The 600°C Rule
In Tokyo, the cherry blossoms are forecast to bloom on March 20th.
This cherry blossom forecast is apparently calculated using something called the "600°C Rule."
It is a method where February 1st is assumed to be the "day of dormancy breaking" and set as the starting point for the forecast, after which the daily maximum temperatures are added up, and the cherry blossoms are predicted to bloom once the total temperature reaches 600°C.
While simple, this seems to be quite accurate.
Now, as of right now, the total of the maximum temperatures from February 1st to March 14th is 536.7°C. And the forecast for maximum temperatures from March 15th onwards is 17°C + 19°C + 21°C + 15°C, which brings the total to 608.7°C on March 18th. In other words, if you start counting from February 1st, the blooming date is March 18th.
However, since the day of dormancy breaking is not necessarily February 1st, it seems necessary to make some slight adjustments. Cherry blossom buds are formed in the summer of the year before they bloom, and they enter a dormant state from autumn through winter to survive the year without growing. Then, after receiving sufficient low-temperature stimulation, they wake up from dormancy when the temperature rises significantly. This is called "dormancy breaking."
Taking all of these factors into account, the forecast for this year's cherry blossom blooming in Tokyo is March 20th.
Note that this varies by region; it seems to be the 700°C rule in Kochi and the 800°C rule in Kagoshima.
There is a rumor that this rule was devised by weather forecaster Masamitsu Morita, but...
It is not certain how true that is.
However, I think the person who came up with this rule is amazing.
Does the Japan Weather Association have a habit of looking at figures accumulated from maximum temperatures over a certain period? Otherwise, I don't think one would normally think, "Let's add up all the daily maximum temperatures for a month!" Also, even if such a habit existed, I think it would be difficult to connect a "total of 600°C" with "cherry blossom blooming" without considerable powers of observation and imagination.
That's right. This is a kind of "mashup."
The two phenomena of "maximum temperature" and "cherry blossom blooming" are things that everyone can see. However, the person who realized that the answer to "cherry blossom blooming" could be found by multiplying those two things together likely has a sense for catching something that is invisible to the average person.
If there are no sudden changes in temperature or long spells of rain, the best time to see the cherry blossoms is about a week after they bloom. This year, March 30th and 31st fall on a weekend, so that will likely be the peak for cherry blossom viewing.
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