Using the Current 500-hPa Chart
In the deepening of lows there must be removal of
air at high levels due to divergence in the 400- to
200-hPa stratum, resulting in stratospheric warming.
Insufficient inflow at very high levels to compensate the
subsidence results in 500-hPa contour falls.
This is roughly the mechanism thought to be
responsible for the development of low-pressure
The high-level decrease in mass
overcompensates the low tropospheric increase in
density; the high-level effect thus determines the
reduction of pressure at the surface when lows are
Stratospheric and Upper Tropospheric
Decrease in Mass
The chief cause of deepening lows is the decrease
in mass in the upper troposphere and the lower
stratosphere. With rapidly deepening lows, it is known
that the change in mass in the stratosphere contributes
as much to the local surface pressure change as do the
tropospheric changes in density, if not more. Warming
is frequently observed in the stratosphere over
deepening surface lows, pointing to subsidence in the
lower stratosphere. This warming is accompanied by
lowering heights of constant pressure surfaces in the
lower stratosphere, indicating a decrease in mass at high
levels. See figure 3-8.
Deepening, to a large extent is controlled by mass
changes in the upper atmosphere. For example, it has
been shown that the lower two-thirds (below about
300-hPa level) of the central column become colder and
denser as the areas of low pressure deepen, while the
upper one-third of the column becomes warmer. The
upper mass decreases by an amount sufficient to
counteract the cooling in the lower layers, plus an
additional amount to deepen the low. The preferred
region for deepening of lows is in the top third of the
atmospheric column or, roughly, the stratosphere. See
Using Facsimile and NODDS Products
Facsimile and NODDS products currently contain
prognostic 500 mb, 1000- to 500-mb thickness, and
500-mb vorticity charts. These charts can be used in
making predictions of advective changes, thickness
patterns, and subsequent changes to the surface pattern.
DEEPENING OF LOWS RELATIVE TO
Weather types were discussed previously under the
section Movement of Low-Pressure Systems. This
method can also be used to forecast changes in intensity
of pressure systems, as each system or type has its own
average movement plus average deepening or filling.
DEEPENING OF LOWS IN RELATION TO
NORMAL STORM TRACK
Lows whose tracks deviate to the left of the normal
track frequently deepen. In general, the normal track of
a low is parallel to the upper flow. If a low deviates to
the left of normal, it crosses upper contours (assuming
an undisturbed upper current) and becomes
superimposed by less mass aloft, resulting in deepening
of the low. As long as this crossing of upper contours is
unaccompanied by sufficient compensatory cooling at
the surface low center, the system will deepen.
RELATION BETWEEN DEEPENING LOWS
There is little basis for the rule that deepening
storms move slowly and tilling storms move rapidly.
The speed of movement of a low, whatever its intensity,
is dependent upon the isallobaric gradient and other
factors. The magnitude of the surface isallobaric
gradients depends upon the low-level advection, the
magnitude of the upper-level height changes, and the
phase relation between the two levels.
FORECASTING THE INTENSITY OF
The following section will deal with atmospheric
factors aloft and how they affect surface
anticyclogenesis. This section will also discuss rules for
forecasting the intensity of surface highs.
In the case of developing dynamic anticyclones,
cooling takes place at about 200 hPa and above. This
cooling is due to the ascent of air, resulting from
convergence in the 400- to 200-hPa stratum.
Incomplete outflow at very high levels causes piling up
of air above fixed upper levels, resulting in high-level
pressure rises. At the same time, warming occurs in the
lower troposphere. This warming sometimes occurs
very rapidly in the lower troposphere above the surface