Chapter 20 Air Masses

Air Mass

�An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given altitude

�When an air mass moves out of the region over which it formed, it carries its temperature and moisture conditions

�As it moves, the characteristics of an air mass change and so does the weather in the area over which the air mass moves

Classifying Air Masses

y Air masses are classified by temperature and surface area over which they form

Classifying Air Masses

NAMEP – Polar

LOCATIONHigh Latitudes towards the poles

TEMPERATURECold Temps

T – TropicalLow Latitudes towards the equator Warm Temps

C – Continental Over land massCold or warm, depending on the latitude

M – Maritime Over waterCold or warm, depending on the latitude

Four Basic Types of Air Masses

cP – Continental Polar•dry and cool•cold and dry in winter and summer

cT – Continental Tropical•dry and warm•hot, drought-like conditions

mT – Maritime Tropical•wet and warm•source of precipitation in the United States

mP – Maritime Polar•wet and cold•mild, humid, unstablecold air from Canada

Weather Patterns in North America

Weather in North America is influenced by continental polar and maritime tropical air masses

Fronts

�When two air masses meet�boundary separating

the two air masses.

Warm Fronty A warm front – warm air

moves into an area formerly covered by cooler air

y Warm air glides up over a cold, dense air mass

y Weather Conditions: Light to moderate rain

Cold FrontCold air mass moves into an area occupied by warmer air.

Weather Conditions: Thunderstorms

Stationary and Occluded Fronts

y stationary front: The surface position of the front does not move (steady rain for days)

y occluded front: when an active cold front overtakes a warm front

Middle Latitude Cyclone

y Large centers of low pressure

y Travel from west to east

yCauses stormy weather

y Also known as Nor’ Easterns

The Role of Airflow Alofty Plays an important role in

maintaining cyclonic and anti-cyclonic circulation

y More often than not, air high up in the atmosphere fuels a middle latitude cyclone

Severe Storms

yThunderstormsyTornadoesyHurricanes

Thunderstorms

y Is a storm that generates thunder and lightning

y Frequently produces gusty winds, heavy rain, and hail

y Associated with cumulonimbus clouds

y Lightning: results from the build up and discharge of electric energy between positively and negatively charged area

y Thunder: the sound of rapidly expanding gases usually associated with lightning

Occurrence and Development of Thunderstorms

y Occurrence{At any given time, there are an

estimated 200 thunderstorms in progress on Earth

{Mostly in the tropicsy Development{ Thunderstorms form when

warm, humid air rises in an unstable environment

{ 3 StagesÙ Cumulus: build-up of

clouds and moistureÙ Mature: Heavy rain fall,

most active timeÙ Dissipating: light rain,

storm is calming down

Tornadoes

y Tornadoes - violent low pressure windstorms that take the form of a rotating column of air (vortex).

y The vortex extends downward from a cumulonimbus cloud producing rain and hail

y Move counterclockwise

Occurrence, Development, and Intensity of Tornadoes

y Occurrence{ 770 occur each year

y Characteristics{ Diameter 150 to 600 m{ Speed – up to 300 mph{ Travels in a NE path{ Forms in Cold Fronts

y Tornado Season {April to June{Associated with severe

thunderstormsy Intensity{Fujita Tornado scale {Based on the amount of

damage

Fujita Tornado Scale

Category Winds (MPH) Winds (KPH) Damage

F0 <73 <116 Light damage

F1 73-112 116-180 Moderate Damage

F2 113-157 181-254 Considerable Damage

F3 158-206 254-332 Severe Damage

F4 207-260 333-419 Devastating Damage

F5 >260 >419 Incredible and SpeechlessDamage

Tornado Warnings vs. Watches

y Watches : Possible of a tornado to developed in the area

y Warning: Tornado has been soon by people or indicated by radar

Hurricanes

y Whirling tropical cyclones (low pressure system) producing winds of at least 119 km per hour (73 mph)

y US – Hurricanesy Pacific – Typhoonsy Indian Ocean – Cyclonesy Most powerful storm on Earth

Occurrence and Development of

Hurricanes

Occurrencey Form between 5 and 20 degrees

north and south latitudey Season: June 1 to November

30th

Development of Hurricanesy Hurricane develop most often in

the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air

y Low Pressure: Spins counterclockwise

Parts of a Hurricane

Eye: Center of storm, warmest part, winds cease, rain ceases

Eye wall: Strongest winds and rain

Spiral Rain Bands: extend out from the eye wall

Hurricane Intensity

y Storm Surge: is a dome of water that sweep across the coast causes by the hurricane’s winds

y Caterized on the Saffir-Simpson Scale (based on wind speed)

Hurricane Warning and Watches

y Hurricane Watch: could see hurricane related hazards within 48 hours

y Hurricane Warning: Could see hurricane winds within 36 hours