GEOG 2050 CH.6 The Restless Sky: Storm Systems and El Nino Flashcards

Flashcards for chapter 6 of Geography of the Atmosphere 2050

Air masses classification based on temperature

• P = polar

• T = tropical

• A = arctic

• E = equatorial

• AA = antarctic

Air masses classification based on humidity

• m=maritime

• c=conditional

cP (North America)

•  form only in NH and are most developed in winter (and cold weather months)

  • Major player in middle and high latitude weather

  • Cold, dense cP air displaces warm, moist air

    • Produces lifting, cooling, and condensation

    • Area covered by cP air in winter = cold, stable air, clear skies, high pressure, and anticyclonic wind flow

mP (North America)

• n the NH these exist over the northern oceans

  • Cool, moist, unstable conditions

  • Aleutian and Icelandic subpolar low pressure cells exist within these mP air masses (especially in winter)

mT (North America)

•  2 exist that influence North America

  • Over the Gulf/Atlantic

    • Influences East and Midwest - unstable and active form late spring to early fall

  • Over the Pacific

    • Stable to conditionally unstable, generally lower in moisture and energy than Gulf/Atlantic mT

    • Influences western U.S. with lower average precipitation than rest of U.S.

cT (North America)

• forms only in the summer from inland heating 

  • Brings very hot, dry air into different regions

  • Creates a sharp gradient in dew points

    • Can help create impressive thunderstorms

Air Mass Modification

• As air masses migrate, their characteristics will slowly change based on surface characteristics.

  • E.g., the mT Gulf/Atlantic may carry humidity to Chicago and on to Winnipeg, but it will gradually lose its warmth and humidity with its movement northward

  • E.g., cP changes as it moves south and east to produce snow belts that lie to the east of each of the Great Lakes.

    • Lake-effect snow

  • As cP passes over the warmer Great Lakes, it absorbs heat energy and moisture

Storm Energy Sources

• Storm systems derive energy from:

  • Solar heating of Earth’s surface

  • Condensation of water vapor in the atmosphere

Thunderstorms

• A cumulonimbus cloud that produces lightning and thunder

  • The SE (particularly Florida) has highest frequency of thunderstorms in the United States because of mT air masses from Gulf of Mexico

  • Globally thunderstorms are most frequent in the tropics over land

3 types of thunderstorms

• Single-cell thunderstorms

• Multicell thunderstorms

• Supercell thunderstorms

  • 2 most important factors that determine thunderstorm type are atmospheric humidity and wind shear 

Wind Shear

changes in wind speed and direction with altitude

Single-cell

• Relatively mild and short-lived

• Form within mT air masses where wind shear is week

• Develop in late afternoon

• Typically experience a predictable sequence of growth, maturation, and dissipation

Multicell

• Form under conditions of moderate wind shear (wind speeds of 40 to 65km/h or 23 to 40 mph)

• Form along fronts rather than within air masses

• Arranged in clusters (mesoscale convective systems) or in squall lines

• multicell thunderstorms often produce severe weather

Squall line

Line of multicell thunderstorm cells that typically forms along a cold front on a midlatitude cyclone

Severe Thunderstorm

• Produces either hail 2.54cm (1in) in diameter, a tornado, or wind gusts of 93 km/h (58mph) or greater

Supercell Thunderstorms

• Contain rotating cylindrical updraft

• Usually produces severe weather

• Produce almost all powerful tornadoes

• Form over land where there is humid air and strong wind shear

Lightning

• Electrical discharge produced by thunderstorms

  • Cloud-to-cloud, within a single cloud, cloud-to-ground

  • Follows channels of least resistance (ex wet solid, tree roots)

  • Lightning can fuse silica in sand, forms glassy hollow tube (fulgurite)

  • Superheats the air to over 30,000 degrees Celsius

Thunder

• Acoustic shock wave produces when lighting rapidly heats and expands the air around it

What Causes Lightning?

• Cumulonimbus clouds develop a separation of electrical charges

• Earth’s surface has a negative charge, and upper atmosphere has a positive charge

• Opposite electrical charges build up

• Two oppositely charged regions develop an electrical connection, and a bolt of lightning is discharged

Staying Safe in Lightning

• Time lag between lightning and thunder … possible to calculate distance of lightning

• 30/30 rule

  • Not safe outdoors if lightning within 10km (6 miles) (30 secs between lightning and thunder)

  • Wait 30 mins after storm has passed to go back outside

Tornadoes

• Violently rotating column of air that descends from a cumulonimbus (wall) cloud and touches ground

  • Form in thunderstorms, hurricanes, and cold fronts

Funnel Cloud

• The same as a tornado, but not in contact with the ground

Ranking Tornadoes

• Tornadoes are ranked using the enhanced Fujita scale (EF scale)

• Tornado strength is estimated by damage done to the landscape

Tornado Geography

• United States has most frequent and strongest tornadoes in the world (Florida has the most)

• April-July - most active period for tornadoes

• Great Plains - warm, humid (mT) air masses interact with dry, cold air masses from W and N… “Tornado Alley”

Tornado Watch

• Alert issued by National Weather Service (NWS) when conditions are favorable to tornadic thunderstorms

Tornado Warning

• Warning issued by NWS after tornado has been seen and called in to local authorities or is suggested by Doppler radar hook echo signature

Hurricanes

• North American name for tropical cyclone with sustained winds of 119km/h (74 mph) or greater

  • Rotate counterclockwise (in Northern Hemisphere) around region of low pressure

• Tropical cyclones go by other names…

  • Typhoon in Southeast Asia

  • Cyclone in countries bordering Indian Ocean

  • Tropical Cyclone in Australia (or willy-willy on the north coast)

Air Movement within a Hurricane

• Tropical cyclones have:

  • Calm eye

  • Eyewall of heavy wind and rain

  • Rain bands

• Winds are fastest in eyewall and slowest in the eye

• The eye has the lowest pressure

Hurricane Strength

• Hurricane strength depends on how much water vapor condenses to liquid (the latent heat exchange)

• Condensation releases latent heat into the storm, creating a positive feedback

• Hurricanes must have warm seawater to persist

Ranking Hurricane Strength

• For a hurricane to persist and strengthen, must have ample supply of warm [about 26°C (80°F)] water that readily evaporates

• Must be little to no wind shear, which tears hurricanes apart

Saffir-Simpson Scale

• Based on measured wind speeds

• Used to describe 5 categories of hurricane intensity

Hurricane Geography

• Hurricanes are restricted to tropical oceans

• Subtropical highs guide all hurricanes

• Do not occur within 5 degrees latitude of the equator due to lack of Coriolis force

Hurricane Hazards

• Worldwide, hurricanes are the main meteorological killer

• Coastal storm surge is most dangerous aspect of a hurricane

Storm Surge

• Rise in sea level caused by strong winds and low atmospheric pressure of a hurricane

• Indian Ocean 

  • Particularly vulnerable because large populations live near coast, and poverty makes evacuation difficult

Anatomy of a Midlatitude Cyclone

• Most midlatitude cyclones are composed of a warm front and a cold front

Cold Front

• Region where cold air advances on relatively warm air

Warm Front

• Region where warm air advances on and flows over cooler, heavier air

Effects of Midlatitude Cyclones on Weather

• Midlatitude cyclones bring storms to midlatitude regions (ex US and Canada) fall through spring

• Warm fronts are usually associated with nimbostratus clouds that bring steady precipitation

• Cold fronts are usually associated with cumulonimbus clouds that bring short bursts of rainfall and potentially severe weather 

Life Cycle

• Midlatitude cyclones experience growth, maturation, and dissipation over about 1-2 weeks

El Nino

• Periodic shift in Earth’s climate caused by the temporary slackening and reversing of Pacific equatorial trade winds and increased surface temperatures in seas off coastal Peru

  • Occur randomly every 3 to 7 years on average

  • Develop in March-June and reach peak intensity December-April

Effects of El Nino

• Alters moisture and weather patterns, causing drought and flooding in many regions

• Brings fewer hurricanes for Atlantic Ocean

• Often weakens Asian monsoon

La Nina

• Often follows El Nino and creates “enhanced normal” meteoritical conditions for affected areas

La Nina and the Enso

• El Nino and La Nina events and changes they cause are collectively called ENSO

  • El Nino - Southern Oscillation

  • Scientists do not know what triggers El Nino, and cannot reliably predict it more than 6 months in advance

Hurricane Vulnerability

• Hurricane Katrina and Superstorm Sandy inflicted massive damage

• Almost all coastal regions in the Gulf of Mexico and up the East Coast of North America have been struck by hurricanes

• New Orleans and Miami are largest metro areas at risk for a hurricane disaster

Climate Change and Hurricanes

• There is a natural cyclical pattern of hurricane activity

• Climate change could result in more and stronger hurricanes but so far no such trend has been observed in frequency

• However, it seems the most intense events are getting worse