Air Masses and Fronts Chapter 8. AIR MASSES l The troposphere can be further divided into separate regions known as air masses. l In most cases, these.

Air Masses and Fronts Air Masses and Fronts Chapter 8Chapter 8

AIR MASSESAIR MASSESThe troposphere can be further

divided into separate regions known as air masses.

In most cases, these air masses do not gradually merge with each other, but remain separated by relatively narrow transition zones or fronts.

Definitions Definitions Air Mass - Large body of air

(portion of the troposphere) that has fairly uniform properties of temperature and moisture in the horizontal.

Source Region - The region which an air mass acquires its particular properties of temp and moisture.

SOURCE REGIONSSOURCE REGIONS

These areas can be large snow or ice covered polar regions, cold northern oceans, tropical oceans, and large desert areas.

CLASSIFICATION OF CLASSIFICATION OF AIRMASSESAIRMASSES

Air Masses are classified according to the temperature and moisture characteristics of their source region. Based on moisture content, air masses can be considered either continental (dry) or Maritime (moist). According to their temperature, they are either Tropical (warm), Polar (cold), or Artic (extremely cold)

CLASSIFICATION OF CLASSIFICATION OF AIRMASSESAIRMASSES

In Meteorology everything is shorthand including the identification of Air Masses.

A small letter such as c or m indicates the moisture conditions of the air mass.

Following the small letter a capital letter T, P, or A represents the temperature characteristics of the air mass.

CLASSIFICATION OF CLASSIFICATION OF AIRMASSESAIRMASSES

What your left with is 6 possible types of air masses.

cP, cA, cT, or; mA, mP, mT

Continental Air MassesContinental Air Masses Continental Polar (cP) and Continental

Artic (cA) Air Masses form over large, high-latitude land masses such as northern Canada or Siberia. In the winter, these regions have short days and low solar angles. They also are usually snow-covered during the winter and therefore reflect much of whatever solar radiation does reach the surface

Continental Air MassesContinental Air MassesThis combination of circumstances

virtually guarantees that the air will lose more radiant energy in the winter than it recieves. The cooling of the air from below leads not only to low temperatures but also to radiation inversions and highly stable conditions.

Continental Air MassesContinental Air Masses Low temps in the winter also very dry,

sunny days. In cities pollutants hard to dispurse very little mixing.

cA air mass differes from the Polar airmass in that it has much less of a vertical extent. (much shallower)

Maritime Polar Air MassesMaritime Polar Air Masses Similar to continental polar air masses

but are more moderate in both temperature and dryness. Maritime polar air forms over the North Pacific as cP air moves out from the interior of Asia. The warm Japan current adds heat and mositure to the cold, dry air and converts it from cP to mP. The air masses then moves eastward across the Pacific. Low Pressure

Continental Tropical Air Continental Tropical Air MassesMasses

Forms during the summer over hot, low-latitude areas, such as the southwestern United States and Northern Mexico. Usually form in the dessert regions where there is little surface water and vegetation.

Very high ground temperatures, very dry often cloud free.

Maritime Tropical Air MassesMaritime Tropical Air Masses

Develop over warm tropical waters (Gulf of Mexico & Atlantic). Warm but not as hot as cT.

mT air is very moist, and unstable near the surface - ideal for the development of clouds and precipiation TS.

Enormous influence on the southeastern U.S., especially during the summer.

Air Masses & FrontsAir Masses & Fronts

Air Mass Modification pg.64Air Mass Modification pg.64

When an air mass moves away from its source region, it takes on the characteristics of its new region

Degree of Modification depends on 3 factors– Speed of the air mass– nature of the region– temp. difference between the new surface and

the air mass

Air Mass ModificationAir Mass Modification 4 ways air masses are modified

– Warming from below (instability and showers)

– Cooling from below (stable, if cooled to much fog)

– Subtraction of water vapor (condensation and precipitation)

– Addition of water vapor (cold air over warm water i.e. evaporation)

DefinitionsDefinitionsFront - Zone between 2 different

air masses. Or a boundary that separates the two unlike air masses.

Frontolysis - when a front dissipates or merges into the adjacent air mass (it DIES)

Frontogenesis - formation of a front (it is CREATED)

DefinitionsDefinitionsOccluded Front - A composite of

two fronts as a cold front overtakes a warm front or quasi-stationary front

Quasi-stationary front - is stationary or nearly is moving at a speed of less than 5 knots.

Trowal - Trough of warm air aloft

Chart SymbolsChart Symbols

FrontsFronts

Discontinuities are changes in properties from 1 front to another– Temperature - usually a temp change

– Dew Point - Temp dew point spread will usually change

– Wind always changes across a front usually both in speed and direction

Beware of wind shear in this area

FrontsFronts Also Pressure

– Cold front pressure drops as the front approaches and with passage pressure rises

– Warm front pressure generally falls until frontal passage then pressure remains steady or may fall slightly in the warm air.

– Generally speaking with frontal passage pressure increases.

PressurePressure

Warm Frontal SlopeWarm Frontal Slope

Slope below is 1 in 150. An aircraft flying at around 6000 feet would encounter the frontal surface 150 nm past the surface based front

FrontsFronts

Follow Low Pressure Systems.Low pressure Systems follow

the Jet Streams

Warm FrontWarm Front

Warm fronts, leading edge of a warm air mass

Warm Front WxWarm Front Wx

Much more stable Shallow slope Stratiform clouds and fog continuous precipitation Smooth air Fair to poor visibility

Clouds Associated with a Clouds Associated with a Warm FrontWarm Front

Cold FrontCold Front

Cold fronts, leading edge of a cold air mass

Cold Front WeatherCold Front Weather

Generally speaking cold fronts are: fast moving More severe weather Cumuliform clouds Showery Precipitation Rough Air (turbulence) Good Visibility

Cold Front Weather in Cold Front Weather in FlightFlight

Windshifts– The windshift occurs at the frontal surface

– A change in temperature tells you when you have passed through the frontal surface.

– The windshift is such that an alteration to starboard (to the right) is required to stay on course, no matter which way you fly through the front.

– Windshift more significant at lower levels

Cold Front Weather in Cold Front Weather in FlightFlight

Windshifts - to right need the crab.

Cold Front Weather in FlightCold Front Weather in Flight Ceiling and visibility - Fast moving cold

front typically brings good visibility the faster it is moving.

Turbulence - can be a problem. If you know where the frontal boundary is slow to Va or below in turbulence for the crossing

Precip and icing - can be severe however you are typically in clouds for a shorter period of time.

Warm Front Weather in FlightWarm Front Weather in FlightWindshift - same as in a cold frontCeiling and visibility - low ceilings and

visibility commonTurbulence - more stable less turbulence

compaired to cold frontPrecip and icing - precip is steady icing

is lighter however you are in the icing layer typically longer resulting in more ice.

Naming FrontsNaming Fronts A front is named two ways. 1. The temperature of the advancing air 2. The colder of the 2 airmass cA, mA, mP

Warm Front OcclusionWarm Front Occlusion

Type of wx associtated with the Type of wx associtated with the warm and cold front occlusionwarm and cold front occlusion

Upper Fronts (basin)Upper Fronts (basin)

Frontal WavesFrontal Waves They usually form on slow moving

cold fronts or stationary fronts A. Winds blowing parallel cause a

disturbance (remember stationary, so winds are parallel)

B. Wave starts C. Start of a Cyclonic

(counterclockwise) circulation developes

Frontal WavesFrontal Waves

D. At the peak pressure falls it then transforms into a Low which reinforces the cylconic circulatory pattern

E. Cold front catches up to the warm front they from an occluded front

Frontal WavesFrontal Waves

F. As it grows in length the circulatory pattern diminishes, the air masses start to mix

G. The fronts merge, break off & disappear

Frontal WavesFrontal Waves

Frontal WeatherFrontal WeatherWeather along the front depends

upon numerous things:

–The amount of moistureMust be present for clouds and precip to form

Dry cold front meeting a moist warm front

Frontal WeatherFrontal Weather

Weather along the front depends upon numerous things continued:– Degree of stability

Stable stratiformUnstable Cumuliform

Frontal WeatherFrontal Weather

– The slope of the frontShallow gives large areas of precipitation and or fog

Steep front gives thinner line of precip but usually more intense cumiliform

–The speed of the frontFaster it moves the more energy and intensity experienced usually

Dry fronts may only have cirroform clouds

Frontal WeatherFrontal Weather

Frontal WeatherFrontal Weather

–Upper wind flow pattern (Jet Stream)When perpendicular the front moves same direction

Parallel to the front it moves very little

Watch the Jet it is the primary moving force

QuestionsQuestions

#1. When a cold front passes, the temperature generally _____________

Falls #2. When a cold front approaches and

passes, the pressure at a station generally______.

Falls continuously as the front approaches, and rises rapidly after it has passed.

QuestionsQuestions

#3. Cold air behind a cold front is generally unstable because?

It is warmed from below as it crosses relatively warm regions (warm air rises)

#4. The horizontal extension of clouds and precip. Associated with a slow-moving cold front is generaly ____ than in the case of a rapidly moving cold front.

greater

QuestionsQuestions

#5. You can tell when you are crossing a cloud-free frontal surface at a low altitude because_____.

The winds will vear (need to crab to the right)

#6. Given two air masses: one warm and one cold. Generally the ______ air mass will contain more water vapor

warmer

QuestionsQuestions #7. Name 7 things you can plan to maybe

experience when crossing a cold front icing severe over short period of time precipitation heavy Turbulence thunderstorms wind shifts low ceiling low visibility

QuestionsQuestions #8. What is the effect on a station’s

altimeter setting while a warm front approaches and passes over?

As the warm front approaches, the thickness of the cold air over the station decreases, and so the MSL pressure and the altimeter setting fall. After the warm front has passed, the uniform warm air mass has uniform thickness and characteristics; the MSL pressure and altimeter remain constant

QuestionsQuestions #9. A warm front indicates that the cold air

mass is (stationary/retreating/ advancing) retreating #10. The slope of a warm front is generally

(steep/gentle) gentle #11. When a front passes a station, the

wind (backs/veers) Veers

QuestionsQuestions #12. A trowal is what? An upper-air trough of warm air #13. In an air mass ______ and _______

are distributed fairly uniformly in the horizontal

Temperature and Moisture #14. An Airmass that forms over an

expanse of water is called ______ Maritime

REVIEW FOR TEST #1REVIEW FOR TEST #1

TAKE NOTES

TroposphereTroposphere

Where is the troposphere the thickest and when?

How can you identify the tropopause?

What is the driving force of the Earth’s weather?

What is relative humidity and dew point temperature?

When the temp/dew point temp spread decreases what happens to relative humidity?

How can you increase relative humidity?

What is a temperature inversion?

Is a temperature inversion a stable or unstable phenomenon?

When is it common for an inversion to be produced?

What would you expect the visibility to be like beneath a low level inversion?

What is the approximate height of the atmosphere?

Where is the atmosphere approximately 1/2

Name two situations when your true altitude may be lower than your indicated altitude

What is the name of the force in the Northern hemisphere that deflects the winds to the right until it is parallel to the isobars?

What is ISA?

How much does pressure decrease per 1000ft?

What is needed to create a cloud?

Define condensation

What is the main factor in determining how much water vapor air can hold?

Cumuliform cloud basesCumuliform cloud bases

Surface temp = 31 degrees C D.P.T.= 23 degrees C At sea level What would the cloud base be

approximately?

Cumuliform cloud basesCumuliform cloud bases

Surface temp = 31 degrees C D.P.T.= 22 degrees C At sea level What would the cloud base be

approximately? Take spread = 8 degrees /2.2 or multiply by

.45 = answer of around 3600 feet

Cumuliform clouds = stable or unstable atmosphere?

What are the four cloud families?

What does nimbus mean?

What kind of clouds are found around mountain waves?

What kind of turbulence can be expected around mountain waves?

How thick do clouds usually have to be to produce significant precip?

What are the approximate bases of low, mid and high clouds?

How can you determine atmospheric stability?

What is the DALR? Approximately what is the SALR? Is the SALR a constant?

What determines the structure of a cloud formation?

What would you expect the visibility to be in an unstable air?

What are some characteristics of stable and unstable air

If you have a good lifting agent and an unstable air mass what type of clouds would you expect?

Name an atmospheric process that would increase the stability of an air mass? Name process that would decrease the stability

Which clouds are the meanest on the block?

Name a type of cloud that forms due to convection?

When can you expect more convective updrafts over what types of terrrain?

What can you always expect with frontal passage?

What is a cold front?

Be able to identify or classify an airmass. If the airmass is moist you would call it

what? If the airmass is warm you would call it

what?

Be able to explain a Chinook