401M: Air Masses

What is the definition of an air mass?

A large section of the troposphere with relatively uniform properties of temperature and moisture content in the horizontal.

Which atmospheric property is explicitly stated as not being uniform within an air mass?

Pressure.

How does an air mass form?

When a large body of air remains over an area for several days or weeks, acquiring the thermal and moisture properties of the surface beneath it.

What type of air mass is formed when it lies across the frozen regions of the Arctic?

A cold and dry air mass.

What type of air mass is formed over the tropics?

A very warm and humid air mass.

In the two-letter classification for air masses, what does the first, small letter identify?

Where the air mass formed (c for Continental/land, m for Maritime/water).

In the two-letter classification for air masses, what does the second, large letter designate?

The warmth of the air mass (A for Arctic, P for Polar, T for Tropical).

What does the letter 'c' (Continental) signify about an air mass's moisture content?

It is a dry air mass, formed over land.

What does the letter 'm' (Maritime) signify about an air mass's moisture content?

It is a moist air mass, formed over water.

What temperature range is indicated by the letter 'A' (Arctic) in an air mass classification?

A cold air mass.

What temperature range is indicated by the letter 'P' (Polar) in an air mass classification?

A temperate or mild air mass.

What temperature range is indicated by the letter 'T' (Tropical) in an air mass classification?

A warm air mass.

What are the characteristics of a Continental Arctic (cA) air mass?

Dry and very cold.

Over what region does a Continental Arctic (cA) air mass typically form in winter?

Over the frozen Arctic.

What are the characteristics of a Maritime Arctic (mA) air mass?

Humid and cold.

How does a Maritime Arctic (mA) air mass form in the summer?

It forms over the open waters of the Arctic Archipelago or the numerous northern lakes of North America.

What are the characteristics of a Maritime Polar (mP) air mass?

Humid and mild.

What are the characteristics of a Maritime Tropical (mT) air mass?

Humid and warm.

Over what region does a Maritime Tropical (mT) air mass typically form?

Over the tropical oceans, such as the Gulf of Mexico, South Atlantic, and South Pacific.

What happens to the properties of an air mass as it moves away from its source region?

Its properties change based on the conditions of the surface over which it passes, a process called modification.

A Continental Arctic (cA) air mass moves south over the northern Pacific Ocean. What new air mass type does it become?

It becomes a Maritime Arctic (mA) air mass, changing from very cold and dry to cold and moist.

List the six major changes that can lead to the modification of an air mass.

1. Changes in latitude,

2. change of seasons,

3. ocean currents,

4. movement over water/land,

5. diurnal variations (radiation warming and cooling),

6. topography.

What is the definition of atmospheric stability?

A measure of the atmosphere's tendency to encourage or deter vertical motion.

Which air mass is considered the most stable in its source region?

The Continental Arctic (cA) air mass.

An air mass formed over a colder surface tends to be ______ , while one formed over a warmer surface tends to be ______ .

stable; unstable

What happens to the stability of an air mass when it is heated from below?

It becomes unstable.

What happens to the stability of an air mass when it is cooled from below?

It becomes stable.

Why is a Continental Arctic (cA) air mass so stable in its source region?

Because the air at the surface is very cold, resulting in a weak temperature lapse rate.

Which air mass is the most unstable in its source region?

The Maritime Tropical (mT) air mass.

Why is a Maritime Tropical (mT) air mass unstable in its source region?

Because the air at the surface is very warm, resulting in a strong temperature lapse rate.

What is the term for the transition zone between two different air masses?

A front.

What phenomenon is responsible for most weather changes at the boundary of two air masses?

The interaction of the two air masses along their frontal zones.

What kind of weather is associated with a Continental Arctic (cA) air mass?

Bitterly cold and dry, with stability trapping any moisture in low levels, causing fog or low stratus.

What kind of weather is associated with a Maritime Arctic (mA) air mass?

Cold and moist, with reduced stability allowing for showers of snow and possibly cooler weather thunderstorms.

Describe the typical weather associated with a Maritime Polar (mP) air mass.

Frequent showers at any time of year, with hail and thunder common in winter as it makes landfall.

Describe the typical weather associated with a Maritime Tropical (mT) air mass as it moves over cooler surfaces.

Low cloud, drizzle, and possibly fog, particularly around windward coasts and hills.

Heating an air mass from below ______ the air, favours _____development clouds, and is associated with ________ visibilities.

destabilises; vertical; good

Cooling an air mass from below ______ the air, favours ______ type clouds, and is associated with ________ visibilities.

stabilizes; layer; poor

What effect do inversions have on vertical movement and visibility?

Inversions tend to block vertical movement, and visibilities would tend to be poor.

A true Maritime Tropical (mT) air mass always has a dew-point temperature value in excess of what value?

15.5 C