Monday, October 5, 2009

No Measurable Precipitation

Excel Math is near San Diego, an area of Southern California that is considered desert.

What's a desert? Not something sweet after dinner, but a landscape or region that receives almost no rain. Deserts are areas with an average annual precipitation of less than 250 millimetres (10 inches) per year.

Here is the recent rainfall history for downtown San Diego:

2004-2005  22.49 inches
2005-2006   5.42
2006-2007   3.85
2007-2008   7.25
2008-2009   9.15

The average over 20 years is 10.77, so actually that's a bit MORE than a desert would receive, but you can see that 2004-2005 was a BIT high.

It seems to me that it rains more here, but I keep seeing this quote No Measurable Precipitation in the paper and on the television. What is measurable precipitation anyway?

Is this rain?

Not according to our weatherman, it's not.

But if you ask a picnic-bound family, it is.

Is this rain?

Not according to our weatherman!

I was soaking wet!

It seemed like rain to me!

Surely this is rain, you can see the drops.

Yes, this was rain.

One-third of an inch fell on this date.

These constrasting opinions led me to ask how do we measure precipitation?

With a rain gauge [A before the U, please!] also known as a pluviometer from Latin pluvia, which means rain. It's a graduated cylinder that collects the falling rain.

Of course there are many types - one measures the weight of the water then infers the rainfall, one is completely electronic and spots the raindrops whizzing by, another counts them as they fall on a flat surface, one has a tipping bucket which sends a signal when it fills (then empties itself), etc.

Here's a standard rain gauge. A funnel attached to a graduated cylinder fits inside a larger container. If water overflows from the top of the graduated cylinder, the outside container will catch it so you can measure the overflow separately. The cylinder is marked in mm or inches. The gauge is on a vertical post away from trees or buildings.

Everyone uses the same sort of rain gauge, so results measured in one part of the country will be comparable to others. Of course, here in the desert, we have to worry about the water evaporating before we get out there to check the results, and in other parts of the country like Hawaii, you have to check frequently because of overflow.