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Old 12-08-2000
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Low-Pressure Systems


Low-pressure systems redistribute hot and cold air, which often results in strong winds and seas.
What is a low-pressure system? And why should we be interested in "lows"? A low is a weather system that draws air inward along the earth's surface and then pushes that air upward until it reaches the jet stream. There, it is siphoned off downstream, which permits a continued inward flow of air on the surface.

Why does this occur? Because nature needs methods of redistributing hot and cold throughout the atmosphere and lows are a very efficient means of accomplishing this mission. Nature takes cold, dry air and warm, moist air on the earth's surface, brings these two quantities together and starts mixing them.

Cold, dry air sinks and hugs the earth's surface while warm, moist air rises. Interestingly enough, when the warm, moist air rises, encouraged by the cold, dry air settling beneath, it cools. When warm, moist air cools, its moisture condenses and forms clouds, and when moisture is condensed heat is released. This heat re-warms the cloud mass, encouraging further rising. Thus we have positive reinforcement, and after rising just a few miles above the earth's surface, this hot-but-cooling air meets the Jet Stream where this air is sucked off downstream making way for continued air inflow at the surface. (For more on this, see Reading Weather Fax Charts.)

So what makes one low a Perfect Storm and another low barely noticeable? It is all related to the rate of air inflow at the surface and outflow aloft. If air is exhausted faster aloft, which happens when the Jet Stream sucks air downstream at a rate faster than air can be dragged in at the surface, then a deficit of air occurs. You might even think of this as a partial vacuum. When this occurs, surface pressure drops dramatically, and surface wind speed increases. Winds aloft that measure 100 knots and greater are a good indication of building surface winds, dropping pressure, and a developing gale or storm.


Rate of airflow at the surface and outflow aloft is what makes one low barely noticeable and the other a Perfect Storm.
However, if upper-level Jet Stream winds are light, or the neighborhood of 30 to 50 knots, then there is no need to expedite air inflow at the surface since air is being sucked out aloft at a leisurely pace. However, an initially slow outflow aloft can evolve into an expedited airflow, so all lows bear watching carefully.

Why do we concern ourselves with lows more than any other type of weather feature? Because when strong lows form, the inward wind flow at the surface produces large seas and waves. It is these building winds and seas that cause us the most concern. Rough seas can tire a crew, fatigue gear, and threaten our vessels' stability. Generally, the worst-case scenarios, capsizes, pitchpoles, and broaches, are all wave-induced.

"Not only does the longest fetch occur behind a cold front, the winds there have the best angle of attack to produce waves. Cold air sinks and thus "digs" into the ocean just as a shovel digs into the earth when you're digging a hole."

Waves form where the wind blows for extended periods over a consistent fetch. The longest fetch area in a low-pressure system is always found behind the leading edge of cold air, known as the cold front. Not only does the longest fetch occur behind a cold front, the winds there have the best angle of attack to produce waves. Cold air sinks and thus "digs" into the ocean just as a shovel digs into the earth when you're digging a hole. By contrast, warm air ahead of a cold front wants to rise; it has no desire to sink and dig into the ocean. Thus we can say that when comparing warm and cold air influencing the same region, the cold air will always produce larger seas.

Consider also that when cold winds blow against an ocean current, such as the Gulf Stream, seas can build to a much a greater height than outside the current. The exact increase in size is dependent on the strength of the wind, the speed of the current, and the temperature difference between the air and the water. A good rule of thumb is that when the air and water temperature differential is 20 degrees C or greater during low-pressure formation (typical late fall and early spring conditions over the Gulf Stream), seas and winds will be 50 percent greater than otherwise predicted.


When air and water temperature differ by 20 degrees Celsius during the formation of a low, seas and wind will be 50 percent greater than predicted.
Once a low-pressure system forms, it rotates counterclockwise and moves in an easterly direction, its specific direction is controlled by the flow of upper level Jet Stream winds. Lows can move in other directions than east if the Jet Stream shows what is called a meridional flow pattern. In meridional flow the Jet Stream has a significant south-to-north component and lows will move in this direction as well. When lows bring snow to the East Coast of the US, this activity is due to a low moving south to north just east of the coast, thus maximizing its duration of encounter with the coast and allowing large amounts of moist, maritime air to flow over land where it cools and condenses. So, here is a forecasting tip: when lows are moving directly west to east (this is called zonal flow) there is little opportunity for snow or precipitation to accumulate along a north-south oriented coast.

When lows become stationary and move very little it is because they are under the influence of light, upper-level winds, the same winds that are exhausting (or sucking out) the rising air that flows in at the surface. Fortunately the National Weather Services Marine Prediction Center produces both surface and upper-air analyses every day and these show the direction and speed of movement for low-pressure systems. Forecast charts go out to 120 hours (five days), and since these are available on both Weatherfax and the Internet (www.mpc.ncep.noaa.gov), they should be consulted prior to and during every voyage.

One final tip on low-pressure systems: the cold front within a low always moves faster than the warm front, and when the cold front catches the warm front, it overrides it and forms an occluded or stationary front. Why does the cold front always catch the warm front? Because the cold front swings south of a low's center as it rotates counterclockwise and this is an area where upper-level winds are stronger and thus propel the cold front across the surface at a rapid clip. While the cold front is zooming along, the warm front is moving more north and northwest into an area where upper-level winds are always weaker, and so the warm front quickly slows down and stalls. A good analogy here is the bend in a river. A warm front moves to the inside of the bend where currents are weak while the cold front stays to the outside where currents are strong.

Satellite imagery provides a wonderful view of lows and I would encourage readers to visit the Marine Prediction Centers webpages where annotated satellite images are posted every six hours. I also recommend my book "Weather Predicting Simplified: How to Read Weather Charts and Satellite Images," which contains numerous charts and satellite images of low-pressure systems.


Suggested Reading List

  1. Avoiding Heavy Weather by Michael Carr
  2. Reading Weather Fax Charts by Michael Carr
  3. At Odds with the Weather by John Kretschmer
  4. SailNet Buying Guide - HF Radios

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