Log entry: environmental studies: earth science2

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Chapter 6 Weather Systems

Visualizing Physical Geography by Timothy Foresman and Alan Strahler

Chapter Overview

Air Masses

Fronts

Midlatitude Cyclones

Tropical Cyclones

Thunderstorms and Tornadoes

Air Masses

Weather refers to the state of the atmosphere at one location and at one time:

Weather variables include:

Temperature

Moisture (dew point and RH)

Precipitation

Winds

Weather conditions change from day to day or even from hour to hour.

Air Masses

Weather systems are patterns of atmospheric circulation that lead to distinctive weather events, such as cyclones or thunderstorms.

Air Masses

What is the current high temperature, wind, moisture (dew point), and precipitation forecast for today?

Based on today’s weather, where do you think the air mass came from?

Air mass = an extensive body of air in which temperature and moisture characteristics are fairly uniform over a large area.

Source Regions of Air Masses

Source region: region where an air mass acquires its characteristics

Moisture characterized by whether it came from the:

Ocean = moist = m

Continent (land) = drier = c

Air Masses

Source Regions of Air Masses

Air masses are classified by the latitude and surface type of their source regions:

Maritime polar (mP)

Maritime equatorial (mE)

Maritime tropical (mT)

Continental tropical (cT)

Continental polar (cP)

Continental Arctic (cA)

Continental Antarctic (cAA)

Air Masses

Source Regions of Air Masses

Maritime equatorial (mE)

Maritime tropical (mT)

Continental tropical (cT)

Air Masses

Source Regions of Air Masses

Maritime polar (mP)

Continental arctic (cA)

Continental antarctic (cAA)

Continental polar (cP)

Air Masses

Movement of Air Masses

Air mass modification

When an air mass moves to a new area, its properties change due to the influence of the new surface environment.

Continental U.S. is not a source region—it is a battleground

What air mass is impacting your region today?

North America: A Battleground for Air Masses

General Characteristics

Diverse range of latitudes, influence of continentality, oceans and mountains

Mountains run north/south

High Severe Weather Potential

No other area of the world experiences such frequent juxtaposition of radically different air masses as mid-latitude North America

Physiography and midlatitude location contribute to potential

Physiographic features of North America with a mature midaltitude cyclone over the Great Plains.

Fronts

Front = the surface or boundary of contact between two different air masses associated with a midlatitude cyclone (MLC).

Four types of fronts:

Cold

Warm

Stationary

Occluded

Fronts

Cold Front

A cold front is a moving weather front along which a cold air mass moves underneath a warm air mass, causing the warm air mass to lift rapidly.

Air rises steeply.

Cumulonimbus clouds and thunderstorms may form.

Fronts

Warm Front

A warm front is a moving weather front along which a warm air mass slides over a cold air mass, producing stratiform clouds and precipitation.

Air rises gradually.

Nimbostratus clouds may form.

Which type of clouds first indicates an approaching warm front?

a. nimbostratus

b. stratus

c. altostratus

d. cirrus

Fronts

Occluded Front

A weather front along which a fast-moving cold front overtakes a warm front, forcing the warm air mass aloft

Triangles and semicircles pointing in the same direction

Stationary Front

Little to no relative motion

Blue triangles and red semicircles pointing in opposite directions

Midlatitude Cyclones

Cyclonic and Anticyclonic Systems

Cyclones

Air spirals inward, converges, and moves upward.

As the air rises, it cools adiabatically.

If the air becomes saturated at the LCL, condensation or deposition occurs, forming clouds and possibly precipitation.

Winds turn around a low pressure center.

Three types: midlatitude cyclone (MLC), tropical cyclone, and mesocyclone.

Anticyclones

Air spirals outward and diverges as it sinks.

The air warms as it sinks.

Fair skies and high pressure mark anticyclones.

Midlatitude Cyclones

Cyclones and Anticyclones

Is a cyclone or anticyclone impacting the region shown in on the left?

Is a cyclone or anticyclone impacting letter C?

Is a cyclone or anticyclone impacting letter A?

Midlatitude Cyclones

Evolution of a Midlatitude Cyclone

Midlatitude cyclones (MLCs) are also known as wave cyclones.

They are dominant in middle and higher latitudes.

Large inspiral of air repeatedly forms, intensifies, and dissolves along the polar front.

In the northern hemisphere, a cyclone normally forms at a polar front and moves eastward as it develops, propelled by prevailing westerly winds aloft.

Midlatitude Cyclones (MLC)

Evolution of a Midlatitude Cyclone

Initial conditions

Cyclogenesis

Open stage

Occluded stage

Dissolving stage

Midlatitude Cyclones

In the open stage of the cyclone (step 3), a ______ air mass could be behind the warm front, and a _____ air mass could be behind the cold front.

a. maritime tropical; continental polar

b. continental polar; maritime tropical

c. maritime polar; continental tropical

d. continental polar; continental tropical

Midlatitude Cyclones

Weather associated within a midlatitude cyclone

Midlatitude Cyclones

Weather Changes within a Midlatitude Cyclone

Cyclonic storm = an intense weather disturbance within a moving cyclone that generates strong winds, cloudiness, and precipitation.

Nor’easters bring strong winds, high tides, and high surfs.

Tropical Cyclones

Tropical cyclone = an intense traveling cyclone of tropical and subtropical latitudes, accompanied by high winds and heavy rainfall.

Anatomy of a Tropical Cyclone

Hurricanes in western hemisphere

Typhoons in the western Pacific

Cyclones in the Indian Ocean

Develop over oceans approximately 8 to 20o N/S

Winds greater than 74 mph

Courtesy NASA

Tropical Cyclones

Anatomy of a Tropical Cyclone

Characteristics:

Eye = relatively calm, sinking air at its center

Eyewall = strongest section, with intense rising air and wind

Spiral rain bands

Tropical Cyclones

Formation and Movement of Tropical Cyclones

Four conditions need to be in place for formation:

Low pressure

Weak Coriolis effect (faster winds to balance PGF)

High humidity (latent heat)

Warm sea surface temperatures

Tropical Cyclones

Formation and Movement of Tropical Cyclones

Tropical cyclone originates as a slow-moving band of low pressure, which intensifies and grows into a deep, circular low.

Easterly wave is a slowly moving trough of low pressure within the belt of tropical easterlies (trades) that grows over the warm ocean.

Cyclones do not form over the equator (±5 o latitude) since there is no Coriolis effect (no way to rotate air)

Data from McGraw-Hill

Tropical Cyclones

Formation and Movement of Tropical Cyclones

High humidity: Always form over the warm ocean with high water vapor content.

Warm sea surface temperatures (SST) greater than 81oF.

Season: Peaks in late summer/early autumn due to warmest SST.

Tropical Cyclones

Movement of Tropical Cyclones

Form in areas of greatest heating

Move west with trade winds

Then move northwest, north, and eventually northeast following the winds from the subtropical high and the upper level westerlies

Last until they hit land or pass over cooler water

Tropical Cyclones

Movement of Tropical Cyclones

Figures show hurricane tracks for 1985–1994 (blue) and 1995–2004 (red).

The intensity of storms increases when sea surface temperatures increase.

Tropical Cyclones

Movement of Tropical Cyclones

Naming of tropical cyclones: Male and female names alternate alphabetically.

Hurricane Andrew (1992) was the second most damaging.

Figure shows 2005 hurricanes, including Katrina, which was the most costly Atlantic hurricane on record.

Tropical Cyclones

Tropical Cyclone Intensity and Damages

Saffir-Simpson Scale

Tropical Cyclones

Tropical Cyclone Intensity and Damages

Precipitation

Earthflow

Storm surges

Which of these can human activity make worse?

Tropical Cyclones

Tropical Cyclone Intensity and Damages

Storm Surge

A rapid rise of coastal water level accompanying the onshore arrival of a tropical cyclone

Deadliest aspect of tropical cyclone

Tropical Cyclones

Tropical Cyclone Intensity and Damages

Hurricane Katrina flooding (2005) in New Orleans

Courtesy NOAA

Thunderstorms and Tornadoes

Thunderstorms and Unstable Air

Thunderstorms may form as a result of intense heating of the ground, resulting in the heated parcel of air warmer (and less dense) than the surrounding air.

Air cools to the dew point, clouds form (cumulus) and may grow into cumulonimbus (thunderstorm) clouds.

Clouds and storm may dissipate with the loss of heating.

Thunderstorms and Tornadoes

Thunderstorms and Unstable Air

Thunderstorm = an intense local storm associated with a tall, dense cumulonimbus cloud in which there are very strong updrafts of air.

Thunderstorms and Tornadoes

Severe thunderstorm:

Intense winds, precipitation, and hail

Strong updrafts and downdrafts

Anvil shape

Wind shear:

Change in wind velocity with height

Required in a severe thunderstorm

May result in rotation (mesocyclone)

Thunderstorms and Unstable Air

Thunderstorms and Tornadoes

Lightning:

Upward and downward motions in a thunderstorm create areas of positive and negative static charge within the cloud that are discharged by lightning

Thunderstorms and Unstable Air

Thunderstorms and Tornadoes

Forecasting Thunderstorms

Hot, summer weather in central and SE United States

National Oceanic and Atmospheric Administration (NOAA) and National Weather Service (NWS)

For a thunderstorm outlook, go to: www.spc.noaa.gov/products/exper/enhtstm/

Thunderstorms and Unstable Air

Courtesy NOAA

Thunderstorms and Tornadoes

Tornadoes

A small, very intense wind vortex with extremely low air pressure in the center, formed below a dense cumulonimbus cloud as apart of a cyclone, typically an MLC

Mesocyclone (rotating thunderstorm)

Courtesy NOAA

Thunderstorms and Tornadoes

Tornadoes

Enhanced Fujita Scale

Thunderstorms and Tornadoes

Tornadoes

Most common in central (tornado alley) and SE United States where cold and warm air clash

Thunderstorms and Tornadoes

Which stage of a thunderstorm leads to tornado formation?

a. cumulus

b. mature

c. dissipating