Natural Hazard--Hail

The knight Owain shelters from a hailstorm.
From: The Mabinogion stories (Welsh).
https://commons.wikimedia.org/w/index.php?curid=22447033

Natural Hazard--Hail

Hail is formed during thunderstorms when updrafts within the storms repeatedly lift water droplets to high altitude, where they freeze at very low temperatures, and then eventually fall back to the ground as ''Hailstones''. Most ''Hailstorms'' occur in plains terrain, in the spring and early summer, and in the late afternoon or early evening.   Falling hailstones can be very dangerous and cause significant destruction to crops, forests and buildings.  In terms of size, hailstones range between 0.2 inch (5 mm) and 6 inches (15 cm) in diameter.  Large hailstones can weigh more than 1.1 lb (0.5 kg). Hailstones 1-2 inches in diameter may fall at speeds of 25-40 mph, while larger hailstones 2-4 inches in diameter may fall at 44-72 mph.  The largest hailstones (more than 4 inches in diameter) may fall at speeds of 100 mph. Hail can fall up to 2 miles (3.7 km) away from the parent storm. Swaths of fallen hail from a single storm can range in size from a few acres to an area 10 miles wide and 100 miles long. 

Green hailstorm cloud.
(Image source: Facebook post: Rob Lord. Merewether Baths, Newcastle, Australia. 
https://www.abc.net.au/news/2018-12-21/what-makes-thunderstorms-green/10646304)

OMEN/SIGN: ''You notice a large thunderstorm approaching, and the lower clouds are an eerie shade of green.'' (Druids and Rangers may make a DC 8 Intelligence(Nature) check to recognize that such a storm has a high chance of producing dangerous hail.)

Large hailstones.
(Image source: https://commons.wikimedia.org/w/index.php?curid=23003409)

EFFECTS: Because Hailstones can be large, heavy, and fall at high speeds, they can cause injury and significant destruction. However, larger, more damaging Hailstones occur less frequently. When Hail is encountered, roll 1d20 on the table below to determine the Hailstone size and damage effects.

HAIL DAMAGE TABLE

d20	Hailstone Size	Hailstone Description	Cumulative Damage Effects
1-6	up to 0.75 inch	Smaller than a penny	25% crops destroyed
7-10	0.75 inch	Penny	50% crops destroyed
11-13	1.00 inch	Quarter	75% crops destroyed, glass windows broken, paint/wood scarred
14-15	1.50 inches	Ping-pong Ball	100% crops destroyed, thatched roofs damaged, 1d4 damage to unsheltered creatures
16-17	1.75 inches	Golf Ball	Tiled roofs damaged, 1d6 damage to unsheltered creatures
18	2 inches	Egg	Wagons/cart damaged, 1d8 damage to unsheltered creatures
19	2.5 inches	Tennis Ball	Wooden roofs damaged, 1d12 damage to unsheltered creatures
20	3.5+ inches	Softball	Most roofs destroyed, 1d20 damage to unsheltered creatures

Hail injuries.
(Image source: Tim Hartmuller. 2010.
https://www.news9.com/story/5e3500ebe0c96e774b36aa89/runner-caught-in-storm-pelted-by-hail)

Hail injuries.  (Image source: Pat Crawford. 2019. 

https://weather.com/storms/severe/news/hail-dangers-costs-20130403)

Hailstorm damage to corn field.
(Image source: https://commons.wikimedia.org/w/index.php?curid=16300230)

Hailstorm damage to vineyard.
(Image source: https://commons.wikimedia.org/w/index.php?curid=115021554)

OPTIONAL EFFECTS: After falling, hailstones might make the ground slippery, perhaps DC 10 Dex save or fall prone.  If a hailstorm occurs while players are in a forest, replace direct damage from hail with direct damage from falling, splintered, tree limbs (perhaps 1d6 piercing damage).  A hailstorm at sea could shred sails, leaving a sailing vessel adrift. 

DM TIPS: Add Hail to your Random Encounters table for plains/steppe terrain.  A hailstorm during combat adds an exciting complication to the encounter. After falling, hailstones make the ground slippery and can be thrown as rocks. If direct injury to PCs does not fit the situation, hailstones might instead injure horses, pets/familiars, etc., or destroy a random item of gear, a helmet, a shield, a cart, wagon, etc. As a plot twist, a hailstorm (or sleet storm) might counteract a wildfire (on plains, or in a forest) that threatens the PCs.

NOTE 1: Sleet is another form of frozen precipitation, but sleet "stones" are typically about the size of peas--much smaller than hailstones.  Furthermore, sleet is caused by a different process: a  temperature inversion--a layer of warm air sandwiched between two layers of cold air--causes snow to melt and then refreeze as it descends.  Sleet is too small to cause direct damage to PCs.

NOTE 2: Freezing rain is yet another form of frozen precipitation.  Freezing rain occurs when snowflakes descend into a warmer layer of air and melt completely before reaching frozen ground. Upon contact with frozen ground, a layer of Glaze is typically formed. Glaze is a smooth coating of ice on the ground and on objects such as trees, roads and rooftops. An accumulation of Glaze is heavy and can cause tree limbs and even roofs to break. A period of freezing rain that lasts several hours or more is called an Ice Storm.

NOAA Winter Precipitation Diagram.
(Image source: https://www.weather.gov/images/iwx/events/education/misc/snowsleetfzrarain.png)

SOURCES:

• https://en.wikipedia.org/wiki/Hail 
• https://www.nssl.noaa.gov/education/svrwx101/hail/
• https://www.torro.org.uk/research/hail/hscale 
• https://www.montana.edu/pdc/archive/2012/hail-storm-projects.html 
• https://weather.com/safety/thunderstorms/news/2020-05-12-15-things-to-know-about-hail
• https://www.weather.gov/ffc/hail 
• https://www.weather.gov/mlb/hail_rules 
• https://www.weather.gov/lsx/hail_climatology 
• https://weather.com/storms/severe/news/hail-dangers-costs-20130403
• https://wmo.asu.edu/content/world-highest-mortality-hailstorm 
• https://wmo.asu.edu/content/world-heaviest-hailstone
• https://www.spc.noaa.gov/publications/schaefer/hailfreq.pdf 
• https://news.wisc.edu/curiosities-why-does-the-sky-turn-green-before-a-tornado/