PHOENIX — You've likely heard meteorologists say, "We have a 40% chance of rain." That may sound like a non-definitive forecast, but it's not.
Speaking from a meteorological viewpoint, the chance of rain is called "probability of precipitation" or "POP". If the area has a 40% POP, for example, it means 40% of the area will have measurable rainfall.
The numbers are derived from the last 10 times the same storm system went through that given area. Under those same conditions, 4 out of the last 10 times, there was measurable rainfall in any given area.
And that's another key element, it must be measurable rainfall.
Anything below 0.5mm in diameter is classified as immeasurable on the Doppler radar. Even if you see drops hitting the ground or can collect the rain in a cup, if the precipitation is classified as immeasurable while falling, there will not be any accumulated total for that area.
This means a "T" will be noted for the climate data in the rainfall category.
Sometimes raindrops are so large they split in half. Usually, that happens when the precipitation is larger than 4 millimeters.
Also, a raindrop does not look like what you see used as an icon, with a tapered top and rounded bottom. Instead, it resembles a shape more like a hamburger bun— flat on the bottom with a rounded top.
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