Flash Floods and Hail Flashcards
What is a downburst?
A strong downdraft in a thunderstorm that strikes the Earth surface and diverges horizontally as a violent wind pulse
What is a macroburst?
A downburst over a large area (>4km) typically lasting >10min
What is a miroburst?
A downburst over a small area (<4km) typically lasting <10min
Why is a microburst dangerous for aircrafts on takeoff or landing?
Because it triggers wind shear
What atmospheric conditions is a flash flood most likely to occur?
- High values of precipitable water
- High relative humidity
- A warm thunderstorm base
What thunderstorm characteristics cause hail?
- Strong updrafts
- Great vertical development
- An abundant supply of supercooled water droplets
What is a derecho?
A family of straight-line downbursts
What is a wet microburst?
A microburst that occurs in the wet regions of the world
What is a dry microbust?
Occurs in the dry regions with high bases of convective clouds
What are the stages of a microburst?
1) Midair microburst
2) Touchdown - high winds
3) Surface microbursts - high winds
What is a flash flood?
A short-term localized rise in stream level due to heavy rainfall
What is a giant rain storm?
A rainstorm that produces >200mm of rain
What is hail?
A lump or ball of ice with layered internal structures
What is a hailstone?
Hail that is >5mm
What happens in the first stage of hailstone growth?
-Falling ice crystals collect supercooled water droplets forming conical snow pellets called graupel
What are hailstone embryos?
- Graupel particles
- Large raindrops
- Frozen raindrops
What happens in the transition stage of hailstone growth?
-Graupel and large raindrops are entertained into the storm’s main updraft and are carried above the freezing level and become frozen raindrops
What happens in the second stage of hailstone growth?
-Hail embryos grow by acceleration of supercooled cloud droplets in the main updraft
What are the conditions needed for hailstones to form?
- Temperature 0 to -40°C
- Liquid water content 1 to 10 g/m^3
- Fallspeed 10 to 50 m/s
What does a dark layer and a white layer in a cross-section of a hailstone mean in transmitted light?
Dark: lots of air bubble (dry growth)
White: few air bubbles (wet growth)
What does a dark layer and white layer in a cross-section of a hailstone mean in reflected light?
White: Dry growth
Dark: Wet growth
What does dry growth mean?
There is no liquid water flowing over the hailstone surface as it grows
What does wet growth mean?
There is liquid water flowing over the hailstone surface as is grows
What does spongy growth mean?
Liquid water is being trapped by frozen water as the hailstone grows
