What is Hail and Hailstorms?
Hail, a type of precipitation, consists of solid ice forming within thunderstorm updrafts. Thunderstorms that produces hail and reaching the ground are termed hailstorms.
Typically lasting less than 15 minutes, these storms can inflict injuries on individuals and damage structures, vehicles, aircraft, and crops. Accumulated hail can lead to power outages, tree damage, and trigger flash floods and mudslides, particularly in hilly regions. Hailstorms may also coincide with other severe weather phenomena like cyclones and tornadoes.
Image: Hail (Source: Facebook)
Figure: Hailstorm (Source: Dhaka Tribune)
How Hail Forms?
Hailstones form when raindrops are lifted by thunderstorm updrafts (upward currents of air) into extremely cold atmospheric regions where they freeze. These hailstones then enlarge by colliding with liquid water droplets that freeze upon contact with the hailstone’s surface.
Figure: Hail formation (Source: Bureau of Meteorology, Australian Government)
The freezing process determines the appearance of the ice: if water freezes rapidly upon collision, cloudy ice forms due to trapped air bubbles. Conversely, slow freezing allows air bubbles to escape, resulting in clear ice.
Figure: Clear and Cloudy/Foggy hailstone (Source: playingwithrain.com)
Hailstones can develop layers of clear and cloudy ice when they encounter varying temperature and liquid water content within a thunderstorm. These conditions change as the hailstone moves horizontally across or near an updraft. However, the layering doesn’t solely result from the hailstone’s vertical movements within the storm. Thunderstorm winds aren’t just vertical; they include horizontal components from rotating updrafts, such as those in supercell thunderstorms, or from horizontal winds in the surrounding environment. Additionally, hailstones don’t increase in size solely by being carried to the top of the thunderstorm. At high altitudes, where temperatures drop below -40°F, all liquid water freezes into ice, hindering further hailstone growth since liquid water is necessary for substantial enlargement.
Hail descends when the thunderstorm’s updraft can no longer support the weight of the hailstone, either because it grows too large or the updraft weakens.
How does hail fall to the ground?
Hail descends when its weight surpasses the force of the thunderstorm updraft, drawing it toward the earth due to gravity. Horizontal winds within the storm can displace smaller hailstones away from the updraft, causing larger hail to fall nearer to the updraft. In cases where surface winds are stronger, hail may descend at an angle or nearly horizontally. Wind-powered hail can wreak havoc, damaging house siding, shattering windows, infiltrating buildings, and breaking car windows, potentially causing severe harm or fatalities to individuals and animals.
Hail Fall Speeds
The rate at which hail falls is complicated, dependent on factors such as hailstone size, air friction, local wind dynamics (both horizontal and vertical), and the extent of hailstone melting. Here are approximate fall speeds associated with different hailstone diameters:
Small hail (<1 inch in diameter): 9 to 25 mph
Common severe thunderstorm hail (1 to 1.75 inches in diameter): 25 to 40 mph
Large hail in intense supercells (2 to 4 inches in diameter): 44 to 72 mph
Very large hail (>4 inches in diameter): Over 100 mph
However, these estimates carry considerable uncertainty due to variations in hailstone shape, melting degree, fall orientation, and environmental conditions.
Estimating Hail Size
Hail size is often gauged by comparing it to familiar objects:
Pea: 1/4 inch diameter
Mothball: 1/2 inch diameter
Penny: 3/4 inch diameter
Nickel: 7/8 inch diameter
Quarter: 1 inch or larger hail is considered severe
Ping-Pong Ball: 1 1/2 inches diameter
Golf Ball: 1 3/4 inches diameter
Tennis Ball: 2 1/2 inches diameter
Baseball: 2 3/4 inches diameter
Tea Cup: 3 inches diameter
Softball: 4 inches diameter
Grapefruit: 4 1/2 inches diameter
Figure: Hail size estimation (Source: nbc26.com)
Hail Detection
Radar technology plays a crucial role in detecting hail. Doppler radar typically depicts hail as intense precipitation, resembling heavy rainfall. Advanced dual-polarization radar can differentiate between hail, ice pellets, and rain, providing insights into hail size and precipitation characteristics with greater precision.
Hail Forecasting
When forecasting hail, meteorologists seek out deep moist convection alongside three fundamental components:
Sufficient updraft strength to sustain hailstone aloft for a significant duration.
Presence of supercooled water in close proximity to the hailstone, facilitating its growth during ascent through the updraft.
Availability of a nucleus such as ice, snow, or dust for hailstone accretion.
Distinguishing between hail-producing and non-hail-producing storms is not always straightforward. Virtually all severe thunderstorms likely generate hail aloft, although it may dissolve before reaching the ground.
Multi-cell thunderstorms often generate numerous hailstones, albeit typically small in size. Their mature phase is relatively brief, limiting the time available for hailstone growth. Conversely, supercell thunderstorms sustain robust updrafts, fostering the formation of large hailstones by repeatedly lifting them into the frigid upper reaches of the storm cloud, where additional ice layers accumulate. Generally, hail measuring 2 inches (5 cm) or larger in diameter is indicative of supercell activity.
References
IFRC. (2024, March 18). Hailstorms | IFRC. https://www.ifrc.org/our-work/disasters-climate-and-crises/what-disaster/hailstorms
NOAA NSSL. Hail Basics . NOAA National Severe Storms Laboratory. Retrieved April 2, 2024, from https://www.nssl.noaa.gov/education/svrwx101/hail/
UNDRR. (2023, June 7). Hail | UNDRR. http://www.undrr.org/understanding-disaster-risk/terminology/hips/mh0036
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