You know, the year that was before AD 1 and after 1 BC.

Well, there wasn’t one.

There is no Year Zero. Our modern calendar starts with Year One. Anno Domini means “in the Year of Our Lord” so it marks the birth of Jesus of Nazareth. Whether Christ was actually born in that year is irrelevant. A 6th-century scholar named Dionysius Exigenus created the Anno Domini system and most of the modern world uses that marking point. Nowadays we call it the Common Era as opposed to the Christian Era, so we say CE 1 and 1 BCE (Before the Common Era), but the starting place is the same. The Romans would have called that year 754 AUC. That stands for ab urbe condita or “from the founding of the city.”

So if Year One was the first, and there was no Year Zero, when did the first decade end? Year Ten, of course. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. That’s ten years, that’s a decade.

So that means the second decade started in Year Eleven. And went for ten years: 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. It ended in Year Twenty.

I’m sure you see where I’m going. The first decade was AD/CE 1-10 and the second was AD/CE 11-20. Then 21-30, 31-40, 41-50, etc. There’s a rule: decades start on years with a “one” at the end and they stop with years with a “zero” on the end. That means this current decade started on 01 January 2011 and will end on 31 December 2020.

But people don’t like that scheme. We are much happier to start our counting at zero and end it at nine. Year 2010 starts the twenty-tens or the twenty-teens or whatever it’s called and Year 2019 ends it. Hence all the “Best of the Decade” lists coming out.

I’m not sure why people like that kind of counting better. Maybe it’s the digits. From 2010 to 2019 you only change the ones place. With 2020 you have to change the tens place as well. Perhaps it is more intuitive to visualize a decade that way, flipping over one number at a time until you run out.

There’s nothing wrong with starting at zero when counting. It’s the same number of leaps, the same number of things, you are just using a different numeral to represent the stopping point.

So if people want to count decades from 0-9 instead of 1-10 that’s OK. The only confusion is for those folks from the first decade (CE/AD 1-10). They have to be a nine-year decade, a nonade or novemade or something. Since there is no Year Zero they go 1, 2, 3, 4, 5, 6, 7, 8, 9 and call it good. In Year Nine they would publish their “Best of the Nonade” (the one-and-only nonade) lists. Then we could get on track and call Year Ten the start of the second decade (10-19) and it will all dovetail nicely with our 2019 end-of-the-decade stuff.

Whether you are a pedantic scholar and refuse to celebrate the End of the Decade until next year, or a party animal who loves those lists and can’t wait to ring in The New Decade, I hope you have a wonderful New Year in CE/AD 2020!

I used to tell my students that a weather forecast was pretty simple. If it said “40% chance of rain tomorrow” that meant if ten of them went outside, four of them would get wet. They were usually skeptical of this interpretation, to which I give them a lot of credit. After all, they would hear stuff from teachers all day long, and of the stuff they actually listened to, much of it was bullshit. That’s just the nature of schooling: a lot of bullshit gets spread around. Humans are a bullshitting species—we can’t help ourselves.

But it always got me thinking about what such a weather forecast actually meant. Statements of probability are attractive because they are unambiguous. Or ought to be. “It might rain tomorrow” is not very useful. “There’s a really good chance it will rain tomorrow” is a little better.

People may prefer “it will” or “it will not” rain, but hardly anyone is ever that certain. Besides, life is unpredictable. We know this. Probabilities are the best we can do.

So what does a “40% chance of rain tomorrow” really mean? Does it mean 40% of the area will be rained upon? All forecasts are organized by areas, so that seems a reasonable take. Perhaps it means it will rain 40% of the time. So in a 24-hour day you’d get 9.6 hours of rain. I don’t like that one, and I’m not sure why, but I could see someone interpreting it that way.

I would always follow up my initial foray into probabilities with another version of “40% chance of rain tomorrow.” I told my students that if we could replay the day 10,000 times it would have rained in 4000 of them.

That’s a little whimsical, we get to play god and mess with time, but computer simulations allow us to think like that. You have to check your models against nature, so you better go back and see how well they did!

Turns out the National Weather Service has an official definition for this so we don’t have to fret (PoP is Probability of Precipitation):

Mathematically, PoP is defined as follows: PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measureable precipitation, if it occurs at all. So… in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = “C” x “A” or “1” times “.4” which equals .4 or 40%.) But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. ) In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.

I suspect a lot of folks will find that unsatisfying, but this mathematical view is at least a precise definition. And it seems to cover both the “area” part and the “time” part.

So what does it all really mean? I think a bunch of meteorologists get together at lunch and look out the window and argue about whether or not it will rain tomorrow. Finally they agree to state it as a probability, and experience tells them how often they’ve been wrong. So “40% chance of rain tomorrow” really means “we get this right four out of ten times!”