1. Tiny Gaps in Umbrella Fabric and the Size of Raindrops
Umbrella fabric is a textile made by weaving threads vertically and horizontally. Small gaps that are hard to see with the naked eye remain between the threads. In fabrics with water-repellent treatment, it is explained that these remaining gaps serve as pathways for air [1].
The size of raindrops varies. According to NASA’s explanation, tiny water droplets in clouds are less than 0.1 mm in diameter, while large raindrops during heavy rain can reach up to about 6 mm [9]. Comparing the gaps in the fabric and the raindrops by size alone does not seem to provide an answer. The reason water droplets “cannot enter” is not just a matter of size.
2. Surface Tension: How Water-Repellent Treatment Makes Water Form Balls
Water has a property called surface tension, which makes it try to clump together tightly at its surface. This is why dew on leaves forms round shapes. The water-repellent treatment applied to umbrella fabric is a method of treating the fabric with a water-repellent agent to make the fiber surfaces unacquainted with water (hydrophobic) [1]. As a result, water does not spread on the fiber surface but forms balls and rolls off.
The degree to which water does not mix with the surface is expressed by the contact angle formed between the fiber surface and the water droplet. If the angle is greater than 90 degrees, it is considered water-repellent. Furthermore, it is explained that the denser the fabric structure, the higher the waterproofing effect [2].
3. Why Smaller Gaps Make It Harder for Water to Get Through
The pressure required to force water into a thin tube that repels water increases as the radius of the tube decreases. Written as a formula, pressure is inversely proportional to the radius. If the radius is halved, the required pressure doubles [3].
The gaps in fabric are not round tubes but have irregular shapes, so the values do not follow this formula exactly. However, the direction that “the smaller the gap, the harder it is for water to enter” is thought to be the same. Some umbrellas take this idea to the extreme. “NURENZA,” made by an umbrella manufacturer in Fukui, is introduced as using ultra-high-density polyester fabric with a special weaving method [7]. However, this is based on a corporate introduction article, so it cannot be applied to umbrellas in general.
4. Two Ways to Waterproof Fabric: Blocking the Gaps or Keeping Them Open
There are broadly two types of processing to prevent water from passing through fabric. One is to create a waterproof film on the fabric, blocking the gaps themselves (non-breathable waterproofing). While this can achieve high water pressure resistance, it also stops air and water vapor from passing through [1]. Fabrics covered with film fall into this category.
The other is water-repellent treatment (breathable waterproofing), which leaves the gaps but makes only the fiber surfaces unacquainted with water. Although its waterproofing power is inferior to film, the dictionary explains that it can eliminate the drawbacks of film [1]. The main umbrella in this article uses fabric from this second method.
As a third solution, the membrane used in GORE-TEX clothing for mountaineering is well known. It is said to have more than 1.4 billion holes per square centimeter, each hole being much smaller than a water droplet but larger than water vapor, so it lets water vapor pass but not water droplets [8]. This is not about umbrellas, but it is an example of the idea of “matching hole size to water particle size.”
5. Why Water Can Still Seep Through a Water-Repellent Fabric
Water repellency is about the surface of the fabric, where water forms balls and rolls off. Whether water enters through the gaps is a separate issue, and it is explained that water can seep through during long periods of rain with water repellency alone [4].
A test to measure fabric waterproofing is the water pressure resistance test. This applies water pressure to one side of the fabric and checks waterproofing by the pressure at which water droplets seep through to the back [2]. The higher the pressure it withstands, the higher the waterproofing. For outdoor use, a figure of 10,000 mm water pressure resistance is introduced as a benchmark for heavy rain [4].
Another factor is time. Umbrella water-repellent treatment can wear off after a few years, and the umbrella may no longer repel rain, getting wet all the way to the inside, according to an umbrella manufacturer’s article [5]. Fabric exposed for many years loses both its water-repellent properties and its own strength.
6. Why Umbrella Seams and Needle Holes Can Leak, Not Just the Fabric
Umbrella leaks can occur not only from fabric deterioration but also at the seams. The size of the needle holes, the material of the thread, the spacing of the seams, and fraying are all factors, according to an umbrella manufacturer’s article [5]. The holes made by the needle can become direct paths for water.
Water running along the tip (ishizuki) or the central shaft is also cited as one of the causes [5]. Weather News reports, citing an umbrella specialty store, that spinning the closed umbrella to shake off water can put stress on the upper and lower hubs, potentially leading to a symptom where rainwater enters from the top and runs down the central shaft [6]. The story that began with the properties of the fabric ends up focusing on the fine details of the umbrella as a constructed object.
7. How to Test Water on Your Umbrella and a Cotton Handkerchief
If you want to try this yourself, you can compare how water behaves on your umbrella’s surface versus a cotton handkerchief. Use a spoon to drop one drop of water at a time onto each and observe whether the water forms a ball or seeps in. Do this in a place where getting wet is not a problem, such as a bathroom, and do it together with an adult.
When you go out, try observing the area around the seams and the central shaft of your umbrella on a rainy day. You might find places where water tends to pool or run along.
For official information, look for websites of umbrella or textile manufacturers that are readable in your language. Searching for terms like “water pressure resistance” and “water repellency grade” can lead you to explanations of the types of tests used.