Transcript Air Masses
Lesson 2-2 Weather Elements
Part 1 of 2
Lesson Overview
Types of air masses and fronts
Factors that impact air masses
Fronts
Wind and Atmospheric Motion High- and low-pressure systems Terrain factors that affect weather Normal weather patterns in U.S.
Chapter 2, Lesson 2
Types of Air Masses and Fronts
Weather refers to temperature, moisture, wind velocity, visibility, and barometric pressure Air mass Large body of air (usually 1,000+ miles across Generally same temperature and moisture content within the entire mass Fronts are the boundary between two different air masses Chapter 2, Lesson 2 Courtesy of NASA
Air Masses
Air masses are classified based on their source region Polar air mass (P) is cold Tropical air mass (T) is warm Maritime air mass (m) forms over water and is humid Continental air mass (c) forms over land and is dry Possible Types: mP mT cP A cT
Air Mass Classification
Air Mass Classification
Aviators and meteorologists in the U.S. are mainly concerned with air masses originating from two sources • Masses that move south and east from Polar Regions • Masses that move north and east from Tropical Regions (mainly the Gulf of Mexico)
Temperature Classification of Air Masses
Based upon its temperature in relation to the surface over which it passes A cold air mass is cooler than the Earth’s surface over which it is moving Tends to create updrafts with this air mass A warm air mass is warmer than the Earth’s surface over which it is moving Typically won’t exhibit updrafts
Air Mass Characteristics
Outside factors affecting air mass characteristics include: Source Region Surface qualities of terrain air mass passes over The seasons (fall, winter, spring, summer) As an air mass moves away from its source, its temp and pressure traits change due to the surface it passes over. Can become warmer or colder Can absorb or lose moisture Can be lifted up by mountains or subside into valleys Cold air masses move quicker than warm air masses Weather generally depends on the nature of a prevailing air mass or the interaction of two or more air masses
Air Masses -- Quick Review
Where do air masses originate?
In a source region
How do they form? How do they “adopt” the temperature and moisture traits of their source region?
By remaining over an area long enough
What type of terrain makes an ideal source region?
Flat, uniform areas where temperature and moisture don’t change rapidly
Air Masses -- Quick Review
What is the letter designation for a moist polar air mass?
mP
As an air mass moves away from its source, what causes its traits to change?
The surface the air mass passes over
What traits can change?
Can become warmer or colder
Can become wetter or drier
Air Masses -- Quick Review
What is the process called when an air mass drifts away from its source region and adopts the temp and moisture trais of the surface over which it passes?
Modification
How do you define a cold air mass?
An air mass that is cooler than the surface it passes over
What happens when two different air masses meet?
Fronts develop and weather changes
Fronts
Fronts are the boundaries between two different air masses They always signal a change in weather As fronts move along the surface, one air mass is displacing the other air mass Four types based on temperature of advancing air compared with temperature of the air it is replacing Cold Front => cold air mass replaces warmer air mass Warm Front => warm air mass replaces colder air mass Chapter 2, Lesson 2 Reproduced from NASA
Warm Cold Stationary Occluded
Types of Fronts
Chapter 2, Lesson 2 Courtesy of NASA
Warm Front
Usually connected to a low-pressure cell Usually moves slowly in east-northeast direction If warm air mass did not move, the front would dissipate The warm front slips upward over cool air and forms a wedge Rises slowly which delays condensation
Warm Front
As warm front advances, it replaces colder air Before a warm front arrives, you’ll likely see: High-level cirrus clouds, layered stratus clouds, and fog As the front passes over you’ll see: Stratus clouds and some drizzle and fog After it passes, you’ll see: Stratocumulus and nimbostratus clouds and rain showers
Warm Front
Question: Why is fog so common in warm fronts as they pass over and past your location?
After front passes, there is a rise in temperatures, general clearing and change in wind direction In winter a warm front causes icing conditions at low altitudes In northern latitudes snow may also be produced
Cold Front
Cold fronts generally move from northwest to southeast.
Can travel very far south Amount of energy depends on amount of cold air and how high the pressure is As the cold air mass catches up to the warmer air mass, cold air forces the warm air upward
Cold Front
As cold air pushes the warm air upwards, the cold air replaces the warm air near the surface Before the cold front arrives, you’ll see: Cirrus, cirrocumulus, cirrostratus; clouds get lower as front gets closer As the front passes over: Cumulus or cumulonimbus clouds form => rain, hail, wind, etc After the front passes: Clouds dissipate, clear skies, cooler weather
Cold Front
In western states: The most noticeable change is shift of wind, temperature, and blowing dust In southern and eastern states: Generally lots more moisture, larger areas of cloud development, and more problems for aviators Test Question: Why are there more cumulonimbus build-ups and tornado activity associated with cold fronts than warm fronts?
Cold Front
Along the cold front there will be a low-pressure cell where weather is worst Almost any type of clouds can be found near the cell Squall lines are common ahead of these fast moving fronts Poor visibility, low ceilings and rain in summer Freezing rain and snow in winter
Stationary Front
Forms between two air masses of relatively equal strength When air masses stop, a stationary front develops Weather can be bad for aviation along the front About every form of weather can be found
Stationary Front
Toward the trailing edge of the any front there will be a stationary-front condition The trailing edge stationary fronts are a great distance from the parent cell As the distance become greater the front no longer exists.
Occluded Front
Forms when fast–moving cold front catches up with slow–moving warm front There is a tendency for a horizontal bend or wave to occur along the front After a frontal bend starts, cold air moves ahead of a warm front
Occluded Front
Warm Front Occlusion: Air mass overtaking the warm front is not as cool as the cold air in front of the warm front, and rides over the cold air mass while lifting the warm air Cold Front Occlusion: Air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses
Occluded Front
A wide variety of weather can be found along an occluded front Thunderstorms possible, but usually their passage is associated with a drying of the air mass
Next….
Types of air masses and fronts Factors that impact air masses Fronts
High- and low-pressure systems
Terrain factors that affect weather
Chapter 2, Lesson 2