Date: Fall 2011
I am having a disagreement with my buddy over rainfall. When I returned to Pennsylvania from a bow hunt in Wyoming I sat my cooler which was empty, outside in the level yard with the lid open facing up. Nothing overhead but sky. After the rain was over, the cooler was filled and slightly running over. The coolers dimensions are 16"deep, 20"long and 16"wide. I said we had 16" of rain. He said no way. The weatherman said York county Pennsylvania had 11.5" of rain. I said that was an average and some had more. He said that is not correct since rain must be measured in an actual rain gauge. I disagree. If I would have had the cooler filled with rain gauges they all would have measured 16". I asked the physics teacher in my high school and he said I was right and had an excellent analogy. How much rain did we have at our house according to my cooler and am I correct in my thinking?
I do agree with you. Assuming the rain is falling vertically and this is a typical kind of rectangular cooler with vertical sides, the volume of rain captured will be directly proportional to the surface area of the cooler top. However, the height of the volume of rain water in the cooler will be inversely proportional to the surface area of the cooler top. Therefore, the surface area cancels out.
All rain gauges do have to be calibrated for sources of error. Error can also be incurred due to where it is located and if anything around it or above it is influencing the path of the falling rain. Assuming these errors do not play a role, I think you are on solid ground saying you received around 16" of rain. Give or take :)
John C. Strong
Rainfall will vary locally, so you could easily have different rainfall than a location reported just a few miles a way. Rain gauges only amplify the signal to get very accurate measurements by collecting over a larger area and measuring over a smaller cross sectional area.
The only thing that might make your cooler measurement seem higher than the actual rainfall is if the lid doesn't come all the way off. Rain might fall on that as well and dribble into the cooler or splash off the lid, or gusts of wind could blow some water from the lid to the cooler even if it opens 180 degrees to be level with the top of the cooler opening.
If it is an open cooler with nothing (no lid) above or level with the opening nearby, then your measurement should be pretty good.
The 11.5 inch figure was at the site of measurement, usually the
local airport or a similar site. I am a little concerned about the
top of the cooler deflecting rain into the cooler if the wind were
in the correct direction and the top at an angle, but I think you
could safely say you had more than the reported amount of rain at your cooler.
In any case, we could use some of those inches out here especially
in western Oklahoma and Texas.
It seems to me that since your cooler was overflowing that you actually had
more than 16" of rain, but we do not know how much more.
Otherwise your logic is correct. If your cooler had 11" of rain in it then
you could say you had 11" of rain.
National Weather Service (NWS) rain measurements are taken at specific
spots, mostly at places like airports.
But there are variations to the rain pattern that cannot be accounted for
100%. Their readings are more like point samples in an infinite population
of specific locations.
For example, here in Virginia it may be raining across the street, but my
property gets absolutely nothing.
It is just a matter of where the cloud that is dropping rain is physically
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Update: June 2012