Flash Floods and Hail

Question 1

What is a downburst?

Answer 1

A strong downdraft in a thunderstorm that strikes the Earth surface and diverges horizontally as a violent wind pulse

Question 2

What is a macroburst?

Answer 2

A downburst over a large area (>4km) typically lasting >10min

Question 3

What is a miroburst?

Answer 3

A downburst over a small area (<4km) typically lasting <10min

Question 4

Why is a microburst dangerous for aircrafts on takeoff or landing?

Answer 4

Because it triggers wind shear

Question 5

What atmospheric conditions is a flash flood most likely to occur?

Answer 5

• High values of precipitable water
• High relative humidity
• A warm thunderstorm base

Question 6

What thunderstorm characteristics cause hail?

Answer 6

• Strong updrafts
• Great vertical development
• An abundant supply of supercooled water droplets

Question 7

What is a derecho?

Answer 7

A family of straight-line downbursts

Question 8

What is a wet microburst?

Answer 8

A microburst that occurs in the wet regions of the world

Question 9

What is a dry microbust?

Answer 9

Occurs in the dry regions with high bases of convective clouds

Question 10

What are the stages of a microburst?

Answer 10

1) Midair microburst
2) Touchdown - high winds
3) Surface microbursts - high winds

Question 11

What is a flash flood?

Answer 11

A short-term localized rise in stream level due to heavy rainfall

Question 12

What is a giant rain storm?

Answer 12

A rainstorm that produces >200mm of rain

Question 13

What is hail?

Answer 13

A lump or ball of ice with layered internal structures

Question 14

What is a hailstone?

Answer 14

Hail that is >5mm

Question 15

What happens in the first stage of hailstone growth?

Answer 15

-Falling ice crystals collect supercooled water droplets forming conical snow pellets called graupel

Question 16

What are hailstone embryos?

Answer 16

• Graupel particles
• Large raindrops
• Frozen raindrops

Question 17

What happens in the transition stage of hailstone growth?

Answer 17

-Graupel and large raindrops are entertained into the storm’s main updraft and are carried above the freezing level and become frozen raindrops

Question 18

What happens in the second stage of hailstone growth?

Answer 18

-Hail embryos grow by acceleration of supercooled cloud droplets in the main updraft

Question 19

What are the conditions needed for hailstones to form?

Answer 19

• Temperature 0 to -40°C
• Liquid water content 1 to 10 g/m^3
• Fallspeed 10 to 50 m/s

Question 20

What does a dark layer and a white layer in a cross-section of a hailstone mean in transmitted light?

Answer 20

Dark: lots of air bubble (dry growth)
White: few air bubbles (wet growth)

Question 21

What does a dark layer and white layer in a cross-section of a hailstone mean in reflected light?

Answer 21

White: Dry growth
Dark: Wet growth

Question 22

What does dry growth mean?

Answer 22

There is no liquid water flowing over the hailstone surface as it grows

Question 23

What does wet growth mean?

Answer 23

There is liquid water flowing over the hailstone surface as is grows

Question 24

What does spongy growth mean?

Answer 24

Liquid water is being trapped by frozen water as the hailstone grows