9: Hail

Question 1

What is hail and properties

Answer 1

• ball or lump of ice >5mm
• Hail falls from cumulonimbus clouds
• small/medium hailstones are spherical
• large hailstones are oblate spheroidal
• rough surface often with lobes
• layered internal structure

Question 2

Where was record canadian hailstone

Answer 2

• Narrow Hills, Saskatchewan in 1973

Question 3

How to know if possible hailstorm is coming

Answer 3

• supercell
• wall cloud: low cloud that develops beneath the base of a cumulonimbous cloud from which tornadoes can form
• very dark cloud base
• twoering cumulonimbus cloud

Question 4

two types of clouds in thunderstorm

Answer 4

• Seeder cloud: on top of feeder cloud
• Cloud droplets fall from the seeder cloud into the feeder cloud and in that process will grow in size by collecting water vapor

Question 5

Hailstone Growth - Stage 1

Answer 5

• Hailstone growth begins in the feeder clouds in the flanking line
• falling ice crystals collcet supercooled water droplets forming conical snow pellets called graupel
• Graupel particles can melt to form large raindrops
• Large raindrops and graupel particles are called hailstone embryos

Question 6

Hailstone Growth - Transition

Answer 6

• Graupel and large raindrops are entrained into the storm’s main updraft
• Raindrops carried above the freezing level (Oc) become froezen raindrops
• Graupel and frozen raindrops become the embryos of hailstones

Question 7

Hailstone Growth - Stage 2

Answer 7

• Hail embryos grow by accretion of supercooled cloud droplets in the main updraft
• Conditions: temperature between 0 to -40C, liquid water content 1 to 10g/m^3, fallspeed 10 to 50m/s
• Growth modes: dry, wet, spongy
• Growth layers are the result of fluctuations in the liquid water content (LWC) –> more water = more growth

Question 8

Growth Layers in Transmitted Light

Answer 8

• In transmitted light, a hailstone’s thin section exhibits alternating light and dark layer
• dark/black layers consist of ice containing large numbers of tiny air bubles –> dry growth
• light/white layers consist of ice containing no bubbles or a few –>wet growth

Question 9

Growth Layers in Crossed Polarizing Filters

Answer 9

• Dark layers have small crystals –> dry growth
• Light layers have large crystals –> wet growth

Question 10

Growth Layers in Reflected Light

Answer 10

• opposite of transmitted light
• light layers consist of ice containing large numbers of tiny air bubles –> dry growth
• dark layers consist of ice containing no bubbles or a few –>wet growth

Question 11

3 ways to view hailstone layers

Answer 11

1. transmitted light
2. reflected light
3. crossed polarizing filters

Question 12

3 Types of Growth

Answer 12

• wet
• dry
• spongy

Question 13

What is dry growth

Answer 13

• cloud droplets freeze individually on impact and pile up on hailstone surface
• air trapped between cloud droplets forms numerous tiny air bubbles in ice –> called bubbly ice
• requires hailstone temperature to be less than 0 C
• high heat transfer, small mass flux
• does not mean no water, just that there is no liquid water flowing over the hailstone surface

Question 14

what is wet growth

Answer 14

• cloud droplets spread over the hailstone surface on impact and don’t have time to freeze before the next droplet hits
• a film of unfrozen liquid flows form lower surface where the cloud water impacts to the upper surface where it is shed –> called icicle lobes
• as the liquid film freezes, air dissolved in the water comes out of solution, forming bubbles
• the bubbles remain in the liquid and rae nto incorporated into the ice, giving clear ice
• air temperature close to 0C and high liquid water content

Question 15

What is spongy growth

Answer 15

• if air temperature is very cold but liquid water content is high, dendritic ice crystals grow rapidly onto the surface liquid film, trapping unfrozne liquid within the ice
• can be up to 50% liquid - mushy hail
• as the trapped water freezes, air dissolved in the water comes out of the solution, forming air bubbles that can’t escape. This gives the ice a milky appearance

Question 16

Relationship between hailstones and updrafts and terminal velocity

Answer 16

• bigger updrafts = bigger hailstones
• The terminal velocity of hail is: v = 12√D where D is hail diamter in cm and v is m/s (know this formula)
• Umax (maximum hail size) = v = 12√D –> the D is the Umax

Question 17

kinetic energy formula

Answer 17

E = 1/2mv^2 where m is the mass of the hailstone

Question 18

Model of hailstreak development

Answer 18

• Hail falls in a long narrow strip vertically
• hailstreak: up and down, path in which hail falls
• Hail swatch: width of hail streak
• Wind shear from west to east

Question 19

how to know duration of hailstorm

Answer 19

• formula: speed = distance/time

Question 21

How to determine hailstone mass

Answer 21

• mass = density x volume
• ice has a density of 0.9g/cm^3
• water has a density of 1g/cm^3