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A line connecting points that have equal changes in pressure over a given period of time.


Pressure tendency can provide a good estimation of what?

How the pressure centre will evolve with time


3 factor that affect the central value of a low pressure system:

-Diurnal pressure changes
-diabatic influences



-Higher elevations reduced to MSL can give inaccurate central pressure values


Diurnal pressure changes

-a 1-2 hPa is calculated for for central values for the 1600 lcl low pressure cycle


Diabatic influences

-Difficult to assess quantitatively.


6 factors which contribute to divergence and convergence in a developing low pressure system

-horizontal thermal advection
-heat transfer by vertical motion
-sensible heat transfer
-latent heat transfer
-horizontal vorticity advection
-surface vorticity


Horizontal thermal advection:

Maximum WAA or Minimum CAA

Maximum CAA or Minimum WAA

-Divergence aloft

-Convergence aloft


Heat transfer by vertical motion

-Sinking air (compression) will contribute to -atmospheric divergence
-Rising air (expansion), local cooling will produce convergence. difficult to evaluate


Sensible heat transfer

-Areas of maximum heating will produce divergence -Regions of maximum cooling causes convergence.


Latent heat transfer

-Local heating due to heat release will contribute to atmospheric divergence.
-Local cooling due to latent heat storage will generate convergence


Horizontal vorticity advection

-NVA is associated with divergence
-PVA is associated with convergence


Surface vorticity

-air flows towards a low center (convergence)
-In free atmosphere upper level divergence and lower level convergence tends to lower surface pressure


The atmosphere is said to be baroclinic when there is?

-a horizontal temperature gradient on an isobaric surface.


What type of baroclinic region exclusively favours large changes in surface pressure?

-those which favour thermal development


The atmosphere is said to be barotropic when?

-there is no variation in it's temperature along an isobaric surface. (at a given pressure)


The development of a low depends on what?

The strength of it depends mainly on what?

-the associated baroclinic zone

-the thermal contrast or temperature gradient


The _______thermal contrast, the ________ the baroclinic zone.

-Larger, stronger


5 stages of a life cycle of a baroclinic depression

-formation (baroclinic leaf)



-any development or strengthening of cyclonic circulation around a surface pressure system.


Indicators of cyclogenesis pt.1

-New vorticity centre developing at 500hPa
-Intensification of vorticity and thickness advection over an area where no centre was previously analyzed
-a kink developing on a baroclinic zone or indication of a new cyclonic flow by the surface winds
-awareness of favourable areas for cyclogenesis
eg. warm water in winter
-New shortwave trough and positive vorticity area moving across a baroclinic zone.


Indicators of cyclogenesis pt.2

-New pressure fall or pressure rise centres developing away from a low centre.
-new height rise or falls developing at 500 hPa.
-Topographical influences. eg.new lows can form in the lee of mountains
-to the east of long wave troughs, especially when when a short wave trough connects to it.
-Changing edges of mid level cloud (new s shape curvature


Baroclinic or frontal depressions originate on a stationary portion of a baroclininc zone, other points that should be noted:

-Some areas favour formation more others
---along arctic coast lines
---cA over warm water
--areas of subsidence -lee side of mountains
--areas of decreased friction from land to water-hudson bay, along east and west coast


Most baroclinic lows are associated with what?

-A 500 hPa short wave trough located 3 to 5 degrees of longitude behind the atmospheric low
-Surface lows may or may not be affected by the shortwave trough
-Surface lows are generally guided by the 500hPa flow.and move through long wave troughs


Baroclinic instability occurs when:

there is a lag between thickness and contour patterns of about 1/4 wavelength


Features associated with a mature baroclinic depression:

-the low has instensified considerably
-wave amplitude has grown on the baroclinic zone and is more pronounced
-Isotherms are curved in the direction toward which the air is moving, like sails on a ship
-the amplitude of the upper trough and ridge is increasing and will reach it's peak at this stage.
east of the low the WAA has increased considerably
-Comma shape


Dynamic features of a mature baroclinic depression

-the direction of the motion is parallel to the isobars in the warm sector
-the thermal wave is rotating counter-clockwise about the centre


Indicators that an occlusion is forming: (pt1)

-Deep UPR TROF due to CAA
-a few closed contours form in the UPR TROF
-Shear and curvature increase UPR vorticity centre
-UPR centre departs from the baroclinic zone
-SFC low loses thermal supprt
-SFC low keeps contact with PVA support


Indicators that an occlusion is forming: (pt2)

-Cold front appears to undercut warm front (TROWAL)
-apparent piching of FNTL crest
-CLD stretches along TROWAL/SFC TROF
CLD further wraps around these intense UPR vorticity centre


Indicators of Baroclinic dissipation:

-Surface low moves toward lower 500 hPa heights
-the air column cools down producing a cold low
-Convergence occurs in the entire air column
-Central pressure is usually rising (filling low)
-spiral cloud band s become more diffuse around cold low.
-cirrus leaf moves away with baroclinic zone