Air Masses and Fronts Chapter 8

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

CLASSIFICATION OF AIRMASSES 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 AIRMASSES 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 AIRMASSES What your left with is 6 possible types of air masses. cP, cA, cT, or; mA, mP, mT

Continental 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 Masses This 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 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 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 Masses 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 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 & Fronts

Air 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 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)

Definitions Front - 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)

Definitions Occluded 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 Symbols

Fronts 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

Fronts 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.

Pressure

Warm 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

Fronts Follow Low Pressure Systems. Low pressure Systems follow the Jet Streams

Warm Front Warm fronts, leading edge of a warm air mass

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

Clouds Associated with a Warm Front

Cold Front Cold fronts, leading edge of a cold air mass

Cold 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 Flight 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 Flight Windshifts - to right need the crab.

Cold 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 Flight Windshift - same as in a cold front Ceiling and visibility - low ceilings and visibility common Turbulence - more stable less turbulence compaired to cold front Precip and icing - precip is steady icing is lighter however you are in the icing layer typically longer resulting in more ice.

Naming 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 Occlusion

Type of wx associtated with the warm and cold front occlusion

Upper Fronts (basin)

Frontal 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 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 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 Waves

Frontal Weather Weather along the front depends upon numerous things: The amount of moisture Must be present for clouds and precip to form Dry cold front meeting a moist warm front

Frontal Weather Weather along the front depends upon numerous things continued: Degree of stability Stable stratiform Unstable Cumuliform

Frontal Weather The slope of the front Shallow gives large areas of precipitation and or fog Steep front gives thinner line of precip but usually more intense cumiliform

Frontal Weather The speed of the front Faster it moves the more energy and intensity experienced usually Dry fronts may only have cirroform clouds

Frontal 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

Questions #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.

Questions #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

Questions #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

Questions #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

Questions #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

Questions #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

Questions #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 #1 TAKE NOTES

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

Cumuliform 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