From the Mailbox Today…

Most of the time when there’s a letter in the mailbox these days, it’s someone who isn’t happy about something. Today’s letter, though, probably has some material you’d be interested in if you watch our weather casts closely. I’ve had many teachers over the years that said things like “there’s no such thing as a stupid question” and “if you have a question, ask it, because someone else may be wondering the same thing.”

Following the letter, I’ve addressed each one of this person’s concerns:


1. Rain most assuredly can be measured to the 1/100th of an inch. A calibrated tipping bucket rain gauge is required to get this kind of measurement; standard tube rain gauges (the $10-or-less models at Wal-Mart and Lowe’s) only measure to the 1/10th inch. The tipping bucket is the NOAA standard for precipitation measurements.

Automated precipitation measuring with this type gauge has advantages and disadvantages, but it is the standard and works very well for liquid precipitation. It does not do very well with snow or ice, and that’s why snow is not measured to the 100th place; it is reported to the tenth’s place.

2. Who cares? Are you interested? What’s the value? The importance is something you learn in basic high school Chemistry or Physics: significant figures. The Purdue University defines it this way:

The number of significant figures in a measurement, such as 2.531, is equal to the number of digits that are known with some degree of confidence (2, 5, and 3) plus the last digit (1), which is an estimate or approximation. As we improve the sensitivity of the equipment used to make a measurement, the number of significant figures increases.

In short, the measurement is considered accurate in the scientific community, and that’s how we report it. In the days before the tipping bucket gauge, significant allowed the weather observer to estimate between the marks giving us readings like 0.25″ instead of the flat 0.2″ seen by reading only the marks.

Regarding the rainfall discrepancies from place to place, the measurement is different because a different amount of rain falls! Precipitation is not an even-handed sprinkling of water. There is a lot of chaos in the production of precipitation, and that chaos means that a thunderstorm could literally cause a flash flood in my neighborhood one mile east of Huntsville International while the rain gauge at the airport never measures a drop of rain. The measurements are taken where we have instruments; if it doesn’t rain there, it doesn’t get recorded.

There’s a flip side to this point of view: if we record only to the tenths place or the ones place (0.2 or just plan 0), how much of our annual rainfall total are we losing? If it rains 0.13, 0.14, 0.23, 0.11, 0.12 (that’s 0.73 in total), and we round to the tenths place, the total becomes 0.6 inches. Do that a few throughout the course of the year, and our annual rainfall climatology would show much less rain than we actually receive.

So who cares about that? What does it matter if you measure 42 or 52 inches, if it falls it falls, right? This is where it becomes your personal preference, and if you don’t like the way it is reported, contact your U.S. Representatives and have them pass legislation to stop NOAA’s practice of reporting to the hundreths place. Sounds a little extreme, doesn’t it? I know I don’t have time to worry about the fact that rain is reported to the hundreths place while temperature is rounded to the nearest one degree.

3. The other example. The writer did not make a #3, but I’m sure he/she didn’t mean to be inconsistent with the questioning.

Why do I have the gall to show a map that doesn’t have a rain chance rounded to the nearest ten or a precipitation forecast to the nearest one inch like these:

Because we have the technology to do it – that’s why we use these maps. The chance of rain (or more commonly known in the meteorological community – the POP…”probability of precipitation”) is usually expressed on a forecast graphic or in a discussion in 10% increments; however, the Weather Prediction Center (part of NOAA) has made available very specific data for use in describing that chance of rain. This is called PQPF data, and it is useful to us because it is another tool to help paint a picture of where rain is most likely.

Honestly, I am not a huge fan of having things like 12% and 42%, but those are the specific numbers that the ensemble model guidance gives us. In spite of that, the specific chance of a particular amount of rain is much more useful in describing how heavy the rain is than simply saying “there’s a 20% chance of a shower.” When it’s going to rain 5 inches in some places and none in others, this allows us to be more specific about who we believe will get the heaviest rain.

It is model guidance, and therefore it is not always accurate. When we think it’s incorrect, we can do some manipulation to correct it or choose to just not show it that day. If I show it, then I am confident in it’s prediction.

The chance of rain is a mystery to most people – even a lot of meteorologists. It’s there because people want it; we show the maps because we have had positive feedback. This is actually the first negativity we have had in over a year of producing those graphics.

The rainfall forecast is WPC’s QPF data; what you see in the map is actually a query of the gridded points. We never make the case that it will rain an exact amount; we always show this as an approximation and say things like “this is close to what you can expect on average in these areas.”

The writer’s statement about a 23% probability of 0.46″ in Downtown Huntsville shows that he/she is not listening to what is being said in conjunction with the maps. We never use that kind of specificity side by side.

Closing Statement:

As most folks around here know, my daughter turns one month old tomorrow (August 2nd). Her due date was June 23rd, and although my wife “measured” correctly according to the weeks of pregnancy, Shelby still didn’t come on the “due date.” Why is that? Because there are unknowns and uncertainties.

If a doctor cannot accurately predict the arrival of a child, should a due date even be used? Should the doctor just say, “well, it might happen sometime in the summer?”

This is an on-demand world where people want real answers about things that affect their lives. Most reasonable people understand that unknown factors can cause a prediction to change.

Weather forecasts have this uncertainty element displayed with a rain chance, temperatures, icons, graphics, and so on.

I could go on, but I’m sure you’ve stopped reading by now. If you haven’t, thanks for hanging with me, and I hope it was worth your time.

The writer has some valid points about the expressions in the forecast, but there is no “false accuracy.” A forecast is only accurate or inaccurate once the time period in question has passed. “False accuracy” is a bomb of a phrase in our business because it implies that we are dishonest people, and I for one am getting a little tired of having my character called into question over silly matters like a 20% chance of rain versus a 23% chance of rain. In your own words, “who cares?”

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