Presentation Information
[O12-P105]Are there rainy days during the week?
*Erika Yarai1 (1. Meikei High School)
Keywords:
weather
1. Objective
The objective of this study is to statistically analyze day-of-the-week patterns in precipitation using historical meteorological data and to examine whether the phenomenon that “rain tends to fall on specific days of the week” actually exists. If such a tendency is identified, the study aims to explore its possible causes. If no clear tendency is observed, the study will further investigate other possible periodicities or regularities and discuss their explanations.
2. Methods
This study employed statistical methods. Daily precipitation data for Tateno, Tsukuba City, were obtained from historical records provided by the Japan Meteorological Agency. The study period covered ten years, from 2016 to 2025.
First, a calendar was created in Excel based on the dates, and daily precipitation (mm) was recorded. The precipitation was then classified into four categories: 0-5 mm, 5-15 mm, 15-25 mm, and >=25 mm. The number of days in each category was counted for each day of the week. Furthermore, the data were classified into rainy days (days with rainfall) and non-rainy days (days without rainfall), and their frequencies were counted by weekday.
To investigate seasonal variations, the data were divided into four periods: the cold season (January-February and November-December), the transitional period (March-May), the rainy season (June-July), and the warm season (August-October). For each period, total precipitation was calculated for each weekday. Precipitation values less than 0.5 mm were treated as 0.25 mm. Based on these data, the number of times each indicator (e.g., number of rainy days and total precipitation) reached its maximum value for each weekday was counted and analyzed.
3. Data
For each year, the weekday corresponding to the maximum value of each indicator was identified.
4. Results
Yearly analysis revealed that the weekday with the highest number of rainy days and the greatest total precipitation varied from year to year, with no consistent pattern observed. However, when the data for the 10-year period (2016-2025) were aggregated, several tendencies emerged. Monday most frequently had the highest number of rainy days. Saturday often showed the highest or near-highest total precipitation.
Furthermore, when comparing the weekday with the highest annual precipitation across years, a tendency was observed in which the peak appeared to shift from Tuesday through Sunday toward Saturday; however, this pattern did not exhibit clear regularity.
5. Discussion
The results of this study indicate that total precipitation varies considerably from year to year, and no clear weekly cycle was identified. This suggests that precipitation is influenced by factors other than a seven-day cycle. One possible explanation is heavy rainfall associated with typhoons or stationary fronts. Such events produce extremely large amounts of precipitation and may occur independently of typical patterns, without being concentrated on specific weekdays.
On the other hand, the number of rainy days showed a slight bias, with Mondays most frequently having the highest counts. However, since no statistical tests were conducted in this study, it cannot be determined whether this difference is due to chance or represents a statistically significant pattern.
References
Japan Meteorological Agency. "Historical Weather Data Search". accessed January 15, 2026. https://www.data.jma.go.jp/stats/etrn/index.php.
Acknowledgments
The author expresses sincere gratitude to Mr. Takamichi Akimoto for his invaluable guidance throughout this research.
The objective of this study is to statistically analyze day-of-the-week patterns in precipitation using historical meteorological data and to examine whether the phenomenon that “rain tends to fall on specific days of the week” actually exists. If such a tendency is identified, the study aims to explore its possible causes. If no clear tendency is observed, the study will further investigate other possible periodicities or regularities and discuss their explanations.
2. Methods
This study employed statistical methods. Daily precipitation data for Tateno, Tsukuba City, were obtained from historical records provided by the Japan Meteorological Agency. The study period covered ten years, from 2016 to 2025.
First, a calendar was created in Excel based on the dates, and daily precipitation (mm) was recorded. The precipitation was then classified into four categories: 0-5 mm, 5-15 mm, 15-25 mm, and >=25 mm. The number of days in each category was counted for each day of the week. Furthermore, the data were classified into rainy days (days with rainfall) and non-rainy days (days without rainfall), and their frequencies were counted by weekday.
To investigate seasonal variations, the data were divided into four periods: the cold season (January-February and November-December), the transitional period (March-May), the rainy season (June-July), and the warm season (August-October). For each period, total precipitation was calculated for each weekday. Precipitation values less than 0.5 mm were treated as 0.25 mm. Based on these data, the number of times each indicator (e.g., number of rainy days and total precipitation) reached its maximum value for each weekday was counted and analyzed.
3. Data
For each year, the weekday corresponding to the maximum value of each indicator was identified.
4. Results
Yearly analysis revealed that the weekday with the highest number of rainy days and the greatest total precipitation varied from year to year, with no consistent pattern observed. However, when the data for the 10-year period (2016-2025) were aggregated, several tendencies emerged. Monday most frequently had the highest number of rainy days. Saturday often showed the highest or near-highest total precipitation.
Furthermore, when comparing the weekday with the highest annual precipitation across years, a tendency was observed in which the peak appeared to shift from Tuesday through Sunday toward Saturday; however, this pattern did not exhibit clear regularity.
5. Discussion
The results of this study indicate that total precipitation varies considerably from year to year, and no clear weekly cycle was identified. This suggests that precipitation is influenced by factors other than a seven-day cycle. One possible explanation is heavy rainfall associated with typhoons or stationary fronts. Such events produce extremely large amounts of precipitation and may occur independently of typical patterns, without being concentrated on specific weekdays.
On the other hand, the number of rainy days showed a slight bias, with Mondays most frequently having the highest counts. However, since no statistical tests were conducted in this study, it cannot be determined whether this difference is due to chance or represents a statistically significant pattern.
References
Japan Meteorological Agency. "Historical Weather Data Search". accessed January 15, 2026. https://www.data.jma.go.jp/stats/etrn/index.php.
Acknowledgments
The author expresses sincere gratitude to Mr. Takamichi Akimoto for his invaluable guidance throughout this research.