Singular Days in Japan

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:

• Fine day = A day without precipitation equal to or more than 1 mm, and with cloud amount smaller than 8.5/10.
• Cloudy day = A day without precipitation equal to or more than 1 mm, and with cloud amount equal to or greater than 8.5/10.
• Precipitation day = A day with precipitation equal to or more than 1 mm.

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