Weather & Climate Systems | Exam #3 | Dr. Lauren Michel

overrunning.

The process best illustrated in the diagram of a front above is:

warm front.

The lifting of air and the resulting formation of clouds and rain is more gentle (gradual) for a:

the steep slope and fast forward motion of the front

The more violent nature of weather produced by a cold front can be attributed to which two factors?

the lifted air mass is moister than the intruding air mass.

A dryline causes uplift to occur because:

rotate counterclockwise.

Middle-latitude cyclones in the Northern Hemisphere:

All of the warm air has been lifted aloft.

Why does occlusion lead to the demise of a mid-latitude cyclone?

the dry slot

The clear skies that separate the head of the comma from the tail in a mid-latitude cyclone are associated with what feature of the storm?

Alberta Clipper.

A midlatitude cyclone forms in western Canada, and very rapidly moves through the Dakotas and passes over the Great Lakes region, bringing very cold temperatures with it. This storm would probably be called a(an):

the conveyor belt model.

The modern view of cyclogenesis, updated from traditional polar front theory to provide a better three-dimensional view, is known as:

an occluded front.

Line A-D probably represents:

cP

Which air mass type is generally found on the poleward side of a midlatitude cyclone?

similar values of temperature and moisture at any given altitude.

An air mass is a body of air with:

is characterized by a general stagnation of atmospheric circulation.

An air-mass source region:

have higher water vapor content.

Compared to continental air masses with the same air temperature, maritime air masses:

cold spells in winter

What weather is associated with cA air masses?

the north Pacific.

Most mP air masses that influence the U.S. originate over:

a strong storm forming in Canada and moving into New England from the North

All of the following accompany a classic New England nor'easter EXCEPT:

An atmospheric flow that transports vast amounts of moisture to regions outside of the tropics.

What is an atmospheric river?

cT

In North America the ________ air mass typically forms only in the summer.

The air mass is likely to become more unstable.

If an air mass passes over a surface that is warmer than it is, what can be said about the changes likely to happen within that air mass?

on the leeward shores of the Great Lakes

Where would you expect to find the greatest amount of lake-effect snowfall?

developing thunderstorms

dynamic thermal instability of air masses =

moist air has ELR between the dry and wet adiabatic rates

Conditional instability prevails when:

thunderstorm conditions

• warm, humid air

• conditionally unstable environment

• air lifted past LFC

localized convective lifting

• unequal surface heating causes localized pockets of air to rise

• cause of single-celled or ordinary celled thunderstorms

convergence

• horizontal air flow results in upward movement

• often associated with low pressure

• lifting mechanism to generate thunderstorms

orographic lifting

• elevated terrains act as barriers to airflow

• adiabatic cooling can generate clouds and copious precipitation

• lifting mechanism to generate thunderstorms

frontal lifting

• warm air rises over cooler, denser air

• lifting mechanism to generate thunderstorms (especially severe)

• wind shear

wind shear

• change in wind speed and direction with increasing height above the surface

• biggest difference between normal and severe thunderstorms

storms and lightning strikes

• greatest proportion occur where warm air, abundant moisture, and instability are greatest

• found in tropics year round

• largely warm season in mid-lats

mT air of the east Gulf Coast and Florida region.

Thunderstorms in the US are most frequent in:

air-mass thunderstorms

• ordinary cell/ ordinary [term]

• associated with warm, humid air that rises in unstable environments

• cumulonimbus clouds

• mT air masses

• single-celled, short lives, predictable life cycle

severe thunderstorms

• high winds, hail, flash floods, tornadoes

• result of uneven heating, frontal lifting, diverging winds

thunderstorm stages of development

• cumulus

• mature

• dissipating

cumulus stage

• largely a product of uneven surface heating and convergence

• cumulonimbus tower develops as water vapor moves from the surface to greater heights

• dominated by updrafts

• updrafts suspend raindrops

• downdrafts further aided entrainment (influx of cool, dry air)

mature stage

• downdraft leaves base of cloud, carries precipitation with it and triggers [term]

• cool downdrafts exist, as well as updrafts

• formation of anvil top

• gusty winds, lightning, possibly small hail

• occasional microbursts

microbursts

powerful, downward burst of air

dissipating stage

• rain turns updraft into downdraft, causes storm to be cut off from rising, moist air

• dominated by downdrafts and entrainment causing evaporation

experience a greater number of air-mass thunderstorms

Mountainous regions, such as the Rockies, and the Appalachians,:

National Weather Service thunderstorm criteria

• one of these must be satisfied:

  • winds in excess of 58 mph

  • produce hail larger than 1 in

  • generate tornado

overshooting top

updrafts strong enough so top pushes into stable lower atmosphere

gust fronts

leading edges of downdraft or outflow boundary

straight line winds

travel behind gust front

roll clouds (shelf clouds)

• arcus clouds

• horizontal tube-shaped cloud

• form at leading edge of storm between cold downdrafts and warm inflow of air

shelf clouds

form in a similar environment to roll clouds, but are low, wedge-shaped clouds

squall lines

• narrow bands of thunderstorms that look like a line with a bulge on radar

• mammatus skies sometimes precede

• cT air pulled into warm sector of a mid-lat cyclone

• commonly create straight-line winds, heavy rain, lightning; small hail or tornadoes

• can also form along dryline (abrupt change in moisture)

mesoscale convective complexes (MCC)

• consists on many individual t-storms

• organized into large oval to circular cluster

• cover an area of at least 100,000 km²

• slow-moving complex that may last for 12 hours or more

• form mainly in Great Plains

supercells

• can produce extremely dangerous weather

• single, powerful cell that can extend to heights of 20 km (65,000 ft) or more

• massive cumulonimbus clouds (20-50 km / 12-30 mi dia)

• single-celled structure that is complex and varies between one structure to the next

• large anvil, overshooting tops, mammatus clouds may form

vertical speed shear

• difference in air velocity between surface and the midlevel

  • change in wind direction with height from the ground

• caused by intense rotating updraft

mesocyclone

vertical winds may cause the updraft to rotate, forming a column of cyclonically rotating air; tornadoes often form

lightning

• discharge of electrical energy

  • produces a negative flow of current from region of negative to positive charge or vice versa

• vast majority is sheet [term]

  • produces bright but diffused illumination

• cloud-to-ground variety kills people

flash

• total discharge

• entire amount of electrons or energy

• lasts few tenths of a second

• visible; contains multiple strokes; downward movement

leader

ionized air, which forms a conductive path

step leader

• extends earthward in a short, nearly invisible burst

  • “flash travels down [term]”

return stroke

extends upward from ground to cloud

thunder

• expands explosively

  • produces sound waves that travel at 330 mps (1000 fps)

  • air is heated quickly to as much as 33,000 C

• if lightning is more that 20 km away, this is not heard

tornadoes (twisters, cyclones)

• violent windstorms with rapidly rotating column of air, or vortex, that extends from a cumulonimbus cloud

  • pressures can be as much as 10% lower than immediately outside the storm

• may consist of single or multiple vortices

• color indicates dust and debris

multiple-vortex tornadoes

• have smaller intense whirls inside, called suction vortices

  • orbit the center of the larger circulation

• dia of 10 m

• form and die in less than 1 min

• responsible for narrow paths of extreme damage

an important precondition to tornado development.

Development of a mesocyclone is:

Dixie Alley

• new tornado region

• Missouri, Kentucky, Tennessee, etc

• mainly night tornadoes, some in late afternoon

tornado season

• Spring, with a March/April peak

• lowest stat likelihood in December/January

profile of a tornado

• average diameter of 150-600 m

• travels ~45 kph

• most travel NE

• exist between under 3 min to ~ 3 hrs

• wind speed between 150 kph - 500 kph

Tri-state Tornado

• single deadliest tornado in US

• 695 dead

• March 18, 1925

• Missouri-Illinois-Indiana

expand unexpectedly (1.5 mi dia to 3+ mi dia in minutes).

Tornadoes can rapidly:

Enhanced Fujita Scale

• assessed on damage

• includes wind speed (tornado intensity)

tornado watch

• ALERT the public

• conditions are favorable, weather event is possible

• cover large areas and can last 3 hrs or longer

tornado warnings

• issued when event is actually sighted or conditions are just right

• usually for a smaller area

• short duration, 30-60 min average

Doppler radar

• measures motion and speed of wind

• two or more units are optimal for more accurate forecasting

• tornadoes have hook-shaped echoes

warm; unstable or conditionally unstable

Thunderstorms form when ________, humid air rises in a(n) ________ environment.

Wind shear

________ is defined as a significant change in wind speed and/or wind direction with increasing height above the surface.

mT

Ordinary, or air-mass, thunderstorms usually form in association with ________ air masses.

cumulus stage

Updrafts dominate the cumulonimbus cloud during the ________ in the life cycle of an air mass thunderstorm.

It causes the storm to become tilted.

What key change does wind shear promote in a thunderstorm that helps extend its life span?

supercell.

Some of the most dangerous weather is produced by a type of thunderstorm called a(n):

lightning.

The terms leader, flash, and stroke are used when describing:

streamer

During the formation of a lightning strike, a ________ is emitted from the surface and connects with the channel mid-air in order to complete the path for electrons to travel into the ground.

cyclone.

A tornado is also sometimes known as a:

tornado watch — a tornado has been sighted on the ground

Which association is incorrect?

middle latitude (midlatitudes)

area of NA roughly between southern Alaska and Florida where prevailing westerlies control the weather

fronts

• boundary surfaces between air masses of different temps

• usually, one air mass is warmer and contains more moisture than the other

  • violent clash when they meet

• shown as lines on weather maps

overrunning

process of warm air gliding up and over a cold air mass

five basic types of fronts

• warm front

• cold front

• stationary front

• occluded front

• dryline

warm fronts

• warm air moves into area previously occupied by colder air

• red line w/red semicircles on line in direction of travel

• gentle slopes

• all precip happens before the front arrives

cold fronts

• cold air advances into area occupied by warm air

  • zone of discontinuity

• blue line w/blue triangles pointing in direction of travel

• slopes twice as steep as warm fronts

• advance twice as fast as warm fronts

stationary front

• airflow is neither toward cold air nor a warm air mass

• surface position does not move, or it moves sluggishly

• red semi-circles and blue triangles on alternating sides

• remain over an area for a long time

occluded front

• rapidly moving cold air overtakes a warm front

• purple line with alternating semicircles and triangles pointing in direction of movement

occlusion

as cold air wedges a warm front upward, a new front forms between the advancing cold air and the warm air

cold-type occluded front

cold front lifts both the warm front as well as the cool air mass that lies ahead of it

warm-type occluded front

the air behind an advancing front is warmer than the cold air it is overtaking

drylines

• dry air is more dense than humid air

• boundaries which develop most often over the southern Great Plains

• occur when dry cT air meets moist mT air from the Gulf of Mexico

• most often generate t-storms

midlatitude cyclones

• synoptic-scale (macro)

• low-pressure systems

• dia often exceed 1000 km

• can impact weather for over a week

• west to east

• counterclockwise

• warm front (E) and cold front (W) extend from center (early stages of dev)

Norwegian cyclone model

• polar front theory

  • midlat cyclones dev along polar fronts

• cold air moving towards equator collides with warm air moving towards poles

cyclogenesis

development and strengthening of cyclones

formation

• step 1 / step A

• clash of two air masses of different temps (densities) move parallel to polar front in opposite directions

• cP from polar easterlies, mT from westerlies

• boundary becomes wave shaped under right circumstances

• waves change shape as storm intensifies

development of cyclonic flow

• warm air → poleward & forms warm front

• cold air → EQ & forms cold front