There are four of us stuck in our bubble called home and last night during dinner we got into a discussion about the meaning of weather forecasts wherein the forecast states a percentage chance of rain in a particular area at a particular time. Here, as an example, is the BBC’s weather forecast for today in my area:
The forecast says there is a 27% chance of rain at 3 o’clock this afternoon. What does the 27% mean? Last night, I maintained that forecast percentage figures for rain were an unnecessary gradation for rain prediction. I asked how do you process a figure such as 27%? How does it differ from, say, 25% or 30%? What does the figure say to you? My three opponents, for all three were opposed to my suggestion that percentage forecasts of rain were an overly detailed way of expressing the probability of rain, were in support of percentages, arguing that the figures had meaning and were a useful indicator of the risk. My counter-argument was that in the good old days, it was sufficient to say, ‘There’s a slight risk of rain this morning,’ or ‘Patchy/moderate rain is possible at lunchtime,’ or ‘Heavy rain is expected this afternoon; snow on high grounds,’ or ‘Thunderstorms will hit the south coast at tea time today – stay indoors,’ or similarly worded imprecise but informative forecasts. I mean, why do I need to know the risk down to a rounded-up (or down) integer percentage? If there is a risk of rain, I act accordingly in terms of whether I venture out and, if so, what I will wear. I do not think to myself that a 25% chance means leave the raincoat at home whereas a 30% chance means take the raincoat.
I first noticed the tendency to express forecasted attributes of the weather – sun, rain, wind, snow, humidity, etc. – in percentages when I used to watch weather forecasts in hotel rooms in the United States back last century. American television weather forecasts were more like a variety show, full of flashy graphics presented by enthusiastic immaculately-groomed male or female meteorologists in the style of a game-show host – ‘Come on down. You’ve won a snowball!’ – and accompanied by homely advice about what to wear and how to survive if your car breaks down in some remote spot. If I ever tired of the endless God-channels on American television (pure comedy), I would switch to an all-day weather channel and marvel at the facts and figures thrown at me by these Caesar Flickerman wannabees.
But, back to the topic. What do the percentage rain forecast figures really mean? Does 27% chance of rain at 15:00 mean that at 15:00 precisely, there will be just over one in four chances of rain; or that for the next hour, 15:00 to 16:00, a one in four chance that it will rain for the whole hour; or that in the hour between 14:00 and 15:00, a one in four chance that it will have rained continuously; or what? And is that rain, if it occurs, distributed evenly over the specified area or will it be wet in some parts and dry in others? And what type of rain – light, medium, heavy or absolutely tipping down, cats and dogs all over the place (we know what these descriptions mean!)?
[If you want to get into the answer to these questions, take a look at this American WMC Action News 5 website. Here’s a quotation from the explanation of how the percentages are derived. In the quotation, the acronym PoP stands for Probability of Precipitation.
“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 = 0.5 x 0.8 which equals 0.4 or 40%.)”
So now you know!]
You see my problem? Some data-crunching meteorologist somewhere has looked at wind speeds, cloud density, cloud height, air temperatures, barometric pressures and probably a host of other factors and, because he or she can, has come up with the percentages. Okay, maybe some situations such as the imminent launch of a satellite-bearing rocket or an attempt on the world’s air-speed record might need such detailed information, but us lesser mortals? When I look at the progression of rain-percentage figures along the bottom of today’s forecast above, I immediately see the analog summary – rain all morning easing off at lunchtime with patchy rain through the afternoon, increasing as the evening progresses. Okay, I’ll go for a walk this afternoon rather than this morning and I’ll be sure to wear my waterproof walking jacket rather than just a heavy sweater, just in case. Done.
And when I go out for my walk, I will reset my digital pedometer to record the distance walked in millimetres, not metres. Because I can!