I see the term height falls used fairly frequently in forecast discussions. They seem to be associated with troughs. But that doesn't make sense to me. I mean, won't the various millibar levels--i.e. H7, H5, and so forth--lower in height in response to being compressed by high pressure? What am I missing here?
First, a little background is required:
AMS Glossary of Meteorology said:
constant-pressure surface—(Or isobaric surface.) In meteorology, a surface along which the
atmospheric pressure is everywhere equal at a given instant.
Constant pressure surface slope due to changes of temperature (see above image).
AMS Glossary of Meteorology said:
constant-pressure chart—(Also called isobaric chart, isobaric contour chart.) The
synoptic chart for any
constant-pressure surface, usually containing plotted data and analyses of the distribution of, for example,
height of the surface,
wind,
temperature, and
humidity. Constant-pressure charts are most commonly known by their
pressure value; for example, the 1000-mb chart (which closely corresponds to the
surface chart), the 850-mb chart, 700-mb chart, 500-mb chart, etc.
Compare constant-height chart;
see mandatory level.
(Height) contour lines are commonly analyzed on constant pressure charts:
AMS Glossary of Meteorology said:
contour line—(Also called contour, isohypse, isoheight.) A line of constant
elevation above a certain reference level (usually
mean sea level) on a previously defined surface, which may be the earth's surface, a
constant-pressure surface, an
isentropic surface, etc. A contour line of a given value is the intersection of the surface in question with the
constant- height surface of the same elevation as the value of the contour line. In meteorology, a contour line frequently refers to a line of constant height on a
constant-pressure chart.
Compare isobar.
AMS Glossary of Meteorology said:
height-change chart—A
chart indicating the change in height of a
constant-pressure surface over a specified previous time interval; comparable to a
pressure-change chart.
See height-change line.
Meteorologists use
height changes to track the movement of
pressure systems (e.g., highs/lows/troughs/ridges) on constant pressure charts.
Upper level ridges and highs move towards height rises, while
upper level troughs and lows move towards height falls.
Here is an example of a 500 millibar Constant Pressure Chart (height contours=solid blue lines) and 6-hour Height Changes (shaded):
Moving beyond theory to practical application:
* How do I recognize height falls on the upper atmosphere maps?
National Weather Service constant pressure DIFAX Charts contain an analysis of height changes plotted in deca-meters on the lower right of the station plot.
http://weather.noaa.gov/fax/barotrop.shtml#mb500
Earl Barker's Model Page contains NAM model forecasts of height changes at:
http://www.wxcaster.com/conus_0012_us_models.htm
* If height falls are in fact related to troughs and lows, then can I correctly assume that severe weather will normally occur east and/or southeast of advancing height falls?
Not necessarily. Thunderstorm formation requires three ingredients:
1. Sufficient water vapor (e.g., "dewpoints") to produce condition 2.
2. Unstable air. Showers and thunderstorms almost always form in an unstable airmass characterized by
Conditional instability.
3. A lifting mechanism to release the
Conditional instability. The lift can be from
fronts,
drylines,
local winds (e.g.,
sea breeze,
lake breeze, land breeze, valley breeze),
upslope wind, and
outflow boundaries from prior showers/thunderstorms.
Thunderstorm type is related to
vertical wind shear, type/nature of lifting mechanism and
mesoscale influences which are not well understood.
However, height falls can be
indirectly associated with the above ingredients. For example, height falls might lower the surface pressure (which increases the vertical wind shear) and act to destabilize the atmosphere. Thus, height falls often
set the stage for severe thunderstorms.
For those who wish to respond: I realize I'm probably dealing with some fairly technical stuff here, but the more accessible and here's-how-to-apply-it you can make your answer, the more helpful it's likely to be.
Thanks!
I hope this helps!
