Singular Days in Japan

in Japanese

Here, singular days are examined, using observational data by the Japan Meteorological Agency (JMA). Firstly, "fine day", "cloudy day" and "precipitation day" are defined as follows:

Then, we define "fine weather ratio" as a possibility at which a day of the year is a fine day, and "precipitation ratio" as one at which a day of the year is a precipitation day. After that, we compare the fine weather ratio or precipitation ratio of the day to that averaged over the 10-day period which consists of 5-day periods both before and after the day. If a large difference is detected between them, we call the day a "singular day". The difference is estimated by a statistical method using a statistic Z. The larger a statistic Z is, the more significant the difference is. Here, days with the absolute value of a statistic Z equal to or larger than 1.96 are picked out. The period for the statistics is a 40-year period from 1961 to 2000. Data shown here are no more than statistical results. Please remember that they are not a weather forecast.

Data at selected sites: | Sapporo | Sendai | Niigata | Tokyo | Osaka | Fukuoka | Naha |
Locations of the sites

Original version: September 8, 2002.
Supplement: Latest version (2019)