1. How the 5mm size line separates hailstones from arare
According to the Japan Meteorological Agency’s forecast terminology, hailstones are defined as ice masses falling from cumulonimbus clouds with a diameter of 5mm or more. Arare are defined as ice pellets falling from clouds with a diameter of less than 5mm [1]. Since 5mm is 0.5cm, arare are very small grains.
Hailstones are said to range from about 5 to 50mm in diameter. Some explanations state that large ones can reach falling speeds of 100km per hour (about 62 miles per hour) [7]. Other sources describe hailstones exceeding 5cm in diameter and 100km per hour in speed, so the descriptions vary depending on the source. However, the 5mm line is a category decided by people to distinguish between them. The size of a pellet is related to how long it stayed and kept growing inside the cloud.
2. Why the top of a cumulonimbus cloud is cold even when the ground is warm
Cumulonimbus clouds can exceed 10km (about 6 miles) in height and sometimes reach the stratosphere [2]. Hailstones and arare are formed inside these developed cumulonimbus clouds.
Even on days when the ground is warm, the inside of a high, developed cumulonimbus cloud has low temperatures, causing water droplets to freeze into ice pellets [5]. Regarding the times they are most likely to fall, arare are more common from winter to early spring, while hailstones are said to be more common in spring or autumn during heavy rain accompanied by thunder [5]. The specific seasons vary depending on the source.
3. How arare grows when cloud droplets freeze onto snow crystals
Inside clouds, there are supercooled water droplets (cloud droplets) that do not freeze even when the temperature is below 0°C. When these cloud droplets collide with snow crystals or frozen droplets, they freeze on the spot and adhere. This process is called accretion (riming), and arare are ice pellets that have grown this way [4].
As cloud droplets freeze one after another, the original snow crystal becomes invisible, turning into a mass of frozen cloud droplets. This is what is called arare [3]. They are described as being about 2mm to less than 5mm in diameter, spherical, and whitish [4]. This matches the Japan Meteorological Agency’s category of less than 5mm.
4. How strong updrafts let hailstones grow large
According to the Snow Science Museum, for hailstones to grow large, strong updrafts comparable to the falling speed of the hailstone are important. Thanks to these updrafts, hailstones can stay inside the cloud for a long time and become large masses [3]. Therefore, extremely developed cumulonimbus clouds are necessary for hailstone growth.
Ice pellets grow by having water droplets adhere to them or by colliding with other ice pellets. They grow while moving up and down inside the cumulonimbus cloud, riding the updrafts [6]. If the pellets become too large, they do not melt completely during the fall and reach the ground as ice [6].
5. Why a cut-open hailstone shows overlapping layers
When a large hailstone is cut open, you can sometimes see a cross-section where transparent layers and semi-transparent layers alternate [8].
However, some hailstones do not show visible layers. It is not certain that the number of layers can be read as the number of times the hailstone moved up and down inside the cloud.
6. The two types of arare, and how forecasts call them "snow" or "rain"
Arare is divided into snow arare and ice arare. Snow arare is white, opaque, and brittle; it may bounce and break when it hits the ground. Ice arare is semi-transparent and is said to be stronger than snow arare [7].
In the Japan Meteorological Agency’s forecast terminology, snow arare is included under "snow" in forecast statements, and ice arare is included under "rain" [1]. However, this is a rule for how they are treated in forecast statements, not a comparison of their properties. The detailed differences in how snow arare and ice arare are formed are not clearly explained in the materials used for this article.
7. How to investigate the size of hailstones and arare yourself
You can measure 5mm with a ruler and place it next to everyday objects to compare sizes. You might feel that the boundary between hailstones and arare is surprisingly small.
If hailstones or arare are falling, do not go outside. Look at them from a window inside a building. It is especially dangerous when thunder is heard, so please stay indoors.
The Japan Meteorological Agency’s "Forecast Terminology (Precipitation)" page and "What kind of cloud is a cumulonimbus cloud?" page are the foundation of this article. Reading the original texts allows you to verify the word definitions and cloud descriptions.