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Rain (Rainfall) Facts

RF Cafe - Weather Radar MapWhen you listen to the weather report and hear that there is a chance for light rain or that it is currently drizzling, there is actually a scientific method for defining the terminology. Now, if the person making the report is just a local broadcaster looking out the window to render an opinion, then the definitions listed below are not guaranteed to apply, but if the forecast is coming from a certified weatherman, then there is a pretty good chance that the number of drops per second per square foot really does fall within the ranges below.


Weather radar displays present rainfall intensity legends that report units of dBZ, which are decibels relative to "Z," with "Z" being the reflectivity factor.


  Number of Drops
(per ft2 per sec)
Diameter of Drops
(mm)
Intensity
(in. per hr.)
~dBZ
Cloudburst 113 2.85 4.0 60
Excessive Rain 76 2.4 1.6 50
Heavy Rain 46 2.05 0.6 40
Moderate Rain 46 1.6 0.15 30
Light Rain 26 1.24 0.04  
Drizzle 14 0.96 0.01 20
Mist 2,510 0.1 0.002 10
Fog 6,264,000 0.01 0.0005  


One interesting tidbit not included in the table above is that technical difference between rain and a shower. Rain refers to precipitation from stratiform clouds like stratus and altostratus, and tend to occur over a large area for a long time. Showers originate from cumulus and cumulonimbus clouds and cover a relatively small area for a short time.

So, when somebody remarks that, "we have had a ton of rain," just how much rain is that?
   Answer: 0.01 inches of rain over an acre of land equals 62,726 cubic inches, or 1.1 tons.

How fast does rain fall?
   Answer: Around 30 km per hour (18.6 mph), depending on the updraft.


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