14.2: Precipitation and Hail

Thunderstorms consist of deep convective clouds that can produce large raindrops (2–8 mm diameter) within 5–10 km wide rain shafts that move horizontally across the ground with precipitation that last from 1–20 minutes over a fixed point. The rainfall rate may be heavy, as much as 10 to 1000 mm of rain per hour. The below photo depicts heavy rain shafts produced by a supercell thunderstorm in Montana.

A supercell thunderstorm in Montana (CC BY 2.0).

The cloud tops of thunderstorms extend far up in the troposphere. In the upper parts of the clouds, ice-phase processes occur. Throughout the cloud, different sizes of ice crystals and water droplets exist, and the heavier hydrometeors fall faster than the smaller ones, sometimes colliding and collecting each other. If these heavier ice particles fall through regions of supercooled droplets (liquid water droplets below freezing), they may grow through a process called riming. The liquid water droplets instantly freeze and adhere on contact to the outside surface of falling ice particles. This forms snow pellets called graupel, which are less than 5 mm in diameter. Fallen graupel is pictured below.

Graupel pellets on a car roof (CC BY 3.0).

Alternatively, smaller ice crystals that fall just below the 0°C level may partially melt and stick to other partially melted ice crystals through collision, causing them to grow through aggregation. These snow aggregates can grow as large as 1 cm in diameter. Snow aggregates and graupel can sometimes reach the ground still frozen or partially frozen, even in the summer, if they fall within cooler, saturated downdrafts. More frequently, these larger ice particles will melt completely into large raindrops before hitting the ground.

Hailstones are irregularly shaped balls of ice larger than 0.5 cm in diameter that are produced in a cumulonimbus cloud—particularly in intense, severe thunderstorms. Hail is formed when graupel or other particles act as embryos that grow through the accretion of supercooled liquid droplets in a cloud. One raindrop takes about one million cloud droplets to form, but a single hailstone may take as many as 10 billion cloud droplets to form. A golf ball-sized hailstone, as pictured below, must travel through the cloud for 5–10 minutes to be able to grow to this size. Strong, violent updrafts within the storm are able to carry smaller ice particles far above the freezing level where collisions with supercooled droplets allow them to grow into hailstones. Rotating updrafts, such as those found in a supercell, are capable of sweeping hail horizontally through the storm, which may allow them to grow much larger. As these growing ice particles pass through areas of the cloud with high liquid water content, extra coatings of ice form around them. Larger hailstones may ascend very slowly in a strong updraft, almost “floating” in place as many supercooled water droplets continue to collide into them. When the hailstone is carried away from the updraft, or if it gets too heavy, it will fall because it is no longer supported by the rising air.

Just below the cloud, the hailstones will begin to melt in the warmer air. Small hail might completely melt before hitting the ground, but in the strong updrafts of severe thunderstorms, hailstones can grow large enough to reach the surface before melting. Notice in the figure below that the hailstone has concentric layers inside, similar to tree rings. The concentric layers alternate clear and opaque ice. This is related to the path that the hailstone takes when it passes through the cloud, as well as the liquid water content of the different areas the hailstone passes through. In areas with low liquid water content (dry growth regime), supercooled water droplets freeze onto the hailstone instantly, which produces the milky opaque ice that contains many air bubbles. In areas with high liquid water content (wet growth regime), supercooled water droplets collect very rapidly onto the hailstone. The transition from liquid to ice releases latent heat, which keeps the surface temperature of the hailstone at 0°C. As a result, supercooled droplets will form a coat of water around the stone rather than freezing instantly. This allows porous areas to be filled in as the water coat freezes slowly and air bubbles escape from the surface. This leaves a layer of clear ice around the hailstone.

Golf ball sized hail cut in half so the inside concentric layers can be viewed (Public Domain).

Large hail can cause severe damage to crops, vegetation, aircraft, roofs, windows, and create a traffic hazard if hailstones accumulate on a roadway. Damage can be greater if the strong winds in a thunderstorm cause hailstones to move horizontally as they fall. Larger hailstones have higher terminal fall velocities and the potential to reach speeds higher than 50 m·s^-1 (112 mph).