasters to estimate the amount of "precipitable moisture" in the clouds or, as it's called, the QPF (quantitative precipitation forecast). If we were to hear a September forecast calling for one inch of rain during the next 24-hours but instead only half that amount falls, I don't think most people (except for meteorologists) would notice--or care. On the other hand, in January, when snow is expected, that same underestimate could result in the difference between 20 inches of snow (if it's cold enough) or as little as 5 inches (when surface air temps are just above freezing). Obviously such an underestimate can make a huge difference in the amount of disruption to a metropolitan area, with the corresponding overexpenditures on stand-by snow clearing crews, etc.

So what was the wettest calendar day in local weather records, you ask (at least at Reagan National Airport or prior DC official measuring stations)? According to PRESTO, the National Weather Service' s "PREcipitation Summary and Temperature Observations" publication for the Washington, DC & Baltimore, MD Area," it was June 21st, 1972 when Tropical Storm Agnes inundated us with 7.19 inches of rain. (I suppose people were saying on that day that it was really raining "cats and dogs."*) It's possible that there's been a wetter 24-hour period, however.

Just for good measure, the snowiest calendar day was probably January 28th, 1922 when the so-called Knickerbocker Storm dumped 21 inches of the white stuff, to be followed by 7 more inches the next day--more than the past 3 winters total snowfall combined!

*Why do we use that phrase (rain like "cats and dogs")? One theory, among others, is that in Old England, particularly in the poorer sections of London, people used thatch to roof their houses. Stray animals, including dogs and cats, tended to congregate on these thatched roofs for the residual warmth. But every now and then, heavy rainfall would cause the thatch to give way and--you guessed it--the dogs and cats would come crashing down on the folks below. Thus, it was raining "cats and dogs."

By Don Lipman  | September 15, 2009; 10:30 AM ET
Categories:  Education, Lipman, Local Climate  
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Interesting how an inch of rain is perceived in different areas of the country (or the world).

Soil type has a lot to do with what actually happens to that inch. Clay soil is harder for water to penetrate, but once it's saturated, retains water for a long time (which can lead to root rot in crops). Sand drains water very well, but doesn't retain it. Impervious surfaces drain rain water into storm drains so quickly that we often don't see it, unless there's a very heavy rainfall.

Plants in different ecosystems absorb water at different rates, too, as you mentioned. I find desert plant adaptations fascinating, in that they have shallow (but horizontally far-reaching) root systems to absorb as much water as possible as quickly as possible and waxy leaves to prevent water loss.

The type of rain gauge used also makes a difference in measuring that inch. I believe that the standard gauges have an 8 inch diameter.

Posted by: Ann-CapitalWeatherGang | September 15, 2009 6:33 PM | Report abuse

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