EAS 308, Meeting 2
Depressions and anticyclones:
Our British author calls regions of low pressure “depressons” and regions of high
pressure “anticyclones”. Rigorously, they are named “cyclones” and “anticyclones”.
But, I must modify the word cyclone because cyclones form at all latitudes. Hence, they
are named tropical cyclones, mid-latitude cyclones and polar cyclones. Here we will
study all three with the emphasis on the mid-latitude cyclone.
The author begins our study explaining how a “little depression” can form as illustrated
in Fig. 15. This same reasoning can explain the formation of both tropical cyclones and
polar cyclones, cyclones which form in one air mass. An air mass is a region of the
atmosphere in contact with the surface with relatively uniform temperature and humidity
(moisture content). In the case of the tropics, the air mass is warm and moist and in the
case of the Polar Regions the air mass is cold and dry.
Then, the author describes the formation of “large depressions” namely the mid-latitude
cyclone which form along the boundary of the polar and subtropical air masses. Our
study of this phenomenon, first described by the Norwegians in the early 1900’s, will
challenge our 4D visualization skills, three space dimensions and one dimension in time.
Mastering of this conceptual model of the mid-latitude cyclone will greatly increase your
understanding of winter weather in NYC.
Vorticity:
The first concept to master is that of vorticity, the spinning motion imbedded in a moving
fluid (Fig. 16). There can be both cyclonic vorticity and anticyclonic vorticity; that
illustrated in Fig. 16 is cyclonic. Cyclonic vorticity is associated with converging and,
hence, rising air and anticyclonic vorticity is associated with diverging and, hence,
sinking air. Literally a good “rule of thumb” to remember is the right-hand-rule from
physics. Rotate your right hand in the direction of the vorticity and your thumb points in
the direction of the vertical motion of the air.
Vorticity and converging and diverging air:
When the horizontal air motion is converging this leads to an increase in cyclonic
vorticity and rising air. Likewise, when the horizontal air motion is diverging this leads
to a decrease in cyclonic vorticity (increase in anticyclonic vorticity) and sinking air.
These concepts are illustrated in Figs. 17 and 20.
In Fig. 17, the converging surface air increases cyclonic vorticity producing the
ascending air. Likewise, aloft the diverging air enhances this rising motion (Fig. 20a). In
Fig. 17, the converging air aloft leads to sinking air (Fig. 20b) which is dissipated in the
surface diverging air (not shown on Fig. 17 but will be sketched on the board). So, this
“tilting” of the surface and upper air troughs leads to a pattern of surface convergence
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and rising air into diverging air aloft and air-aloft converging and sinking into diverging
air at the surface. This, pattern you will soon see is critical to the formation and
maintenance of the mid-latitude cyclone.
Let’s observe the surface and upper air charts on the excellent University of Wyoming
weather web site (weather.uwyo.edu) to find an example of this 3-D structure of the midlatitude cyclone.
Formation of fronts:
The surface convergence draws cold polar air and warm sub tropical air together forming
fronts, the boundaries between two air masses (Figs. 21 and 22). When cold air advances
into warm air, a cold front is formed and vice versa for a warm front. We will study
fronts in more detail in the next chapter.
Life history of a mid-latitude cyclone:
The author’s narrative and illustration (Fig. 23) of the formation, maturation and
dissipation of this wonderful atmospheric phenomenon need to be read and understood.
The juxtaposition of the surface and upper-air flows and the corresponding fronts
illustrates well the life cycle. An added bonus, are the schematics of what would be
observed on a satellite image. The evolution also is illustrated in Fig 28. These are
infrared images from Earth-orbiting satellites, so what is cold is white (high clouds) and
what is warm is dark (low clouds or land/ocean). There are better images of the midlatitude cyclone (but not in an orderly sequence) in Figs. 148 – 153 (pp. 136-141).
A note. I was in Bergen Norway June 2006 to make a brief pilgrimage to see the earliest
weather charts drawn in the early 1900’s. I was astonished how well their analyses were
considering the little data that were available.
The track of the mid-latitude cyclone is illustrated in Figs. 26 and 27. The typical track
is from west to east because the jet stream flows in that direction. Why does the jet flow
in that direction? Remember the balance of forces: northward-directed pressure-gradient
force balancing a southward-directed Coriolis force leading to an eastward moving air
flow.
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