Pressure Systems
What is Wind
The title of this topic might have been “What causes the wind.” Quite simply, wind is moving air. Air doesn’t seem like much, however, it has weight and it has substance, and is actually considered a fluid. This becomes apparent when air moves. While moving, you can see many of the same characteristics you might find in a river. Things like moving as a “unit,” parting and coming back together as it flows around an obstacle, and eddies or back-flows.
Air moves from an area of higher pressure into an area of lower pressure. The closer together the high and low pressure is and the greater the difference in pressure, the faster the air flows.
Air has weight
The temptation is to believe air has no weight, which isn’t true. Logically, it must have weight. If not, you wouldn’t be able to feel the wind on your face while sailing, and your sails would hang limp even in a fresh breeze.
If you weigh the column of air directly above you, it would weigh approximately 14.696 pounds per square inch. This is also known as the standard atmospheric pressure. As the graphic demonstrates, there are a number of labels to use when expressing standard pressure. All represent the same value.
In the maritime world, millibars (mb) are used. Standard pressure in millibars is 1013.25(mb). Throughout these lessons, we will be using millibars to express atmospheric pressure, however, you may have occasion to check weather briefings and forecasts that use other units of measurement. For example, aviation forecasts typically use inches of Mercury (Hg), where standard pressure is 29.9212 inches (Hg).
Atmospheric pressure is also known as barometric pressure.
In the Island in the Sun analogy in An Introduction to Marine Weather, heated air ascended traveling away from where it rose where it was added to the “pile” of air at another location, creating a higher pressure. There are other ways in increase atmospheric pressure in a given area. For example, cold air is heavier the warm air. Moisture laden air is heavier than dry air. Consequently, all of these factors are added together to determine pressure in a given area.
High Pressure vs. Low Pressure
Relatively small variations in pressure determine the wind and storm patterns around the world. The highest pressure every recorded was 1083.98 mb at a location in Siberia. Whereas the lowest, 877.07, occurred in the eye of a South Pacific typhoon. In contrast, most readings fall between 975 mb and 1050 mb, a pressure difference that can easily result in wind speeds in excess of 50 kts!
Pressure gradient
Pressure gradient is the change in pressure measured across a given distance. Air flows from the high to the low because the pressure gradient creates a net force that starts the movement of air. The greater the pressure change over a given distance the greater the net force.
Coriolis Effect
Air moves from higher pressure into lower pressure, however, not in a straight line as demonstrated above. Instead, something called the Coriolis Effect causes the air to deflect to the right around the high pressure area.
The Coriolis Effect describes the bending of the trajectory of an object as it travels. In the Northern Hemisphere, air that is moving out of a high pressure system towards an area of lower pressure bends to the right.
Clockwise Around the High
Here’s some good news. Unless you are a scientist or a mathematician, it really doesn’t matter to weather forecasting? Just remember, in the northern hemisphere high pressure systems rotate clockwise and low pressure systems rotate counter clockwise. The wind travels slightly out of the highs and slightly into the lows.
The opposite is true in the southern hemisphere.
Around and Out
We now know the net force resulting from the pressure gradient gets the air moving out of the high, towards the low. Once moving, the Coriolis Effect begins to turn the moving air to the right. The air continues deflecting to the right until the net force of the pressure gradient and the turning force of the Coriolis Effect balance each other out. As a result, the air flows clockwise around and out of the high by 15° to 30°.
Isobars
Isobars are the contour lines of atmospheric pressure. They are a series of lines on a map connecting points having the same atmospheric pressure. Each isobar is labeled with its represented pressure, generally with a four millibar spacing.
In this surface analysis forecast example, a high pressure area can be seen on the left side of the image, slightly below center at latitude 40N. The “H” indicates the center of the high. The expected high pressure there is 1034mb. It’s important to note the “1034” label is underlined, and is not found at the actually location of the highest pressure. In this case, it is slightly above and to the right. The first isobar out from the “H” is 1032mb and includes just the last two digits of the pressure (32). Each successive isobar is 4mb less than the one before; 28, 24, 20, etc.
Moving to the upper right on the map, is a low pressure area with an atmospheric pressure of 999 in the position of the X next to the “L”. Although not marked, the small circle surrounding the “Lx” represents a pressure of 1000 isobars. Take a moment to locate and identify the pressure for each isobar between the areas of high pressure and low pressure.
