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