Surface air pressure can be measured by a device called a barometer. A barometer indicates pressure in units known as millibars (mbs) or inches of mercury. Without getting into the mechanics of how barometers operate, it's safe to say that barometers are the standard for measuring surface air pressure for weather purposes. If you live at sea level near the equator, standard 'normal' sea level air pressure would be about 1000 mbs. If a relatively strong high-pressure system were to move over your home, the barometer might indicate the pressure at around 1020 mbs. Conversely, if a strong low-pressure system were to move in, the barometer might register about 970 mbs. (Surface air pressure tends to be less as you travel north or go up in elevation). Of note, both high and low pressure systems typically cover at least many hundreds of miles in area, and move rather slowly. So you typically won’t see a change in pressure from one extreme to the other in a matter of hours, but more over a time periods of days or weeks. And it's highly doubtful you'll feel the change in air pressure, but you'll certainly see the weather changes associated with pressure change. Under high pressure, weather is typically fair, skies cloudless. But under low pressure, rain, wind and inclimate conditions prevail.
The greater the difference in air pressure between two competing systems, the greater the wind. And the greater the surface area covered by low pressure, the greater the size of the area of wind will be. Since strong wind is typically associated with storms, and since storms are characterized by low pressure, surf forecasters typically track and look for developing areas of low pressure, preferably large ones. The deeper or lower the pressure, the stronger the storm, the stronger the wind, and the greater the probability for surf to be generated. The larger the area of low pressure, the larger the fetch or ocean surface that will be affected by the resulting wind. Of course, to generate a large swell, the three requirements we discussed earlier must be met: High wind velocity, large fetch area, and long lasting winds. The ideal big wave set-up is to have a massive deep storm surrounded by extreme high pressure. And the systems must be positioned so that the fetch is aimed towards your beach for a long time. In reality, this rarely happens, but rather you get either a massive storm surrounded by relatively normal conditions, or a moderate storm surrounded by strong high pressure for a day or two at best. Make a Homemade barometer
The air pressure around us greatly affects our weather. Notice how your weatherman always mentions various pressure systems (low pressure system, high pressure system, etc.) and how they will affect tomorrow's weather. In this experiment, we will create a tool that lets you gauge the pressure of the air around you.
1. Fill the measuring cup or glass with water and add some colored dye to it.
2. Flip the empty soda bottle upside down into the glass measuring cup.
3. Assure that you use a bottle that is just the right size. It should fit snugly in the measuring cup so that the mouth of the bottle does not touch the bottom of the cup.
4. Assure that the level of the water extends into the neck of the bottle.
5. Mark a line on the cup to indicate the water level within the bottle.
6. Re-examine the bottle in a few days.
Notice the change in the water level? The amount of air within the bottle is fixed and cannot change since the water extended into the bottle acts as a 'plug'. Hence, you can consider the amount of air trapped in the bottle as an indicator of the air pressure on the day you plugged the bottle. The pressure on the surface of the water depends on the current air pressure. When the air pressure increases (as it does in drier weather), the pressure on the surface of the water is greater and the water is forced up into the bottle changing the level of the water. If you see the water level drop, look for your umbrella...
George H. Taylor, CCM
State Climatologist, Oregon
Oregon Climate Service
316 Strand Hall
Oregon State University
Corvallis OR 97331 http://www.ocs.orst.edu Source: seps.org