High and Low Air Pressure
Just as the air temperature is a measure of how much energy is present, so is the air pressure.
(Recall that energy is necessary to move mass.) But first what is PRESSURE? If you take a force and spread it over an area, that's pressure. For example, if you evenly apply a force of 10 pounds over a piece of paper with an area of 10 square inches, the pressure is the force (10 pounds) divided by the area (10 square inches). So the pressure is 10 lbs/ 10 sq. in. or 1 pounds per inch.
So why is the air pressure an important part of the weather? Well, pressure differences between air masses causes the air masses to MOVE! So air masses of high and low pressure are constantly moving over the surface of the Earth. This is one way the weather changes, because the characteristics (for example, the temperature, pressure, and humidity) of the moving air masses moves with them. What if the temperature of theses air masses are different? What would happen if these air masses move over you? You would feel the movement of the air (that's the wind) and you would feel the change on the air temperature. So you see, the air pressure plays a big role in what we experience as the weather. There are many high and low pressure areas over the surface of the Earth so there is plenty of energy available for moving air masses and changing our weather.
One other thing about air pressure is that it changes with the altitude. For example, the air pressure is higher at sea level than on the top of a high mountain. This pressure difference also helps air to move up and down. (Watt, Fiona and Francis Wilson, Weather & Climate, Usborne Publishing Ltd.,1992.)
Air also circulates around high and low pressure areas in very specific ways. In the northern hemisphere, air circulates clockwise around a high pressure area. The opposite is true for a low pressure area, that is, air circulates counterclockwise around a low pressure area.
Interestingly, the air circulation is exactly opposite in the southern hemisphere. The direction of circulation is due to the Earth's rotation, specifically to something called the Coriolis effect.
Have you ever noticed that the water rotates the same way every time as it goes down the drain?
This circulation can help explain why the wind direction isn't always what you think it should be. For example, a strong low pressure air mass is moving toward you from the west. You live in the northern hemisphere. You might think that the winds would be blowing from the west, but they come from the south. Can you explain this?
The average air pressure is 14.7 pounds per square in. That means at on average you have a force of 14.7 pounds on every square inch or your body! Usually the air pressure is reported in inches of mercury. This is related to the how high the air pressure can push a column of mercury in a curve tube. To convert pressure reported in in. of mercury, multiply that number by 0.4912 to obtain the number in pounds/sq. in (psi).
Fronts
A front is a boundary between two different air masses. In general, when a cold air mass moves into a warmer area (this is called a cold front), the cold air pushes under the warm air, raising the warm air. Conversely, when a warm air mass moves into colder region, this is called a warm front. In general, warm air raises over cold air at different rates along a front.
This causes temperature and pressure differences along the front. Sometimes this nonuniform behavior can cause very turbulent conditions, with severe winds and precipitation.
Experiments
(Lessons Science Program, Lawrence Livermore National Laboratory, 1986)
1. Can you feel pressure? Try this.
Fill a large bucket with water about 3/4 full with water. Put your are in a plastic bag up to about your elbow. How slowly put your arm into the bucket. Do you feel the pressure? Does the pressure increase as your hand goes deeper? (This works better if the student uses a bag or something else to keep the skin dry.)
2. Lifting a Book with Pressure
Need:
- a large zip-lock bag
- straw
- modeling clay
- a book
In this experiment we will demonstrate how pressure can be used to move heavy objects. Place the straw in the bag as shown
above and seal the zipper. Use the clay to help maintain a good seal.
Place the book on the bag and slowly blow into the straw. You will notice that you can easily lift the book.
This experiment shows how a little pressure acting over a large area can generate a large force. Remember this when a mild wind is blowing and you're trying to carry a poster board to school.
