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 cT A

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

Types of Fronts Warm Cold Stationary Occluded 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: In southern and eastern 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: Cold 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 High- and low-pressure systems Terrain factors that affect weather Chapter 2, Lesson 2