Weather Patterns

and Severe

Weather

Chapter 14 Lecture Outline

Air Masses

Characteristics

• Large body of air

– 1600 km (1000 mi) or more across

– Several km thick

• Similar temperature at any given

altitude

• Similar moisture at any given altitude

• Move and affect a large portion of a

continent

Air Masses

Mass

A Cold

Canadian Air

Air Masses

Source region • Where an air mass acquires its properties

Classification of an air mass

• Two criteria used to classify air masses: 1. By the latitude of the source region

• Polar (P)

– High latitudes: cold

• Tropical (T)

– Low latitudes: warm

2. By the nature of the surface in the source region

• Continental (c)

– Form over land: dry

• Maritime (m)

– Form over water: humid

Air Masses

Four basic types of air masses

Continental polar (cP)

Continental tropical (cT)

Maritime polar (mP)

Maritime tropical (mT)

Air Masses

Air masses and weather

• cP and mT air masses important in N. 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

Air Masses

Satellite image of lake-effect snow storm

Air Masses 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

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

Fronts

Warm front

• Warm air replaces cooler air

• Shown on a map by a line with red

semicircles

• Shallow slope (1:200)

• Clouds become lower as the front nears

• Slow rate of advance

• Light-to-moderate precipitation

Fronts

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

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

Midlatitude Cyclones

• 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

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

Cloud Patterns of

Typical Mature Middle-

Latitude Cyclone

Midlatitude Cyclones

Development

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

Thunderstorms

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

Thunderstorms

Stages of development

• All thunderstorms require

– Warm air

– Moist air

– Instability (lifting)

• High surface temperatures

• Most common in afternoon and

early evening

Thunderstorms

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

Tornadoes

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

Tornadoes

Occurrence and development

• Average of 770 each year in the U.S.

• Most frequent from April through June

• Associated with thunderstorms

• Exact cause is not known

• Formation of tornadoes

– Occur most often along a cold front

– May be associated with huge thunderstorms called supercells

Mesocyclone Development

• Supercell thunderstorms

– Large, long-lasting thunderstorm with a single

violent rotating updraft

– Strong vertical wind shear

– Outflow never undercuts updraft, updrafts may

exceed 90kts and can cause large sized hail

– 3 types:

Classic - CL

high precipitation, HP

low precipitation, LP

Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the southeast.

The storm is moving to the northeast.

Intense thunderstorms often can create flash flood

conditions especially if storms are “training”

A wall cloud photographed southwest of Norman, Oklahoma.

• Lightning and Thunder

– Causes of electrification of clouds

– graupel and hail fail into region of

supercooled water , water freezes,

releasing latent heat and keeping the

hailstone warmer than surrounding ice

crystal nuclei

• Net transfer ….+ ions from warmer to colder,

this leaves larger hail stones negatively

charged and smaller ice crystals positively

charged

When the tiny colder ice crystals come in contact with the much larger and warmer

hailstone (or graupel), the ice crystal becomes positively charged and the hailstone

negatively charged. Updrafts carry the tiny positively charged ice crystal into the upper

reaches of the cloud, while the heavier hailstone falls through the updraft toward the

lower region of the cloud

The generalized charge

distribution in a mature

thunderstorm.

• The Lightning Stroke

– A discharge of static electricity

– Positive charge on ground, cloud to ground

lightning

• Thunder

– Lightning heats air to 54,000deg F – hotter than Sun’s surface

– Explosive expansion of air - shock wave

– Sound travels at 330m/s or 1100 ft/s, so delay… about 5 sec per mile

– Sound is refracted upward in unstable atm and we do not hear

lightning at approximately 15km away … Heat Lightning

Thunder travels outward from the lightning stroke in the form of waves.

If the sound waves from the lower part of the stroke reach an observer before the waves

from the upper part of the stroke, the thunder appears to rumble.

If the sound waves bend upward away from an observer, the lightning stroke may be

seen, but the thunder will not be heard….”heat lightning”

The four marks on the road surface represent areas where lightning, after striking a car

traveling along south Florida’s Sunshine State Parkway, entered the roadway through

the tires. Lightning flattened three of the car’s tires and slightly damaged the radio

antenna. The driver and a six-year-old passenger were taken to a nearby hospital,

treated for shock, and released.

Average lightning flash density per square kilometer per year from 1997 to 2010. Notice

that in the United States, Florida is the most lightning-prone state. (Data from the North

American Lightning Detection Network. Courtesy of Vaisala.)

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 hr

• 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

Tornadoes

Doppler Radar image of tornado near Moore , OK…..May 3, 1999!

• May 20, 2013 Moore OK…..again

Hurricanes

• Most violent storms on Earth

• 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

Hurricanes

Parts of a hurricane

• Eyewall

– Near the center

– Rising air

– Intense convective activity

• Wall of cumulonimbus clouds

• Greatest wind speeds

• Heaviest rainfall

Hurricanes

Hurricanes

Eye

• At the very center

• About 20 kilometers (12.5 miles) diameter

• Precipitation ceases

• Winds subsides

• Air gradually descends and heats by compression

• Warmest part of the storm

Hurricanes

• Hurricane formation and decay

– Form in all tropical waters except the

• South Atlantic(rare)and Eastern South Pacific

• Energy from condensing water vapor

• Develop most often in late summer

– Tropical depression

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

– Tropical storm

• Winds 61–119 km (38–74 mi per hour)

Hurricanes

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

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

• Resonsible for large number of deaths

– Wind damage

– Inland flooding from torrential rains

Hurricanes

2012 Season • In 2012, there were 19 tropical cyclones, 19 tropical storms, 10 hurricanes,

and 2 major hurricanes. Damage in this season was around $77.57 billion

and deaths were around 354. The majority of these damages and deaths

were caused by Hurricane Sandy and Hurricane Isaac.

Hurricane Sandy October 22, 2012 – October 31, 2012

2013 Season With 14 tropical storms, two hurricanes, and no major hurricanes,activity fell far

below the predictions. The season's impact was minimal; although 15 tropical

cyclones developed, several were weak or remained at sea. Tropical Storm

Andrea killed four people after making landfall in Florida and moving up the East

Coast of the United States

2014 Season The season's first tropical cyclone, Arthur, developed on July 1, ahead of the long-term

climatological average of July 9. Early on July 3, the system intensified into a hurricane,

preceding the climatological average of August 10.[16] After continuing to steadily

intensify, it moved ashore between Cape Lookout and Cape Hatteras as a Category 2

hurricane, becoming the first U.S. land falling cyclone of that intensity since Hurricane

Ike in 2008.Upon moving inland, Arthur became the earliest known hurricane to strike

the North Carolina coastline on record.

2015 Season The season officially began on June 1, 2015, and ends on November 30, 2015. These

dates historically describe the period each year when most tropical cyclones form in the

Atlantic basin and are adopted by convention. However, the formation of tropical

cyclones is possible at any time of the year.

The first storm, Ana, developed a month before the official start of the season, becoming

the first pre-season tropical or subtropical cyclone since 2012's Beryl, the earliest-forming

cyclone since 2003's Ana, and the earliest cyclone on record to strike the United States.

Despite an ongoing El Niño event, the season started unusually early, and August and

September so far have featured 8 tropical cyclones. With the classification of Tropical

Storm Ida, this season has featured more named storms than the previous season;

however this season is still slightly below normal according to its Accumulated cyclone

energy Index. In early October, Joaquin became the strongest hurricane in the Atlantic

since Igor in 2010. But Wait………there’s MORE !