9: Hail Flashcards
1
Q
What is hail and properties
A
- 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
2
Q
Where was record canadian hailstone
A
- Narrow Hills, Saskatchewan in 1973
3
Q
How to know if possible hailstorm is coming
A
- 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
4
Q
two types of clouds in thunderstorm
A
- 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
5
Q
Hailstone Growth - Stage 1
A
- 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
6
Q
Hailstone Growth - Transition
A
- 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
7
Q
Hailstone Growth - Stage 2
A
- 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
8
Q
Growth Layers in Transmitted Light
A
- 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
9
Q
Growth Layers in Crossed Polarizing Filters
A
- Dark layers have small crystals –> dry growth
- Light layers have large crystals –> wet growth
10
Q
Growth Layers in Reflected Light
A
- 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
11
Q
3 ways to view hailstone layers
A
- transmitted light
- reflected light
- crossed polarizing filters
12
Q
3 Types of Growth
A
- wet
- dry
- spongy
13
Q
What is dry growth
A
- 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
14
Q
what is wet growth
A
- 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
15
Q
What is spongy growth
A
- 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
