On Jun 13, 7:06*pm, Weatherlawyer wrote:


A small step from that is looking at the Atlantic chart every 6 hours
and watching these small pressure waves build up. I don't pretend to
know their cause but you really can see Low pressure regions develop
as these things converge.

This is what I mean when I use the term "convergance".
I have no real idea what the more knowledgeable understand about it.

This is what one source (RA-Aus) says about convergence, divergence
and subsidence:

Atmospheric vertical motion is found in cyclones and anticyclones,
mainly caused by air mass convergence or divergence from horizontal
motion.

Meteorological convergence indicates retardation in air flow with
increase in air mass in a given volume due to net three dimensional
inflow. Meteorological divergence, or negative convergence, indicates
acceleration with decrease in air mass. Convergence is the contraction
and divergence is the spreading of a field of flow."

http://www.auf.asn.au/meteorology/section1b.html

I on the other hand am talking about an increase in the number of low
pressure waves. As they arrive already in combinations that take the
pressure to 990 to 980 millibars they already consist of a number of
wavlets each worth about 2 or 3 millibars decrease in pressure.

They converge off Iceland and Greenland as if waiting the full
pressure capacity required to get them over the Mid Atlantic Ridge.
This has to be some 970 or so minimum IIRC.

When they hit land once more you can see them separating out into
their constituent air masses. Blowing where they list.

The article goes on to describe what happens without explaing why. It
is difficult to see why an air mas floating free of any vessel has to
build up or con~/di~ ~verge:

"If, for example, the front end of moving air mass layer slows down,
the air in the rear will catch up – converge"

How? And more importantly; why?

"The air must move vertically to avoid local compression.

If the lower boundary of the moving air mass is at surface level all
the vertical movement must be upward. If the moving air mass is just
below the tropopause all the vertical movement will be downward
because the tropopause inhibits vertical motion."

How and why?

"If the front end of a moving air mass layer speeds up then the flow
diverges. If the air mass is at the surface then downward motion will
occur above it to satisfy mass conservation principles, if the
divergence is aloft then upward motion takes place.

Rising air must diverge before it reaches the tropopause and sinking
air must diverge before it reaches the surface.

As the surface pressure is the weight per unit area of the overlaying
column of air, and even though divergences in one part of the column
are largely balanced by convergences in another, the slight change in
mass content (thickness) of the over-riding air changes the pressure
at the surface."