1. Tokyo's Monthly Rainfall: Why October Gets the Most Rain Despite Sunny Days
When looking at the average monthly rainfall in Tokyo from 1991 to 2020, October has the highest amount at 234.8 mm, followed by September at 224.9 mm. In contrast, June, during the rainy season, averages 167.8 mm, meaning the two autumn months receive more rain than the traditional rainy season. The driest month is February at 56.5 mm, so October receives about 4.2 times as much rain as February [2]. The Japan Meteorological Agency explains that September has high precipitation due to the autumn rain front and typhoons, while October is often covered by moving high-pressure systems, leading to many clear days [1]. Typhoons approach more frequently around September [3]. "Many sunny days" and "high rainfall amount" are separate statistics. Even if there are fewer rainy days, if heavy rain falls on those days, the monthly total becomes large. We will look at how October's rain is distributed later.
2. How Rising Air Cools and Forms Clouds
Air contains invisible water vapor. When air is lifted upward, the surrounding pressure drops, the air expands, and its temperature falls. Unsaturated air cools by about 10°C for every 1 kilometer it rises [4]. As it cools, the amount of water vapor the air can hold decreases, and the excess turns into water droplets. This process is called condensation [5]. The US National Weather Service lists four reasons for air rising: heating from the ground, air moving up mountain slopes, surface air converging and being pushed up, and rising along fronts. Autumn rain fronts correspond to the fourth type [4]. However, air cooling alone is not enough to form clouds easily. According to Nagoya University, in clean air without dust, humidity must reach 300–400% for droplets to form. But real air contains salt particles from the sea, yellow sand, and exhaust particles (aerosols). These act as nuclei, allowing droplets to form when humidity slightly exceeds 100% [5][21]. Clouds form when "lifted, cooled air" and "small particles" are present together. Cloud droplets have a radius of only 0.001–0.01 mm [5]. How do these become raindrops?
3. How Many Cloud Droplets It Takes to Make One Raindrop
Cloud droplets fall at only about 1 cm per second [5]. Assuming no wind or evaporation, it would take about 28 hours to fall 1,000 meters. In reality, because there are updrafts and evaporation, cloud droplets have difficulty reaching the ground. Raindrop diameters are roughly 0.5 mm to several millimeters [6]. If a cloud droplet has a diameter of 0.02 mm and a raindrop has a diameter of 2 mm, the raindrop is 100 times larger in diameter. Since the volume of a sphere increases with the cube of the diameter, 100 cubed is 1,000,000. A textbook from the University of Hawaii states that 1 million cloud droplets with a radius of 10 micrometers combine to form one raindrop with a radius of 1 mm [7]. If a cloud droplet were the size of a 2 cm ball, the raindrop would be a 2-meter ball, similar to those used in school sports days. However, both are water droplets, not hard balls. Condensation alone takes too long to grow droplets to this size, so another mechanism is needed to create rain [7].
4. Two Ways Cloud Droplets Grow into Raindrops: The Ice Path and the Collision Path
The first method is the ice path. The upper parts of clouds are below 0°C. There, water droplets can remain unfrozen (supercooled) and mix with ice particles. Ice holds less water vapor than water, so vapor deposits onto ice particles, causing surrounding water droplets to evaporate and ice particles to grow rapidly [7]. These grown ice crystals fall as snow and melt into rain at altitudes warmer than 0°C. This is called "cold rain," and it is thought that much of the rain in temperate zones, where we live, is cold rain [8]. The second method is the collision path, or "warm rain," where droplets grow without ice. Larger droplets fall faster, catch up to smaller droplets, and merge. This chain reaction requires that cloud droplet sizes are not too uniform. The University of Hawaii textbook notes that this is the main method over tropical oceans [7]. The UK Met Office also describes droplets growing by "colliding with other particles" or by "condensation" [9]. Thus, rain in Tokyo is often melted ice or snow from high altitudes. However, this is a general trend for temperate zones; you cannot tell which path a specific drop took just by looking.
5. How Fronts and Pressure Systems Change Tokyo's Autumn Weather
In September, the main drivers are the autumn rain front and typhoons. The Japan Meteorological Agency defines the autumn rain front as a stationary front appearing near Japan during the seasonal change from summer to autumn, bringing prolonged rain [10]. Fronts are where warm and cold air collide, lifting warm air to form clouds. The Met Office calls this "frontal rain" [9]. When typhoons approach, rain can intensify. On September 4, 2005, Typhoon No. 14 was related to the activation of the autumn rain front, causing intense rain exceeding 100 mm per hour in Tokyo and Saitama [3]. In October, the main driver changes to moving high-pressure systems. The Agency defines these as high-pressure systems moving eastward alternately with low-pressure systems, appearing frequently in spring and autumn [11]. At about 10,000 meters altitude, strong westerly winds blow at roughly 100 meters per second almost year-round. High and low-pressure systems are carried by this wind from west to east, which is why weather changes from west to east [12]. In spring, the Agency notes that high and low-pressure systems pass alternately from west to east, changing weather on a cycle of several days, with warm air entering the east side of a low and cold air entering the west side [1]. A similar pattern of change occurred in autumn 2018, when weather across Japan changed on a cycle of several days from October to November [13]. The difference between September and October is the system driving the weather: fronts and typhoons in September, and high/low pressure alternation in October.
6. What You Need to See a Rainbow: Low Sun and Rain in Front of You
According to the US National Weather Service, to see a rainbow, you must have the sun behind you and rain in front of you, with the sun's altitude less than 42 degrees above the horizon. Sunlight must also hit the raindrops [14]. The noon altitude of the sun is calculated as "90 degrees minus latitude plus solar declination," using signed values for latitude and declination. Around the equinoxes, declination is nearly 0 degrees [15]. Tokyo is at about 36 degrees North Latitude, so the noon sun in early autumn is about 54 degrees, which is higher than 42 degrees. Therefore, rainbows are hard to see at noon in early autumn. The chance for a rainbow comes in the morning and evening when the sun is low. Since weather changes from west to east, consider an evening when rain clouds have moved east and sunlight comes from the west. The sun is in the west, and rain is in the east. To see the rainbow, look east. Under those conditions, evening rainbows appear in the eastern sky. Autumn weather changes can create such evenings, though they do not always appear due to cloud breaks and remaining rain.
7. Comparing Yearly Rainfall in Tokyo, London, and Singapore
Tokyo's annual precipitation is 1,598.2 mm. Comparing it with two other cities reveals different patterns. London (Heathrow) has an annual precipitation of 615.0 mm, with monthly amounts ranging from 38.8 mm (March) to 66.6 mm (November) [16]. Singapore has an annual precipitation of 2,113.3 mm, with the least rain in February and the most in November to January [17]. Tokyo's range is from 56.5 mm (February) to 234.8 mm (October) [2].
8. How to Investigate the Weather Yourself Safely
If thunder is sounding or heavy rain is falling, do not go outside; observe from windows or screens. You can look at the Japan Meteorological Agency's weather maps at the same time each day for several days to see if high and low-pressure symbols move east [19]. Use the "Nowcast" feature to see how current rain clouds move east [20]. In the Agency's past weather data search, compare daily precipitation in Tokyo for last October and June. Even if totals are similar, the pattern differs: "heavy rain in one day" versus "light rain over many days" [18].