1. Lecture 19: Air masses & fronts (Ch 9)
answers to numerical problems are now on the web
definition & classification of airmasses
airmass formation/transformation
fronts & their recognition

2. Airmass an idealisation
body of air with rather uniform T, Td over huge horizontal distance; airmasses separated by narrow boundary zones, ie. “fronts”
originate by having stagnated (light winds, anticyclonic conditions) in a source region - generally uniform
in mid latitudes there is strong spatial variation in T, p (etc.) and (thus) strong winds. In mid-latitudes therefore we have a transition zone: air masses invade, confront each other across fronts, are modified… producing “weather”
concept of “airmass weather” – static, because one is in the interior of an airmass: diurnal changes only
passage of a front is a significant weather event – large sudden change

3. Airmass classification
extremely cold cP air is called continental arctic (cA)
though uniform horizontally, an airmass cannot be uniform in the vertical… necessarily there are vertical gradients, affecting airmass stability

4. Airmass source regions
Fig. 9-1
cP by far our most common airmass in Ab.
eg. classic Ab. summer day - dry, cool, light Cu

5. snowcover? – high albedo therefore daily totalized
(p256-7)
Winter-time
hi latitude winter
long night, low sun
snowcover? – high albedo
therefore daily totalized
Q* likely to be negative
airmass cooled from the
base implies inversion (poor mixing – bad air quality), no convection
which may deepen day after day
cold, dry air + subsidence, few clouds
In summer, less extreme
not so dry
daytime heating erases inversion, permits Cu

6. cA airmasses and the arctic front
extremely cold airmasses are usually shallow
sometimes one may distinguish a sharp boundary with less extremely cold and dry air, ie. the “arctic front”
little or no “weather” associated with such fronts (too dry)
Fig. 9-2

7. (p258)
Airmass modification – exemplified here as mechanism to form mP airmass… *
* Barely appropriate to name a particular storm the “Aleutian low”… latter is a climatological feature

8. Criteria to locate fronts (ie. airmass boundaries)
large T over short distance (packed isotherms)
large Td over short distance
sudden change in wind direction
sudden change in sign or magnitude of pressure trend p /t
clouds and precipitation
front located along troughline (ie. along kink or bend in isobars)
As a front sweeps by, the above noted spatial changes are experienced as a rapid temporal change
Classic signs of cold front passage in Alberta: suddenly gusting wind, turns from SE or S or SW towards W or NW; rapid cooling; clearing follows
rare to see all of these signs
somewhat subjective

9. A Idealized frontal structure of a mid-latitude cyclone Fig. 9-5
fronts along/near isobar kinks
low level cross-isobar wind
wind direction change across front
imagine whole picture moves
eastward… observer at A sees initially falling, then steady, then finally rising pressure

10. Ideal cold front, showing:
jump in wind direction
frontal lift (causing cloud, precip)
Fig. 9-6
Map symbol
front itself can move rapidly (up to a nominal 50 kph)

11. Ideal warm front, showing:
overrunning warm air (stable ELR)
gently sloped frontal boundary
progression of stratiform cloud types
Fig. 9-8

12. Would you diagnose a front (or fronts) associated with the Manitoba storm? Where?
Why?
Cloud
type Ns
( , ) St
( )
auto-station
( )
mist Sc

13. The 850 mb isotherms help
CMC 850 mb analysis, 12Z Sep 20, 2004.

14. Would you diagnose a front (or fronts) associated with this N
Would you diagnose a front (or fronts) associated with this N. Alberta storm?
very cold
cold
mild
CMC surface analysis, 12Z Nov 28, Storm trough through C. and NE. Ab, plus wind induced lee trough in the SW; wind warning for SW Ab.

15. CMC 700 mb analysis, 12Z Nov 28, 2003. SW current aloft across Rockies

16. Fig. 9-4
Fig. 9-10
Fig. 9-4
warm air cut off from the surface by the meeting of two cold fronts