Ch20 Form I

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

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

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

In addition to their overall temperatures, air masses are clasified according to the surface over which they form

Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses.

During late autumn & early winter, the difference in temperature b/t the lakes & adjacent land areas can be large; cold cP air mass pushes southward across the lakes; when this occurs, the air gets large quantites of heat & moisture from warm lake; when air mass reach opposite shore, its air mass is humid & unstable, causing heavy snow.

Only occasionally do cT air masses affect the weather outside their source regions ---Indian Summer

air masses are classified according to the surface over which they form.

Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses.

because it has hot, dry air masses....not humid & unstable

they form a front, which is a boundary that separates two air masses.

• when warm air moves into an area formerly covered by cooler air.
• - red line w/ red semi-circle that point toward the cooler air

forms when cold, dense air moves into a region occupied by warmer air.

Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front. In such cases, the surface position of the front does not move, and a stationary front forms

are large centers of low pressure that generally travel from west to east and cause stormy weather.

air high up in the atmosphere fuels a middle-latitude cyclone

When two air masses meet, they form a front, which is a boundary that seperates two air masses of different properties

forms when warm air moves into an area formerly covered by cooler air.

forms when warm air moves into an area formerly covered by cooler air.

• forms when cold, dense air moves into a region occupied by warmer air
• blue line edged w/ blue triangles that point toward the warmer air mass.

the surface position of the front doesn't move, and a stationary front forms

twice as steep & advance more rapidly than warm fronts do.

When a cold front overtakes a warm front

are large centers of low pressure that generally travel from west to east and cause stormy weather.

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

• pg. 569
• A. The formation of a front sets the stage for a mid-latitude cyclone.
• B. Over time, the front takes on a wave shape
• C. Changes in air flow & pressure result in a counterclockwise flow of air.
• D. The cold front closes in on the warm front to produce an occluded front
• E. As the cold front lifts, an occluded front forms
• F. Eventually, the cyclone weakens

when warm air moves into an area formerly covered by cooler air.

A thunderstorm is a storm that generates lightning and thunder. Thunderstorms frequently produce gusty winds, heavy rain, and hail.

Thunderstorms form when warm, humid air rises in an unstable environment.

Whirling tropical cyclones that produce winds of at least 119 km/hr are known in the United States as hurricanes

Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air.

• The eye is a zone of scattered clouds and calm averaging about 20 kilometers in diameter at the center of a hurricane.
• The eye wall is a doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane.

a storm that generates lightning and thunder. Thunderstorms frequently produce gusty winds, heavy rain, and hail.

Thunderstorms form when warm, humid air rises in an unstable enviroment.

• A. Cumulus stage: warm, moist air is supplied to the cloud
• B. Mature stage: heavy precipitation falls
• C. Dissipating stage: cloud begins to evaporate

are violent windstorms that take the form of a rotating column of air called a vortex. The vortex extends downward from a cumulonimbus cloud all the way to the ground.

Most tornadoes form in association with severe thunderstorms.

Whirling tropical cyclones that produce sustained winds of at least 119 kilometers per hour that sometimes develop over the ocean when water temperatures are warm enough to provide the necessary heat & moisture to fuel the storms.

Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air.

• Discuss the formation & growth of hurricanes
• Hurricane begins as a tropical disturbance that consists of disorganized clouds & thunderstorms; it becomes a hurricane when spiraling air has sustained winds of at least 119 km/hr
• When do they occur.
• They develop most often in the late summer when water temperatures are warm enough to provide the necessary heat & moisture to the air.
• How are they related to LP
• The low pressure in the center or the eye causes inward rush of warm, moist surface air to move toward the core of the storm, which in turn causes the air to move upward in a ring o fcumulonimbus clouds, creating the eye wall or the doughnut-shaped wall that surrounds the center. The eye wall is where the greatest wind speeds & heaviest rainfall occur.
• where do they get energy
• It gets its energy from the heat & moisture of surrounding airflow.
• why do they lose strength & dissipate?
• Hurricane weakens when it moves over cool ocean waters that cannot supply adequate heat & moisture; its intensity also drops when storms move over land because there is not sufficient moisture & friction with the rough land surface causes winds to subside.

• Why does the US from TX-Ohio Valley experience the greates frequency of tornadoes on the Earth?
• Also known as the "Tornado Alley", which include Oklahoma, Missouri, Kansas, and Ohio.
• Why?
• because cold, dry air from Canada meets warm, moist air from the Gulf of Mexico which combines with atmospheric instability to produce intense thunderstorms

• When?
• Frequency of tornadoes is greatest from April through June.

• Is it possible for tornadoes to occur in other areas at a different time/year?
• Is it possible for tornadoes to occur in other areas at a different time/year? Yes, tornadoes may strike whenever thunderstoms occur.

• When air mass moves out of the region over which it formed, it carries its temperature & moisture conditions w/ it.
• ie: a cold, dry air mass from northern Canada moves southward; the initial temp is -46^oC, as it moves south, it gets warmer but also causes the areas that it crosses to be slightly colder.

• Continental Polar (cP)Air Masses:
• Continental polar air masses are uniformly cold and dry in winter and cool and dry in summer.

• Maritime Tropical (mT) Air Masses:
• - Maritime tropical air masses are warm, loaded with moisture, and usually unstable.
• - Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

• Continental Tropical (cT) Air Masses:
• - Only occasionally do cT air masses affect the weather outside their source regions. However, when a cT air mass moves from its source region in the summer, it can cause extremely hot, droughtlike conditions in the Great Plains.
• - Movements of cT air masses in the fall result in mild weather in the Great Lakes region, often called Indian summer.

• pg. 569
• A. The formation of a front sets the stage for a mid-latitude cyclone.
• B. Over time, the front takes on a wave shape
• C. Changes in air flow & pressure result in a counterclockwise flow of air.
• D. The cold front closes in on the warm front to produce an occluded front
• E. As the cold front lifts, an occluded front forms
• F. Eventually, the cyclone weakens

• A. Cumulus stage: warm, moist air is supplied to the cloud
• B. Mature stage: heavy precipitation falls
• C. Dissipating stage: cloud begins to evaporate

• Mesocyclone can occur before the formation of a tornado:
• A. strong winds aloft cause lower winds to roll
• B. updrafts tilt the rolling air so that it becomes nearly vertical
• C. When the rotating air is completely vertical, the mesocyclone is established