1. How Much Earlier Does Evening Darkness Arrive Over Seven Weeks in Tokyo?
Looking at sunrise and sunset times for Tokyo in the National Astronomical Observatory of Japan's Calendar Notes (2026), September 8 shows a sunrise at 5:18 and sunset at 17:59, while October 28 shows a sunrise at 5:59 and sunset at 16:51 [2]. During this interval, sunrise becomes about 41 minutes later, and sunset becomes about 68 minutes earlier. The length of the day decreases from 12 hours 41 minutes to 10 hours 52 minutes, a reduction of approximately 1 hour and 49 minutes; this value is calculated from the difference in times [2]. The rate of change is not constant. On June 20, the day length was about 14 hours 35 minutes. Over the 80 days until September 8, it shortened by about 1 hour 54 minutes, or roughly 1.4 minutes per day. From September 8 to October 28 (50 days), it shortened by about 1 hour 49 minutes, or roughly 2.2 minutes per day. The speed at which daylight shortens increases from summer into autumn [2]. The change in evening darkness is greater than the change in morning darkness, which may be why evenings feel "already dark" more easily. But why does the day become shorter?
2. Earth's Tilted Axis Makes Days Shorter in Autumn
Earth orbits the Sun with its rotational axis tilted at approximately 23.4 degrees relative to the orbital plane [1]. Because it rotates while maintaining this tilt, the relationship between the direction of the axis and the position of the Sun changes over the course of a year. In Japan, summer has long days with the Sun high in the sky, while autumn brings shorter days with the Sun lower [1]. The seasons are determined not by the difference in distance to the Sun, but by this tilt; the explanation that "summer is hot because we are closer to the Sun" is incorrect [1]. We now understand why days shorten. However, the longest day is in June. Why do we feel coolness starting from September?
3. Why Temperatures Fall Slowly Even While Days Get Shorter
According to the Fukuoka District Meteorological Observatory, the highest annual temperatures occur not at the summer solstice (the longest day), but around early August [3]. This is similar to how the daily high temperature occurs around 2:00 PM rather than at noon when the Sun is highest; air continues to be warmed without cooling while sunlight persists [3]. Autumn represents the downward slope of this cycle. Tokyo's monthly average temperatures are 26.9°C in August, 23.3°C in September, 18.0°C in October, and 12.5°C in November [4]. The drop is 3.6°C from August to September, 5.3°C from September to October, and 5.5°C from October to November. Although the rate of daylight shortening is nearly constant, the magnitude of the temperature drop increases month by month [4]. The ocean covers about 70% of Earth's surface and has approximately 1,000 times the heat capacity of the atmosphere; this ratio compares the entire ocean to the entire atmosphere, not equal volumes [8]. The ocean, warmed in summer, stores heat easily and is thought to be one factor that moderates the rate of autumn temperature decline. However, if only daylight shortness and thermal lag were involved, temperatures should decrease smoothly every day. In actual autumns, the air itself also changes.
4. How Cold, Dry Air From the Continent Arrives With High Pressure
In explanations for the Tohoku region by the Japan Meteorological Agency, autumn sees the weakening of the Pacific High pressure system, while high pressure from the continent moves south toward Japan [5]. Weather changes cyclically, with high and low pressure systems passing alternately at intervals of about 3 to 4 days; note that dates are not uniform nationwide as this is a regional explanation [5]. The migratory high-pressure systems of autumn bring cold, dry air from the Eurasian continent [6]. Near the center of these high-pressure systems, skies clear and winds weaken, creating pleasant weather. At night, temperatures drop, and frost begins to form in late autumn [6]. During seasonal transitions, fronts may stall similarly to the rainy season, resulting in continuous rain known as the autumn rain front [5]. While the rate of daylight shortening is nearly constant daily, air mass changes progress on a cycle of about 3 to 4 days. The day-to-day feeling that "today feels chilly" is thought to be related to these air mass changes. What happens at night under these high-pressure systems?
5. How Heat Escapes From the Ground on Clear Nights (Radiative Cooling)
Radiative cooling occurs when the ground and grass, unable to receive solar heat at night, emit heat and cool down [6]. If there are clouds overhead, they act like a blanket, making it harder to cool. When wind blows, air cooled near the ground mixes with surrounding air, weakening the cooling effect [6]. Conversely, radiative cooling becomes significant on nights that are clear, dry (low in water vapor), and have weak winds [7]. On cloudy nights, clouds also emit heat, reducing the amount of cooling [7]. In an example from the Fukuoka District Meteorological Observatory, on a clear night with no clouds and weak wind, even if the air temperature at 1.5 m above the ground is about 4°C, the area near the ground can drop below 0°C, causing frost; 4°C is not a fixed value but an example under specific conditions [6]. Tokyo's monthly average daily minimum temperatures are 23.5°C in August, 20.3°C in September, 14.8°C in October, and 8.8°C in November [4]. This is a drop of about 14.7°C from August to November, with single-digit temperatures on November mornings. The arrival of dry continental high pressure, clear skies, and weak winds creates conditions where nighttime cooling intensifies, leading to the crisp air of autumn mornings.
6. How to Check the Reasons for Cooler Weather With Sunset Times and a Morning Thermometer
You can try observing these changes yourself. Once a week, write down the sunset time in a notebook and compare it with values from early September; this information is published in weather forecasts and newspapers. You can verify if it is advancing by about 10 minutes per week. In the morning, record the reading from an outdoor thermometer at home and whether there were clouds in the sky. Comparing clear, low-wind mornings with cloudy or windy mornings allows you to see if radiative cooling conditions are effective in your local area [6]. You can also observe the brightness of the sky at the same time of evening at the same location, comparing how different it is from one month ago. For official information, the National Astronomical Observatory's Calendar Notes list sunrise and sunset times for various locations, and the Japan Meteorological Agency's normals page allows you to check monthly temperatures by location [2][4].