A Spell of Weather III

The North Atlantic as a map reading exercise. …

The first thing that strikes you if you are confronted with a weather map for the first time is that the thick black lines are meaningless and confusing.


Perhaps the next thing are the meaningless letters and numbers.

Image gleaned from: {url}http://www.wetter-zentrale.eu/wz/pics/bracka.gif[/url]

Finally if the lumps and bumps haven't put you off, the next thing will be the isobars. They are the contours or rings joining the places where the air pressure is more or less the same at the time the data was collected.

Their centres are marked with an "X", And besides the "X" is the lowest or highest pressure recorded at that time.

Conventional wisdom has it that highs and lows cancel each other out, that they move across the face of the globe in an effort to neutralise each other.

The fact is that they never meet. They travel the planet, eternally building and collapsing, eternally moving on in a game of chess where all the pieces can be restored.

The Highs come down from the upper atmosphere. Their falling causes the air to heat up as it compresses. Exactly what empowers such a system reaction eludes me. Heat is not created but the effect requires some energy input as yet undefined.

The law of physics at work here is called "Newton's First Law" which states that things don't just happen because it is their birthday.

Lows tend to follow geological "faults". This means they follow one another in similar spells. And it means they catch up to each other in the same spells. It also means that having moved on they tend to separate out from each other. This is an acoustic phenomenon of the type that occurs in the physics of all forms of wave behaviour: "Newton's Rings" and "Interference Patterns".

What will probably escape you on a map of this scale and/or detail, is that the median lines for the isobars are 1012 for Lows and 1016 for highs. On the Canadian North America map the graduations are in steps of 8 millibars not 4 and the 1016mb line is more easily seen:

Image Source: http://www.weatheroffice.gc.ca/ensemble/charts_e.html?Hour=0&Day=0&RunTime=00&Type=pnm

To be continued, meanwhile here are some more of the same:
http://my.opera.com/Weatherlawyer/blog/2008/11/13/a-spell-of-weather
http://my.opera.com/Weatherlawyer/blog/2008/11/13/a-spell-of-weather-ii

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