Sem#2 Chap 2 Flashcards
Thunderstorms
Towering clouds that produce lightning and thunder. May rise to the troppause
Typically <30 km diameter, with speed ~90 km/hr
Most occur over land within the tropics.
Severe thunderstorms (defined by the NWS) may produce:
- Winds >93 km/hr
- Hail >2.5 cm in diameter
- A tornado
Thunderstorm Development, Temperate climates:
- Thunderstorms are frequent during warm seasons.
Thunderstorms are rare in :
- Within high latitudes
- Over subtropical deserts
- Over oceans
Thunderstorm days:
- Days on which at least 1 thunderstorm occurs.
3 conditions must be present for thunderstorm:
- Moist air (mostly from oceans, lakes, and wetlands)
- A lifting mechanism
- Atmospheric instability
5 Lifting Mechanisms Force Air to Rise
Movement along a front:
Gust fronts:
Less distinct boundaries:
High relief (orographic lift):
Convergence:
Movement along a front:
- Advancing cold air forceswarm, moist air up.
Gust fronts:
- Thunderstorms create cold downdrafts.
- Downdrafts act like small cold fronts
Less distinct boundaries:
- Warm areas may contain less dense, buoyant air.
- Cool air may move beneath warm air
High relief (orographic lift):
- Mountains force air upslope.
Convergence:
- Colliding air flows may create lift.
Air May Rise Without Being _________.
Physically Lifted
Air above ground becomes buoyant enough to rise. Which is common during…..
Common during the summer on plains and mountain slopes
Unstable air:
- When an air parcel is less dense than its environment
Adiabatic expansion:
- Air parcel expands as it rises.
- Air in a rising, expanding parcel will cool.
Environmental lapse rate:
The rate temperature drops with increasing altitude.
* Typical rate is 4°–9°C/km
Unsaturated air parcels:
- Do not form clouds as they rise and cool
Saturated air parcels:
- Form clouds as they rise and cool
Dry adiabatic lapse rate (DAR):
- Applies to unsaturated air parcels
- Cool at ~10°C/km
- Relative humidity increases as air parcel rises and cools.
If DAR > than environmental lapse rate:
- Air parcel will cool, become stable, and stop rising.
Moist adiabatic lapse rate (MAR):
- Applies to saturated air parcels
- Cool at ~6°C/km
- Slower rate is due to latent heat released during condensation.
Conditional instability:
- An air parcel’s MAR < than the environmental lapse rate
- Air parcel remains warmer than environment.
- Air parcel remains buoyant and continues rising
Thunderstorm development depends on 2 variables:
- The environmental lapse rate
- An air parcel’s moisture content
Unsaturated air parcels:
- Cool more quickly than environment
- Become dense, stable, and stop rising
Saturated air parcels:
- Cool more slowly than environment
- May become conditionally instable
- Water condenses out, and a thunderstorm may develop
Class Question #1
What happens to a rising air parcel if its moist adiabatic rate is less
than the environmental lapse rate?
a) The air parcel will sink, warm adiabatically, and condense its water
vapor.
b) The air parcel will sink, cool adiabatically, and condense its water
vapor.
c) The air parcel will rise, warm adiabatically, and not condense its
water vapor.
d) The air parcel will rise, cool adiabatically, and condense its water
vapor
(No given answer)
Ordinary Thunderstorms
Do not rotate around a vertical axis
Form where regional winds do not change direction or speed
Rarely produce hail, strong winds, or tornadoes
~2,0000 are active at any time around the world.
The anvil shape on cumulonimbus cloud during a thunderstorm
Common development scenarios: for ordinary thunderstorms
- Late afternoon formation as sunlight warms ground
- Formation on mountain slopes due to orographic lifting
3 Stages of an Ordinary Thunderstorm
Developing, Mature, Dissipating
Developing: Ordinary Thunderstorm
warm, moist air is lifted, expands, and cools.
- Conditional instability develops.
- Cloud spreads laterally at tropopause into anvil shape.
Mature: Ordinary Thunderstorm
ice particles fall and melt into raindrops.
- Rain initiates downdraft due to evaporative cooling.
- Falling rain pushes down air and intensifies downdraft.
Dissipating: Ordinary Thunderstorm
- Downdraft suppresses updraft, cutting off storm’s energy source.
Squall-Line Thunderstorms
Thunderstorms organized in a line:
A shelf cloud often develops:
Most intense thunderstorms arrive after shelf cloud passes:
Thunderstorms organized in a line:
- Line may be a consolidated gust front, or a cold front.
A shelf cloud often develops:
- Forms over the front, often has little precipitation
- Straight-line winds may develop.
Most intense thunderstorms arrive after shelf cloud passes:
- Lightning and heavy rain are common.
- Wind is often gusty (variable velocity and direction)
Warmer-Season Squall-Line Thunderstorms, develop…..
Develop from the gust fronts of many ordinary thunderstorms
Many gust fronts form a single, large, coalesced pool of cool air.
Single, coalesced gust front advanced into warm, moist air.
New thunderstorms develop along advancing gust front
Radar:
- Initially: many individual thunderstorms are visible.
- Later: a well-defined line of thunderstorms is visible.
Frontal squall lines
- Thunderstorms occurring in a very long line
- Often seen along the “tail” of a mid-latitude cyclone
Derechos
Derecho: Spanish for “direct” or “straight”
Severe straight-line winds over a large geographic region
May develop along squall-line thunderstorms
Supercell Thunderstorms
A thunderstorm with a longlasting, rotating updraft
- Often produce tornadoes
- Typically isolated from neighboring thunderstorms
- Base diameter ~50 km
Vertical wind shear must be
present in troposphere.
Supercell Thunderstorm Development
Wind shear produces horizontal, rolling wind tube near ground.
Updrafts lift tube into an arch, and a cumulonimbus cloud develops.
Downdrafts splits the storm into two distinct thunderstorms.
Warm SSE air feeds the SE storm, while NW storm dies out.
Chap#2 Slide 21
Mesocyclones
Counterclockwise-spinning, medium-sized storms
High-altitude winds create asymmetrical anvil cloud.
Wall cloud develops at base of updraft.
Forward flank
NE side of NE-moving supercell
* Strong wind at forward flank
downdraft (FFD)
Rear flank
SW side of NE-moving supercell
* Rear flank downdraft (RFD)
NE tilt of storm
- FFD lies to NE of updraft, so
storm can last for hours.
Class Question #2
Identify one way that a squall-line thunderstorm can develop.
a) from a consolidated gust front of several ordinary thunderstorms
b) from a saturated air parcel sinking, warming adiabatically, and thereby producing intense precipitation
c) from the rapidly-cooling air in a rear-flank downdraft (RFD) of a supercell thunderstorm
d) from rapidly sinking air on the leeward slopes of a mountain range
(might be D as mentioned in class)
Lightning
A charge separation develops
in a thunderstorm’s clouds.
Lightning is a huge
electrostatic discharge.
- Electrons jump from tiny ice crystals to large ice particles.
- Ice crystals get (+) charge, ice particles get (–) charge.
Name the two variations of lighting strikes…
Cloud to Ground and Cloud-to-cloud lightning, which is the most common.
Updrafts carry tiny ice crystals (+ or - ?) to high altitude.
+
Large ice particles (+ or -?) remain
near cloud bottom.
-
Air is an insulator, so charge
separation is ________.
maintained
Stepped leader:
Electrons in cloud base surge to ground in steps ~50–100 m long
Charge separation develops between cloud base and ground.
- Negative charge of cloud base pushes away the ground’s electrons.
- The ground and surface air develop a net positive charge.
Positive streamer
Ionized (+) air molecules rise from ground to meet stepped leader
Electron flow channel
- Forms when stepped leader and positive streamer meet
- Main electrostatic discharge occurs (called a “return stroke”)
- Process may repeat and produce multiple return strokes
Positive-polarity cloud-to-ground lightning is rare:
- Electrons jump from ground to cloud’s positively charged anvil.
- Requires an especially large electrostatic potential
Protect yourself from lightning… list
- Stay indoors, away from conducting materials
- Install lightning rods on rooftops to direct current
Class Question #3
How is an electrical charge separation maintained in a thunderstorm?
a) A push-force is created when the electrons on ice crystals chemically combine with supercooled water droplets.
b) The air acts as an insulator and thereby keeps the positive and negatively-charged areas separated.
c) Updrafts and downdrafts act in unison to keep the east and west sides of a thunderstorm electrically separated.
d) Stepped leaders carry H2O ions up from the ground and into a thunderstorm, thereby creating and maintaining a charge
separation.
(Pretty sure its B)
Thunder
Lightning produces thunder
Lightning strikes instantly heat up and expand the air.
Air suddenly contracts when lightning strike ends
Downbursts
Downbursts are especially
intense downdrafts
Downdrafts created by:
* Evaporation causing air to cool and sink
* Rain pushing air downward
Microbursts
- A small downburst affecting ~5 km2
- Airplane lift is enhanced if taking off into a microburst.
- Airplane lift is diminished if flying out of a microburst.
Hailstones
Spherical lumps of solid ice
Supercooled water droplets
form at high altitudes.
Hailstorm
When hailstones fall to the ground
Hail swath
the path where hail falls to ground
Class Question #4
Identify one variable that causes hailstorms in Mendoza, Argentina, to be so severe that farmers must install hail-nets to protect their crops.
a) Dry air flows downslope from the Andes Mountains, creating counterclockwise-rotating gust fronts.
b) Severe air pollution from Mendoza creates anthropogenic uplift patterns that frequently produce thunderstorms and hail.
c) Moist air blows south, along the eastern slopes of the Andes Mountains, and often creates supercell thunderstorms.
d) Regular weather patterns in Mendoza repeatedly create tornadoes, 30% of which spawn severe hailstorms.
(no given answer)
Tornadoes
A violently rotating air vortex
Extends from thunderstorm’s
base to the ground
World’s largest number of
tornadoes is in ______.
Tornado Alley
Tornado outbreaks
multiple tornadoes in a short time
Supercell contains a _______.
mesocyclone. (A strong rotating updraft)
Supercell contains 2 _______.
downdrafts
- Forward flank downdraft
(FFD) on NE side - Rear flank downdraft (RFD)
on SW side
Rear flank downdraft (RFD) creates _______
roll of rotating air.
- Air spins on a horizontal axis.
When rolling air passes beneath mesocyclone:
- Updraft pulls rolling air upward.
- Rolling air stretches, thins, and wind speed increases.
- Tornado forms if vortex contacts the ground.
Funnel cloud visibility:
- A spinning cloud develops as updraft condenses moisture.
- Debris is carried aloft, darkening the vortex.
Multiple-Vortex Tornadoes
Tornado may break into several smaller vortices.
Very large tornadoes may have especially low pressure at vortex base.
Very low pressure may draw tornado core air downward.
Sides of tornado may be pushed outward from descending, core air.
Non-Supercell Tornadoes
Develop along squall lines and in hurricanes
Significant horizontal wind shear is needed:
* Wind in opposite directions on either side of a front
If shear is strong enough, small vortices may develop.
Updrafts may lift and stretch vortices into tornadoes.
Hurricane rain band shear may also cause tornadoes.
Defining Tornado Intensity
- Scale refined in 2007 to the Enhanced Fujita Scale
Tornado classification is not based on ______.
wind speed
Tornado Classification is based on ______
the damage tornadoes cause
Tornado fatalities also occur from ….
- Vehicles are tossed.
- Roofs collapse.
- Walls are blown away.
- Trees fall onto homes.
Doppler radar
- Detects rotating winds
- Detects tornado hook echoes
Tornado watch
- Weather conditions favor tornado formation.
Tornado warning
- A tornado has been seen or detected by radar.
Class Question #5
Why must hospitals especially emphasize tornado safety drills?
a) Because hospital administrators must lock all exterior doors, and
secure all windows, immediately following a tornado warning.
b) Because staff must move patients to safety, and this takes more than
the typical lead time of NWS tornado warnings.
c) Because backup electricity generators must be manually switched
on following a NWS issued tornado watch, and this takes training.
d) Because federal law mandates that all drugs be securely locked
away within 5 minutes of a NWS issued tornado warning.
(Not given)
