Saturday, March 8, 2014

Normal Weather?

Meteorologists, TV weathermen, and state and local water planners have convinced the general public that there is something called "Normal Weather." The implication of this is that anything other than "Normal Weather" is "Abnormal." But there is no such thing as normal weather in the sense that most people think of normal. Indeed, what meteorologists mean when they say normal is AVERAGE. "Average Weather" makes perfect sense, but unfortunately the general public is so stuck on normal and this terminology is so ingrained in the media that it is unlikely to change. To give due credit to many professional meteorologists and meteorological organizations, they have tried to use the term average instead of normal, but on TV and in newspapers normal is still used almost exclusively.

The problem with using NORMAL is that the general public expects that temperature or rainfall or number of hurricanes or other weather phenomena should, most of the time, fall into the normal category. This is simply not the case. For a given month, day, or year to actually hit right onto the value touted in the media as normal is a fairly uncommon event. Most days, months, and years the temperatures and rainfall are either less than or more than the normal value. And yet the media continue to broadcast these statistics as if we should be getting the normal amount.

In speaking of the recent storms that have helped a bit with the water situation in California, one web site stated: "Thanks to the most recent rounds of rain and snow throughout California, Los Angeles and San Francisco both received at least 86 percent of their normal rainfall for the month of February. However, farther south, San Diego only received 44 percent of the city's normal precipitation for February."

It sounds from these reports that 86 percent of normal or 44 percent of normal is abnormal, like California deserves to at least get a normal amount of precipitation and if they get anything less or more then someone has really messed up. But how likely is it that California will get only 86 or 44 percent of "normal" precipitation? Meteorologists like to calculate a measure called the Coefficient of Variation to help determine this likelihood. The coefficient of variation is related to the standard deviation of rainfall at a particular station and in essence tells us that if the coefficient is 0.24, for example, then at that station the average rainfall would be expected to vary by 24% in 67% of the years (1 standard deviation on a bell curve). This coefficient varies at different locations across the state of California, but here is a list of a few places provided by another web site:

Station in California (Climate Zone)
Coefficients of Variation
Klamath (Marine West Coast)
Eureka (Marine West Coast)
Fort Bragg (Marine West Coast)
Chico (Mediterranean)
San Francisco (Mediterranean)
Monterey (Mediterranean)
Spreckels (Mediterranean)
Santa Barbara (Mediterranean)
Los Angeles (Mediterranean)
San Diego (Mediterranean)

So in Los Angeles, rainfall will vary by up to 47% from average 67% of the time. It also means that 33% of the time (one-third of the years) rainfall will vary by more that 47%. Since Los Angeles received 86% of it's average rainfall in February, then it is only 14% from its average value and well within the coefficient of variation.

I don't suppose this is a battle I will ever win. The media will continue to use normal instead of average, water planners will continue to use normal in their future plans, and the general public will continue to expect "normal" to happen and be disappointed when most of the time it does not. But perhaps a few will realize that if we expect normal to happen, we are fooling ourselves. 

1 comment:

Karen said...

THANK YOU! For putting so clearly what I have been trying to tell people for years!!!! It's just an average!