If you’re an avid reader of Capital Weather Gang’s morning forecasts, you have no doubt noticed that little box with the green number and a snippet of commentary, better known as the “Daily Digit.” We announce it on Facebook and Twitter the night beforehand. And depending on what kind of weather is your cup of tea on any given day, you may find our assigned digit to be too generous, too harsh, or sometimes just right.
What is the daily digit, and how did it come about? For lack of a better definition, we define the digit as a somewhat subjective rating of the day’s weather, on a scale of 0 to 10. It debuted on December 16, 2009, just three days before D.C.’s biggest December snowfall on record. Since then it has become an integral part of our morning forecasts – so much so that many of you have asked questions about the history of this arbitrary number.
Today we’ll look at some overall trends and other interesting trivia in the daily digit’s now 20-month history.
OVERALL FREQUENCY AND AVERAGES
Through August 30, our CWG forecasters have assigned a total of 618 daily digits. An analysis of these data reveals that most of our digits fall within the middle of our 0-10 range, with roughly 34 percent of all digits earning a 5 or a 6. About 29 percent of the digits were 4 or below, while 37 percent were in the 7-10 range, indicating a slight bias towards the upper end of the scale.
On the whole, the average digit assigned was 5.8, which has stayed fairly consistent over time: 5.8 was the average for calendar year 2010, and for 2011 the digit has averaged nearly the same, at 5.9 year-to-date.
The graph at the top of this post illustrates that the digit follows a classic bell curve, with a near consistent drop in frequency on either side of the average. The only exception is in the 9 to 10 category, where our forecasters have assigned a few more 10s than 9s, which I will discuss more in Part 2 of this series.
A look at the frequency of digits by dry versus precipitation days indicates that nearly half of dry days earned a 6 or higher. The average digit assigned on days with little to no precipitation in the forecast was 6.3. Precipitation, on the other hand, lowers the average digit by over 2 points. Roughly in 1 in 4 of the past 600+ forecasts called for a decent chance of some precipitation, and collectively days calling for rain or snow averaged a digit of only 4.2.
While the historical ups and downs of the daily digit might remind you of the mercurial stock market at first glance, there are nonetheless some clear seasonal patterns. It should come as no surprise that higher digits occur during the spring and autumn months, when glorious days in the 70s against a backdrop of deep blue sky easily score a 9 or 10 on the scale.
Digits drop in the heart of summer, but are still interspersed with some nice days, which signal a temporary reprieve from D.C.’s oppressive heat and humidity. Whether or not you like cold weather and snow, our forecasters show a moderate bias towards warmer temperatures, given that the winter months average the lowest digits of any season.
For this analysis I divided the digits using the meteorological definition of the seasons (the seasonal digit averages turned out to be similar when divided by the astronomical definitions).
Spring and fall of 2010 averaged 6.3 and 6.9 respectively, with spring 2011 coming in slightly lower at 6.1. The past two winters have averaged below a 5, with the historic winter of 2009-2010 averaging 4.7, and the less snowy, but still cold winter of 2010-2011 earning a similar 4.8. Despite the record heat we’ve experienced this summer and in 2010, both summers have averaged higher digits than the winter months, with 5.5 last summer, and 6.0 this summer (to date).
TEMPERATURE AND PRECIPITATION
How temperature affects the digit
The graph above offers some clear trends:
* The highest average digit of 7.8 coincided with forecasted high temperatures in the 76-85 degree range, followed closely by an average of 7.1 for forecasted temperatures of 66-75.
* The digit averaged higher for days in the upper 80s to low 90s (6.0) compared to days in the upper 50s to low 60s (5.6).
* There is a steep drop in the digit for forecasted highs in the upper 90s or higher (2.9). This represents the lowest average digit of all eight temperature categories – even lower than the average digit of 3.7 for high temperatures around the freezing mark.
* Despite our forecasters’ bias towards warmer weather, it appears that snow prospects bump up the average digit in the 26-35 degree range.
How precipitation affects the digit…
Overall, there were 466 days with little to no precipitation in the forecast, and 152 days with a decent chance of considerable precipitation (about 1 in 4 days of rain or snow). Not surprisingly, the average digit is noticeably higher in each temperature range on dry days versus precipitation days, with two exceptions:
* In the 26-35 degree range, the digit is virtually identical for dry days (3.7) vs. precipitation days (3.6), undoubtedly due to our forecasters’ affinity for snow.
* In the 96-100+ degree range, the digit is also similar for dry days (2.9) vs. precipitation days (3.0). An oppressively hot day with the chance of a thunderstorm to cool things down will likely score the same, if not better, than a similar day with no rain in the forecast.
Interestingly, the prospect of precipitation is perhaps a better indicator of an overall correlation between the daily digit and temperature. For example, while the 86-95 degree days average a higher digit than 56-65 degree days on the whole (6.0 vs. 5.6), dry, sunny days in the 56-65 category actually average slightly higher than hot, rainless days in the 86-95 range (6.6 vs. 6.4 respectively).
Still, our forecasters agree that days in the low 60s might feel too cool for some, and therefore grade dry, sunny days with highs in the 66-75 and 76-85 degree ranges much more generously. But we’ll save that for next time...
Stay tuned for Part II of “Dissecting the Daily Digit” to find out which of Capital Weather Gang’s forecasters grades the weather most leniently (or critically).
Average digit (overall): 5.8
2010 calendar year avg.: 5.8
2011 calendar year avg. (to date): 5.9
Number of precipitation days: 152 out of 618 (25%)
Avg. digit of days with precipitation: 4.2
Avg. digit of precipitation-free days: 6.3
Frequency of assigned digits by percentage:
0 – 0.5%
1 – 1.8%
2 – 4.7%
3 – 8.7%
4 – 13.3%
5 – 16.7%
6 – 17.1%
7 – 14.9%
8 – 9.7%
9 – 5.8%
10 – 6.8%
9 or above: 13%
8 or above: 22%
7 or above: 37%
In the 5-6 range: 34%
4 or below: 29%
3 or below: 16%
2 or below: 7%
1 or below: 2%
Seasonal Averages (Meteorological)
Winter 2009-2010: 4.7
Spring 2010: 6.3
Summer 2010: 5.5
Fall 2010: 6.9
Winter 2010-11: 4.8
Spring 2011: 6.1
Summer 2011 (to date): 6.0
Average Digit by Temperature (ºF): Dry / Precip. Days
<26-35: 3.7 / 3.6
36-45: 5.1 / 2.9
46-55: 6.1 / 4.0
56-65: 6.6 / 3.0
66-75: 7.9 / 4.8
76-85: 8.5 / 5.4
86-95: 6.4 / 5.0
96-100+: 2.9 / 3.0