CLASS #6 Air Masses and Fronts

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Transcript CLASS #6 Air Masses and Fronts

Air Masses and Fronts
Chapter 8
 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.
 Air
Mass - Large body of air
(portion of the troposphere) that
has fairly uniform properties of
temperature and moisture in the
 Source Region - The region which
an air mass acquires its particular
properties of temp and moisture.
 These
areas can be large snow
or ice covered polar regions,
cold northern oceans, tropical
oceans, and large desert areas.
 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)
 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
 What
your left
with is 6
possible types
of air masses.
 cP,
cA, cT,
or; mA, mP,
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
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
 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
– Speed of the air mass
– nature of the region
– temp. difference between the new surface and
the air mass
Air Mass Modification
ways air masses are modified
– Warming from below (instability and
– 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)
 Front
- Zone between 2 different
air masses. Or a boundary that
separates the two unlike air
 Frontolysis - when a front
dissipates or merges into the
adjacent air mass (it DIES)
 Frontogenesis - formation of a
front (it is CREATED)
 Occluded
Front - A composite of
two fronts as a cold front overtakes
a warm front or quasi-stationary
 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
 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
 Also
– Cold front pressure drops as the front
approaches and with passage pressure
– Warm front pressure generally falls until
frontal passage then pressure remains
steady or may fall slightly in the warm
– Generally speaking with frontal passage
pressure increases.
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
 Follow
Low Pressure Systems.
 Low pressure Systems follow
the Jet Streams
Warm Front
 Warm
edge of a
warm air
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
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
 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
 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
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
Naming Fronts
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
Frontal Waves
 D.
At the peak pressure falls it then
transforms into a Low which
reinforces the cylconic circulatory
 E. Cold front catches up to the
warm front they from an occluded
Frontal Waves
 F.
As it grows in length the
circulatory pattern diminishes, the
air masses start to mix
 G. The fronts merge, break off &
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
 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
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
perpendicular the front
moves same direction
Parallel to the front it moves very
Watch the Jet it is the primary
moving force
#1. When a cold front passes, the
temperature generally _____________
 Falls
 #2. When a cold front approaches and
passes, the pressure at a station
 Falls continuously as the front approaches,
and rises rapidly after it has passed.
#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
#5. You can tell when you are crossing a
cloud-free frontal surface at a low altitude
 The winds will vear (need to crab to the
 #6. Given two air masses: one warm and
one cold. Generally the ______ air mass
will contain more water vapor
 warmer
#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
#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
#9. A warm front indicates that the cold air
mass is (stationary/retreating/ advancing)
 retreating
 #10. The slope of a warm front is generally
 gentle
 #11. When a front passes a station, the
wind (backs/veers)
 Veers
#12. A trowal is what?
 An upper-air trough of warm air
 #13. In an air mass ______ and _______
are distributed fairly uniformly in the
 Temperature and Moisture
 #14. An Airmass that forms over an
expanse of water is called ______
 Maritime
Where is the troposphere the thickest and
How can you identify the tropopause?
What is the driving force of the Earth’s
What is relative humidity and dew point
When the temp/dew point temp spread
decreases what happens to relative
How can you increase relative humidity?
What is a temperature inversion?
Is a temperature inversion a stable or unstable
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
 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
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
Cumuliform cloud bases
Surface temp = 31 degrees C
 D.P.T.= 22 degrees C
 At sea level
 What would the cloud base be
 Take spread = 8 degrees /2.2 or multiply by
.45 = answer of around 3600 feet
Cumuliform clouds = stable or unstable
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
What is the DALR? Approximately what is
the SALR? Is the SALR a constant?
What determines the structure of a cloud
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
When can you expect more convective
updrafts over what types of terrrain?
What can you always expect with frontal
What is a cold front?
Be able to identify or classify an airmass.
 If the airmass is moist you would call it
 If the airmass is warm you would call it
Be able to explain a Chinook