I Can't Stand The Pressure! What's With All This Atmospheric Pressure!!!!

You may watch TV weather and see the Barometric Pressure or maybe it's just referred to as "Pressure", but you have no clue what it is? Until now that is, so let's get Savvy....Weather Savvy!

Barometric pressure is the same thing as atmospheric pressure or air pressure. We measure the pressure with a barometer and anyone can buy a barometer which will tell them what the atmospheric pressure is...but what exactly is that?

Well, think of this...when the wind blows in your face, what blows your hair back? Air, right?

Yep, air. And I mention this because sometimes we think of air as being nothing because it is invisible. But air is something, comprised of Nitrogen, Oxygen, Carbon Dioxide and all sorts of other molecules. Basically air has mass. Even though it's made of tiny particles that you don't stop to notice on a daily basis, air has mass. You perhaps do notice it when your dog sticks his face out the car window and the air molecules push your dogs face back, force his tongue back and to the side and whip the drool into the car behind you. That's the air molecules doing that!

Anything around earth that has mass has weight, because it is under the influence of gravity. Thus, the air around earth has weight. Even though it is invisible, air weighs down on our shoulders, our grass, our houses, and our cars. Everything on earth feels the weight of air!!!! The weight of the air over the earth IS atmospheric pressure.

Imagine all the air above you up to 9 miles above the surface of the earth (most of earth's air is from the surface to 9 miles up...there is less and less air as you go higher). Now, this air isn't stationary, it actually moves. Sometimes there is less air above you, sometimes there is more air above you. This would mean that sometimes there is less pressure on your body and sometimes there is more pressure on your body. Even though our bodies don't notice this (Perhaps except for arthritis sufferers, click here for more on that), these changes in pressure correlate to changes in our weather. So, wouldn't it be great if we could somehow measure these changes in pressure to predict changes in weather?

How Is Pressure Measured?

In 1644 Evangelista Torricelli, a student of Galileo, measured atmospheric pressure with several different liquids. Torricelli is credited with inventing the barometer. The concept of the barometer is similar to water in a straw.

Water in a straw? Huh?

If you place a straw in a glass of water, some of the water will rise into the straw. This is because the weight of the air on the water is forcing it up the straw where there is less pressure. I'll give you a second to digest that. The concept is like a blow pop, you know the tasty treat you get from the good humor ice cream man. If you put pressure on the bottom of the blow pop, it goes up! Well, in this case it's pressure from the air (the weight of the air) on the surface of the water in the glass. Without a straw in the water, the water has no where to go. By putting a straw in the water, you have created an area of low pressure, which is the straw, and the water will fill a portion of the straw. From this you can deduce that water (or air or anything really) will move from areas of high pressure to low pressure.

If there is more air pressure on the surface of the water, the water level in the straw will go higher up the straw. If there is less air pressure, the water level in the straw will sink lower. Now we can measure pressure changes by looking at the water level in the straw! A barometer isn't quite that simple though, I just used that as an example to understand the concept.

Torricelli settled on using Mercury as the liquid for his barometer, and of course he didn't use just a glass of mercury and a straw. Instead of a straw, he used an evacuated tube. In science "evacuated" usually refers to the removing of air. By removing air from this tube, then sticking it in a pool of mercury, he created a much stronger area of low pressure in the tube. Remember that pressure goes up with more air and down with less air. So, a vacuum (no air at all) is the ultimate low pressure.

Another type of barometer, and more common today, measures pressure "without fluid", called an aneroid barometer. The aneroid barometer uses metal that contracts and expands with changes in atmospheric pressure (hence no fluid). So, the contractions and expansions of the metal are simply due to the metal changing as a result of changing air pressure.

How The Air Pressure Changes, Tells Us Something Interesting About Weather

On TV weather maps you always see "H" and "L" and by the way, "H" is for high pressure and "L" is for low pressure. Although the exact number of the high pressure and low pressure aren't important to the average viewer, it's the location and the type of pressure that is important. Low pressure is really an area of lower pressure, relative to the pressure around it. At the center of this area of lower pressure air is rising and this is VERY important to meteorologists.

One of the keys to forming rain is rising air. As air rises it typically cools, and with sufficient moisture, clouds and perhaps rain is all a possibility. So with lower pressure, you have rising air, and this makes clouds and rain a good possibility. Note: This does not mean that rain WILL occur.

When people on the east coast talk about a nor'easter, you'll notice a big "L" just off the East coast, this is the storm. A Hurricane is a large and intense "L". Yep, that's right, a hurricane is an area of low pressure.

The big "H" is an area of higher pressure. At the center of this area of high pressure the air is sinking slowly to the earth. As air sinks it typically warms (remember I said above while talking about low pressure that as air rises it typically cools, well this is just the opposite.) As air warms, it encourages evaporation, which inhibits cloud or rain formation. That's why under high pressure weather conditions are typically mostly clear and tranquil.

Photos from http://www.istockphoto.com