Chapter 19: Air Masses

What are Air Masses?

• Similar Moisture• Can cover a whole continent

Fronts:• Leading edge of advancing air

mass• Polar FrontFrigid, Arctic air

moves South from North Pole (Winter Months)

West to East From Rocky Mountains to Maritimes

• Warm Front• Cold Front• Stationary FrontWarm and Cold

air boundary remains in one place• Occluded Cold overuns warm

mixing the air

Air mass Classifications:1. Place of OriginHigh latitudes Arctic/Polar vs. Low

Latitudes Tropical2. Overland/Water Continent vs. Maritimes3. Stable vs. Unstable Air• Stable Air: Lower layers of air are cooler than the air above,

therefore no vertical movement of air (convection) takes placescharacterized by calm air, clear skies, and no precipitation.ex. When stable air masses blanket large cities such as Toronto, air quality quickly deteriorates as pollutants from factories and vehicle exhaust are unable to rise into the upper atmo. where they can be dispersed by wind currents. (a.k.atemperature inversion)

• Unstable Air: The lower layers of air are warmer than the air above so air can readily risecloudy, wind, and rain.

Air Masses in North America

Continental Polar (aka Continental Arctic in winter)• Originates in North Pole extends as far south as

Floridaaffects food crops• Air mass is very cold, stable and dry.• Cold air is heavier than warm air, so as this air mass

moves south it stays in contact with the ground.• Clear, sunny skies and high pressure

Maritime Polar• Originate over the ocean in temperate and polar regions

and move toward lower lats.• Northern Pacific (Gulf of Alaska) is where it originates• Cool, moist, and unstable therefore moisture in BC coast• Continental arctic is usually kept out by mountain

ranges running south, but when it makes it way into coast and comes into contact with maritime polar heavy blizzard conditions occur.

• Summer this air mass is cool, but stable therefore few rainy days.

Maritime TropicalOriginate in lower latitudes and are much

warmer and far more stable than Maritime Polar air masses.

Locations such as California get little ppt. in winter months.

Those the originate in the Gulf of Mexico, are warmer, wetter and far less stable, and generate frequent storms.

Continental TropicalNorth from the desert plateau of central

Mexico into the central Great Plains region of North America bringing hot, dry conditions

Air mass associated with tornado and hurricane activity (Tornado Alley in the mid United States is where Maritime tropical air meets Continental Polar)

This air mass is primarily responsible for hurricane and tornado activity!

Animations:

• http://www.classzone.com/books/earth_science/terc/content/visualizations/es2001/es2001page01.cfm?chapter_no=visualization

• http://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfm?chapter_no=visualization

Chapter 20: Global Winds

• Winds are named for the direction from which they blow, not the direction they are going.

• For instance, winds blowing from the Pacific Ocean toward the coast of N.A. are called westerly winds because they blow from West to East.

• its Remember: they move

from high to low.Good diagram on p. 88 of

Sovio Fig. 20.1

• Because the earth turns on its axis, from west to east, the two main factors in wind direction are: A)rotation of earth on its axis B) Location of High and Low Pressure Air masses.

• the winds in the northern hemisphere curve to the right and in the south to the left.

• Air flows from an area of high pressure to an area of low pressure. • Continual trade winds, the northeast trades and southeast trades

blowing from sub-tropical, high pressure belts to the constant low pressure region at the equator.

• At the equator, winds are calmdoldrumsArea of continuous low pressure! (popular provincial question)

Look at Fig. 20.2The rotation of the earth on its axis is responsible for the curving of

winds, to the right in the northern hemisphere and left in the southern hemisphereknown as Coriolis Effect.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm?chapter_no=visualization http://www.wiley.com/college/strahler/0471480533/animations/ch07_animations/animation2.htmlhttp://weather.about.com/od/weathertutorials/tp/coriolisvideos.htm

Wind Patterns

You also need to know which winds are found along latitudes:

• Equatorial : Doldrums = low pressure (constant) I ask why?

• 0-30 N/S North East and South East Trades• 30-60 N/S Westerlies (affect B.C.—in the North

provincial) • 30 N and S known as Horse Latitudes (High• Pressure)• 60 degrees N/S (Low Pressure—not continuous)• 60-90 N/S Easterlies• 90 N/S High Pressure (Constant) therefore, very little or

not precipitation

Global Circulation Model

Global Atmospheric Circulation

Jet StreamA stream of fast-moving, atmospheric air that occurs where the

troposphere meets the stratosphereMeanders east at speeds of 300 km an hourlocated in mid-latitudes they migrate north/southWeather systems in the lower troposphere are greatly influenced by

jet streams. (bring cold air either south or northcreating bad weather conditions.

Chinook Winds• Are warm winds that blow down the leeward side of

mountain ranges especially the eastern slopes of the Rockies.

• The sinking air is warmed by compression at a rate of about 1 Celsius per 100 meters.

• Bring warm conditions to Calgary, Alb.

Land and Sea Breezes. (FIG 20.6)-very popular on the provincial exam!

Land warms and cools much faster than water creating temperature and pressure changes between these two locations.

During the summer the land heats rapidly after sunrise and air in contact with the ground also warms and expands. As the warm expanded air begins to rise, cool dense air from the ocean flows in to replace itSea Breeze

Animation: http://www.classzone.com/books/earth_science/terc/content/visua

lizations/es1903/es1903page01.cfm?chapter_no=visualization

As night approaches the land cools rapidly and is now cooler than the ocean. Air over the warmer ocean begins to rise and the cooler air from the land moves out to replace it resulting in a land breeze.

Valley and mountain breezes On hot summer day’s valleys act as wind tunnels generating extremely high winds in mountain passes. As the sun heats the mountain slopes, air begins to rise and is replaced by air rushing up the valley. The narrower the valley the stronger the wind. (opposite at night)

Local Wind SystemsSea Breeze•Sun-heated air over land rises – Low pressure

•Cooler air over water sinks – High pressure

Land Breeze•Night air over land cools fast – High pressure

•Air over water stays warmer – Low pressure

Water Cycle (fig. 21.1)Continual movement of water from the oceans into the

atmosphere, and back to earth (into the ocean) again is referred to as the water/hydrologic cycle.

Evaporation of water leaves behind salt…therefore only freshwater is involved in the cycle.

Most of the water comes from ocean, but we also get evaporation from lakes/rives and transpiration from vegetation.

When moist air rises, it cools, and the water vapous condenses to form cloudswinds carry clouds over the continent and some comes down as snow or air.

Some infiltrates the ground (remember aquifers!) the rest will runoff into rivers and lakes returns water back to the ocean

Thing of three environmental issues that you have heard about that would relate to the water cycle?

http://www.sweetwater.org/education/watercycle.swf

http://epa.gov/climatechange/kids/

water_cycle_version2.htmlInfiltration—water below ground remember aquifersTranspiration—evaporation of plants

(release of water vapour) aka evapo-transpiratio

Condensation—gas to liquidEvaporation—liquid to gasPrecipitation—result of condensationthe water reaching earth

Dew, Frost and clouds

Dew PointHumidity amount of water vapour air containsAbsolute humidity refers to the amount of water vapour present in a body of air.Relative humudity is the percentage of water vapout in the air100 % saturated then dew point has been reached and water vapour begins to

condenseWarmer air can hold more vapour before it saturatesCold air is the oposite…some people say its too cold to snow and that is true

because the air can only hold very little water

DewWhy is the lawn wet in the morning after a clear night?Ground quickly loses much of the heat it gained during the day and begins to

cool the air in contact with it. As air temperature drops, relative humidity continues to increase until 100%

saturated.condensationresults in dew (below freezingfrost or ice)

FogWhen air close to earth is cooled enough that its dew point is reached, water vapour begins to condense and forms a low lying cloud

Radiation FogForms during cloudless/windless nightsseen in low lying valley bottoms as cooler dense air from the surrounding

valley where the air quickly reaches dew point and condensation occurs.most common on land in late fall and winter

Advection FogIs produced as warm moist air cools to its dew point (saturation) while

moving over a cold surface (always involves horizontal movements of air)Example—occurs along Pacific coast in the summer. Surface water is cooler

than air above-air reaches dew point and fog is formed.

Radiation Fog

Advection Fog