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Weather Patterns and Severe Weather

Chapter 14 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Define air mass.

Describe the classification and weather associated with air masses.

Focus Questions 14.1

Air mass characteristics

Large body of air

1600 km (1000 mi) or more across

Several kilometers thick

Similar temperature at any given altitude

Similar moisture at any given altitude

Move and affect a large portion of a continent

Air mass weather

Air Masses

Source region

Where an air mass acquires its properties

Classification of an air mass

Two criteria used to classify air masses:

By the latitude of the source region

Polar (P) and Arctic (A)

High latitudes: cold

Tropical (T)

Low latitudes: warm

By the nature of the surface in the source region

Continental (c)

Form over land: dry

Maritime (m)

Form over water: humid

Air Masses

Five basic types of air masses

Continental polar (cP)

Continental arctic (cA)

Continental tropical (cT)

Maritime polar (mP)

Maritime tropical (mT)

Air Masses

Air masses and weather

cP and mT air masses important in North America

North America (east of the Rocky Mountains)

Continental polar (cP)

From northern Canada and interior of Alaska

Winter: Brings cold, dry air

Summer: Brings cool relief

Air Masses

Air masses and weather

North America (east of the Rocky Mountains)

Continental polar (cP)

Responsible for lake-effect snows

cP air mass crosses the Great Lakes

Air picks up moisture from the lakes

Snow occurs on the leeward shores

Air Masses

Satellite image of lake-effect snow storm

Air masses and weather

North America (east of the Rocky Mountains)

Maritime tropical (mT)

From the Gulf of Mexico and the Atlantic Ocean

Warm, moist, unstable air

Brings precipitation

Continental tropical (cT)

Southwest and Mexico

Hot, dry

Maritime polar (mP)

Brings precipitation to the western mountains

Occasional influence: causes the Northeaster

Air Masses

Compare and contrast typical weather associated with a warm front and a cold front.

Describe an occluded front and a stationary front.

Focus Questions 14.2

Fronts—boundaries that separates air masses of different densities

Air masses retain their identities

Warmer, less dense air forced aloft

Cooler, denser air acts as wedge

Overrunning

Fronts

Warm front

Warm air replaces cooler air

Shown on a map by a line with red semicircles

Small slope (1:200)

Clouds become lower as the front nears

Slow rate of advance

Light-to-moderate precipitation

Fronts

Cold front

Cold air replaces warm air

Shown on a map by a line with blue triangles

Twice as steep (1:100) as warm fronts

Advances faster than a warm front

Associated weather is often violent

Intensity of precipitation is high

Duration of precipitation is short

Weather behind the front is dominated by

Cold air mass

Subsiding air

Clearing conditions

Fronts

Stationary front

Flow of air on both sides of the front is almost parallel to the line of the front

Surface position of the front does not move

Occluded front

Active cold front overtakes a warm front

Cold air wedges the warm air upward

Weather is often complex

Precipitation is associated with warm air being forced aloft

Fronts

Summarize the weather associated with the passage of a mature mid latitude cyclone.

Describe how aloft is related to cyclones and anticyclones at the surface.

Focus Questions 14.3

Primary weather producer in the middle latitudes

Idealized weather

Middle-latitude cyclones move eastward across the United States

First signs of their approach are in the western sky

Require two to four days to pass over a region

Largest weather contrasts occur in the spring

Changes in weather associated with the passage of a middle-latitude cyclone

Changes depend on the path of the storm

Midlatitude Cyclones

Weather associated with fronts

Warm front

Clouds become lower and thicker

Light precipitation

After the passage of a warm front:

Winds become more southerly

Temperatures warm

Midlatitude Cyclones

Cold front

Wall of dark clouds

Heavy precipitation

Hail and occasional tornadoes

After the passage of a cold front:

Winds become more northerly

Skies clear

Temperatures drop

Midlatitude Cyclones

Role of air aloft

Cyclones and anticyclones

Generated by upper-level air flow

Maintained by upper-level air flow

Typically are found adjacent to one another

Midlatitude Cyclones

List the basic requirements for thunderstorm formation.

Locate places on a map that exhibit frequent thunderstorm activity.

Describe the stages in the development of a thunderstorm.

Focus Questions 14.4

Features

Cumulonimbus clouds

Heavy rainfall

Lightning

Occasional hail

Occurrence

2000 in progress at any one time!

100,000 per year in the United States

Most frequent in Florida and eastern Gulf Coast region

Thunderstorms

Stages of development

All thunderstorms require

Warm air

Moist air

Instability (lifting)

High surface temperatures

Most common in afternoon and early evening

Thunderstorms

Stages of development

Continuous supply of warm air and moisture

Each surge causes air to rise higher

Updrafts and downdrafts form

Eventually precipitation forms

Gusty winds, lightning, hail

Heavy precipitation

Cooling effect of precipitation marks the end of thunderstorm activity

Thunderstorms

Summarize the atmospheric conditions and locations that are favorable to the formation of tornadoes.

Discuss tornado destruction and tornado forecasting.

Focus Questions 14.5

Tornadoes

Tornado – local storm of short duration

Features:

Rotating column of air that extends down from a cumulonimbus cloud

Low pressure inside

Winds approach 480 km (300 mi) per hour

Smaller suction vortices can form inside stronger tornadoes

Tornadoes

Occurrence and development

Associated with severe thunderstorms

Product of interaction between thunderstorm updrafts and tropospheric winds

Average of 1297 in the United States between 2000 and 2014

Most frequent from April through June

Tornadoes

Occurrence and development

General atmospheric conditions

Occur most often along a cold front

Associated with huge thunderstorms called supercells

Cold, dry cP air meets warm, humid mT air

Greater contrast = more intense storm

Tornadoes

Characteristics

Diameter 150–600 m (500–2000 ft)

Speed 45 km (30 mi) per hour

Can cut a 10 km (6 mi) long path

Max winds over 500 km (310 mi) per hour

Intensity measured by the Fujita intensity scale, or EF-scale

Tornadoes

Tornado forecasting

Difficult to forecast

Tornado watch

To alert public to the possibility of tornadoes

Issued when the conditions are favorable

Covers 65,000 km2 (25,000 mi2)

Tornado warning

Issued when a tornado is sighted or indicated by weather radar

Use of Doppler radar helps increase the accuracy by detecting the air motion

Identify areas of hurricane formation on a world map.

Discuss the conditions that promote hurricane formation.

List the three broad categories of hurricane destruction.

Focus Questions 14.6

Hurricanes

Most violent storms on Earth

Hurricane

Intense centers of low pressure

Form over tropical oceans

Characterized by intense convective activity and strong cyclonic circulation

Hurricanes

To be called a hurricane:

Wind speed >119 km (74 mi) per hour

Rotary cyclonic circulation

Form between 5º and 20º latitudes

Wind speeds reach 300 kph

Generate 50-foot waves at sea

Typhoons in the western Pacific

Cyclones in the Indian Ocean

North Pacific has the greatest number per year

Hurricanes

Parts of a hurricane

Eyewall

Near the center

Rising air

Intense convective activity

Wall of cumulonimbus clouds

Greatest wind speeds

Heaviest rainfall

Hurricanes

Eye

At the very center

About 20 km (12.5 mi) diameter

Precipitation ceases

Winds subsides

Air gradually descends and heats by compression

Warmest part of the storm

Hurricanes

Hurricane formation and decay

Energy from condensing water vapor

Develop most often in late summer

Form in all tropical waters except the South Atlantic and Eastern South Pacific

Other tropical storms

Tropical depression

Winds do not exceed 61 km (38 mi) per hour

Tropical storm

Winds 61–119 km (38–74 mi) per hour

Hurricanes

Diminish in intensity as:

They move over cooler ocean water

They move onto land

The large-scale flow aloft is unfavorable

Hurricanes

Hurricane destruction

Factors that affect amount of hurricane damage

Strength of storm (the most important factor)

Size and population density of the area affected

Shape of the ocean bottom near the shore

Saffir–Simpson scale ranks the relative intensities of hurricanes

Hurricanes

Hurricane destruction

Categories of hurricane damage

Storm surge

Large dome of water 65 to 80 km (40 to 50 mi) wide sweeps across the coast where eye makes landfall

Wind damage

Inland flooding from torrential rains

Hurricanes