Is the 90-day Rule Truth or Myth?

Every year people ask us this question: Is it true that it always rains 90 days after fog? To the meteorologist, this is an unexpected question, because nowhere does such a rule appear in any textbook. While we always inform the questioner that this isn’t true, to our knowledge nobody has ever actually studied this question. In this blog post we put the legend to the test – does it actually rain 90 days after fog?

Does fog really mean it will rain 90 days later?

While this seems like a fairly easy rule to test, it’s actually not so simple. For one, the 90-day “rule” varies depending on who you talk to. Some say 60 days, some say 80-90 days, some say 90-100 days, and some say something in between, so which is it? Also, what is considered rain? Most people interpret this rule to mean a significant rainstorm is coming, but what qualifies as a significant storm? Is 13 mm (0.5 in) a big storm, or 25 mm (1.0 in), or 50 mm (2.0 in)? Or does this mean any amount of rain?

To account for all the different variations of the rule, we put it to the test using a number of different criteria at three different locations on the Prairies: Winnipeg, Regina, and Calgary. We checked how often there was precipitation (rain or snow) 30, 60, 90, and 120 days after fog. We also varied the criteria of what a significant storm is, by checking how often there was precipitation exceeding five different thresholds: 0.01” (0.2 mm), ¼” (6.4 mm), ½” (12.7 mm), 1” (25.4 mm), and 2” (50.8 mm).

Let’s start by checking the results for Winnipeg. As you can see, on average there is 0.2 mm of precipitation only 33.5% of the time following a fog event. However, this percentage drops off to less than 1% for large precipitation events of 1 or 2 inches. The numbers are also quite consistent for various time ranges, with the frequency of rain only varying by about 1% regardless of whether we use 30, 60, 90, or 120 days as the “rule” for a future storm. But this is just Winnipeg, how do Regina and Calgary fare?

As you can see, the numbers are not that much different for Regina and Calgary. In fact, it appears the frequency of rain is even lower following fog in those cities. On average, Regina received at least 0.2 mm of rain only 32% of the time after fog and Calgary gets 0.2 mm only 31.9% of the time. Like Winnipeg, large amounts rarely follow fog, with both Regina and Calgary receiving 1 or 2 inches of rain 0.5% of the time or less.

As shown by the statistics above, the 90-day “rule” clearly does not work. Even if you vary the criteria, the results are very poor. But then why does this rule often seem to work? There are a few explanations we can think of:

1. Significant rains are fairly common on the Prairies.

During the growing season, most of the Prairies receives over 300 mm on average, so there’s bound to be a few significant rains in a typical year anyway.

2. There is uncertainty in the criteria.

If you mark fog on the calendar, then it rains about 90 days later, it has appeared to work. But the word “about” is the key one. If it rains 85 days later does that count, what about 95 days? The larger the window you give, the more likely it will rain by chance.

3. We only remember when it works.

If it doesn’t rain 90 days after fog, you probably don’t remember it, but when it works you do remember. We used thousands of fog events for our study – so if the “rule” works a few times, that is just pure luck.

One strange thing about this legend is its origin. We tried to find out where this came from and why, but couldn’t find anything. If you have some idea where it comes from, we’d love to hear about it.

In conclusion, the 90-day rule is most definitely a pure myth. It only seems to work because rain was likely to happen anyway. Still don’t believe us? If you’ve marked fog on your calendar consistently, we can run the stats for your location. But we need a big sample! Just showing that it worked once or twice doesn’t cut it. For our stats, we used over 2000 fog days since 1953 as a test. So we’ll need more than a few examples to be convinced!

So if the 90-day rule doesn’t work – how can you tell if it’s going to rain? Luckily, Weatherlogics prepares real weather forecasts for farmers across the Prairies – giving them a reliable heads-up about upcoming rainfall and other significant weather. Check out our agriculture page for more information!

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