10.2: Precipitation during Thunderstorms

Types of precipitation during thunderstorms

Thunderstorms consist of deep convective clouds that can produce large raindrops (2–8 mm in diameter) within 5–10 km wide rain shafts that move horizontally across the ground, with precipitation lasting from 1 to 20 minutes over a fixed point. Rainfall rates may be high, with rates ranging from 10 to 1000 mm per hour. Figure \(\PageIndex{1}\) depicts heavy rain shafts produced by a supercell thunderstorm in Montana. However, thunderstorms can generate a variety of precipitation types.

Figure \(\PageIndex{1}\): A supercell thunderstorm in Montana producing heavy, localized precipitation. (CC BY 2.0; Jedimentat44 via Flickr).

Graupel

The cloud tops of thunderstorms extend far up in the troposphere. In the upper regions of the clouds, ice-phase processes take place. In the cloud, ice crystals and water droplets of different sizes exist, and heavier hydrometeors fall faster than smaller ones, sometimes colliding and coalescing. If these heavier ice particles fall through regions of supercooled droplets (liquid water droplets below freezing), they may grow through a process called riming. Liquid water droplets freeze instantly and adhere to the outer surface of falling ice particles upon contact. This forms snow pellets or soft hail called graupel, which are less than 5 mm in diameter. Fallen graupel is pictured in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Graupel pellets on a car roof from the previous evening's thunderstorm in Elko, Nevada. (CC BY-SA 3.0; Famartin via Wikimedia Commons).

Alternatively, smaller ice crystals that fall just below 0°C may partially melt and adhere to other partially melted ice crystals via collisions, thereby promoting their growth through aggregation. These snow aggregates can reach up to 1 cm in diameter. Snow aggregates and graupel can sometimes reach the ground still partially frozen, even in summer, if they fall within cooler, saturated downdrafts. More frequently, these larger ice particles will melt completely into large raindrops before hitting the ground.

Hail

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 can carry smaller ice particles far above the freezing level, where collisions with supercooled droplets enable 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 regions of the cloud with high liquid water content, additional ice coatings 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 a hailstone is carried away from the updraft or becomes too heavy, it will fall because the rising air no longer supports it.

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. Figure \(\PageIndex{3}\) shows people holding some hailstones which are cut in half. Note in the figure that the hailstone has concentric layers, similar to those found in tree rings. The concentric layers alternate between clear and opaque ice. This relates to the hailstone's path through the cloud and the liquid water content of the regions it passes through. In areas with low liquid water content (a dry growth regime), supercooled water droplets freeze instantly onto the hailstone, producing a milky, opaque ice that contains many air bubbles. In areas with high liquid-water content (wet-growth regime), supercooled water droplets accumulate rapidly on the hailstone. The transition from liquid to ice releases latent heat, thereby maintaining the hailstone's surface temperature at 0°C. As a result, supercooled droplets will form a water film around the stone rather than freezing immediately. 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.

Figure \(\PageIndex{3}\); Golf ball-sized hail cut in half so the inside concentric layers can be viewed. (Public Domain; NOAA via Wikimedia Commons).

Large hail can cause severe damage to crops, vegetation, aircraft, roofs, and windows, and create a traffic hazard if hailstones accumulate on roadways. 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 (112 mph).