1. Why the Pointed Teardrop Shape in Drawings Is Not the Real Shape of Raindrops
When drawing a rainy day, many people draw a "teardrop shape" that is pointed at the top and round at the bottom. The US Geological Survey (USGS) describes this teardrop shape as an imaginary image created by popular culture, stating that real raindrops do not resemble it at all [1]. So, what is the actual shape? According to explanations by a Japanese meteorologist, falling raindrops are not "teardrop-shaped" but "manju-shaped" (like a steamed bun), because air resistance affects them as they fall [2]. However, the manju shape is not the shape of every raindrop. Small drops are nearly spherical due to surface tension. The USGS states that drops with a radius of less than 1 mm (diameter less than 2 mm) are spherical, while larger ones are bun-like. The Japanese Wikipedia also uses a diameter of 2 mm as the dividing line [1][4]. On the other hand, the meteorologist suggests different estimates: drops appear round at about 0.5 mm in diameter, and become manju-shaped at about 5 mm [2]. These numbers are rough estimates and are not fixed to a single value. What forces act on round water drops to flatten them as they fall?
2. How Surface Tension and Air Pressure Pull Against Each Other to Shape a Raindrop
The USGS explains that the shape of a raindrop is determined by a "tug-of-war" between the surface tension of the water and the air pressure pushing up from the bottom of the falling drop [1]. Surface tension is the property of water trying to make its surface as small as possible. Since a sphere has the smallest surface area for a given volume, a drop would be a sphere if only surface tension acted on it [3]. ScienceABC also states that drops smaller than about 1 mm in diameter are nearly perfect spheres supported by surface tension. However, falling drops are pushed from below by air. If a drop is small, it falls slowly, so the upward push is weak, and surface tension wins, keeping it round. As the drop gets larger, its falling speed increases, and the pressure on the bottom becomes greater, causing the bottom to flatten. A depression may even form [1]. Imagine pushing up on a water balloon placed on your palm. The round balloon flattens where it is pushed. But raindrops have no rubber skin; surface tension holds the water-only drop together. This analogy goes only so far. How fast do larger drops fall?
3. How Cloud Droplets Grow into Raindrops and How Size Affects Falling Speed
Raindrops are not large from the start. Fresh cloud droplets have a radius of 1–20 µm (0.001–0.02 mm), while the average raindrop has a radius of about 1 mm [4]. Comparing radii, raindrops are 50 to 1,000 times larger than cloud droplets. This ratio is calculated from the figures in the source. The same source also writes raindrop size as about 1 mm in diameter, so the average should be read with some range. Rain with drops smaller than 0.5 mm is called drizzle. Falling speed increases as the drop gets larger. Values listed in the Japanese Wikipedia, converted to diameter, are about 0.7 m/s for 0.2 mm, 4 m/s for 1 mm, 6.5 m/s for 2 mm, and 9 m/s for 4 mm [4]. In kilometers per hour, this is about 2.5 km/h, 14 km/h, 23 km/h, and 32 km/h. A speed of 6.5 m/s is like running 50 m in about 8 seconds. Beyond 4 mm, the speed hardly changes. Combining this with the tug-of-war in the previous section, we can see with numbers why larger drops fall faster and are pushed harder from the bottom, causing them to flatten. What happens to the tug-of-war if the drop gets even bigger?
4. Why Very Large Raindrops Swell into a Bag Shape and Burst
As large raindrops fall, their bottoms become depressed. According to the USGS, the depression spreads explosively into a bag-like shape, splitting into small droplets. The estimated size for this splitting is a radius of about 4–4.5 mm (diameter 8–9 mm) [1]. This process has been recorded with high-speed photography. In 2009, Villermaux and Bossa from a French research team reported that large raindrops flatten like pancakes as they fall, take in air at the front, swell into a bag-like shape like an "upside-down bag," and burst at a certain size into many small droplets [5]. They also state that the variation in raindrop sizes can be explained by this splitting of single raindrops. The size at which splitting begins varies by source. ScienceABC says drops become unstable at about 4–5 mm in diameter [3], while the Japanese Wikipedia states that raindrop sizes are usually about 0.2–6 mm in diameter, with a maximum of about 8 mm [4]. The size where splitting begins and the maximum size usually seen are figures for different situations. So far, these shape discussions are about falling drops. How did people see the shape of drops falling at such speeds?
5. How Choji Magono's Photographs First Confirmed the Shape of Raindrops
Falling raindrops are fast, and their shapes are not clearly visible to the eye. According to the Japanese Wikipedia and other sources, Choji Magono of Hokkaido University photographed falling raindrops in 1951, and it is said that he confirmed the manju shape for the first time in the world [4]. The paper summarizing this work, "On the Shape of Water Drops Falling in Stagnant Air," was published in the English journal of the Meteorological Society of Japan in 1954. It is important not to confuse the dates: the photography was in 1951, and the paper was in 1954. More than half a century later, in 2009, Villermaux and Bossa, mentioned in the previous section, recorded the swelling into a bag shape and bursting using high-speed photography [5]. This shows that even events happening at invisible speeds can have their shapes and splitting recorded by devising how to take pictures. The teardrop shape in pictures is a familiar image that has been seen for decades. We know it differs from real raindrops because of this accumulation of photography.
6. Observing Water Drops on Windows and Leaves on a Rainy Day
On a rainy day, look from inside the window for water drops running down the glass or beads of water on leaves in the garden. Are there round beads? Are there any that look like teardrops? Is there a difference in shape between large and small beads? However, water drops on glass or leaves have their shapes determined by gravity and how they touch the surface, which is different from falling raindrops. The shape of falling raindrops cannot be seen with the naked eye. Still, the way small water tries to stay round can be a clue for imagining it through the beads. When going outside, use an umbrella and be careful not to slip on wet roads. For those who want to investigate more, Japanese resources include explanations by Japanese meteorologists [2] and Japanese research highlight articles [5].