We Are All Under Pressure, Atmospheric Pressure That Is!
The differences between high and low pressure determine our wind weather patterns
Friday, March 21st, 2014 — Meteorology books typically describe us as living in an ocean of air. Funny thing is air does behave similarly in many ways like water. This ocean of air exerts air pressure. Air pressure is the force exerted on you by the weight of tiny particles of air, air molecules. Although air molecules are invisible, they still have weight and take up space. Since there's a lot of "empty" space between air molecules, air can be compressed to fit in a smaller volume, or area.
How air pressure is related to the basics of weather is described quite nicely by About.com, www.about.com “An important characteristic of the Earth's atmosphere is its pressure as it often determines wind and weather patterns across the globe. By definition, atmospheric or air pressure is the force per unit of area exerted on the Earth’s surface by the weight of the air above the surface. The force exerted by an air mass is created by the molecules that make it up and their size, motion, and number present in the air. These are important factors because they determine the temperature and density of the air and thus its pressure.”
Earth's atmosphere is pressing against each square inch of you with a force of 14.7 pounds per square inch or 1 kilogram per square centimeter. The force on an area a little larger than a square foot or 1,000 square centimeters is about a ton! What saves us from being crushed? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and you are not squashed.
Have you ever noticed your ears popping as you climbing or driving up a mountain? You also need to breathe more often then when you did at a lower elevation. As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit. As you climb higher, air temperature decreases. Typically, air temperatures decrease about 3.6° F per 1,000 feet of elevation. This is called a “lapse rate”. Air at the surface is denser; it has more air molecules so it is able to retain more heat. The air thins out, has fewer air molecules the higher you go, so it cannot retain as much heat. That is why even though you are closer to the sun on a mountain top; the air is much cooler or colder.
When air is compressed, it is said to be "under high pressure". Air at sea level or ground level is normal or what we're used to. We are so used to feeling the normal air pressure that we forget we're actually feeling air pressure all the time!
Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (Mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars. Planes use “inches of mercury” in their altimeter to determine how high they are flying above sea level.
I like to use the “balloon” metaphor when it comes to explaining one of the ways pressure produces wind. When you blow up a balloon, what kind of pressure are the air molecules under inside the balloon? High pressure of course! Where do they want to go? The molecules want to go to an area of low pressure where they are not compressed. So when you pop they balloon the air molecules all rush out to where there is lower pressure. The same thing happens in out atmosphere, the air speeds up when flowing from high pressure to an area of low pressure, producing wind.
Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.
As the name says, a "high" is an area where the air's pressure is higher than the pressure of the surrounding air. A "low' is where it's lower. There is not a specific number that divides high from low pressure. It is the relative changes or differences that matter
The pressure is high at the surface where air is slowly descending. This usually happens over a large area, like a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally, but not always , is associated with good weather.
The air that descends in high-pressure areas has to get to high altitudes in some manner, and this is accomplished by rising in areas where the pressure at the surface is low.
As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there's enough water or ice, rain or snow begin to fall. This is why low pressure is associated with storms or bad weather.
The air descending in high pressure flows out in a clockwise spiral in the Northern Hemisphere. Air flowing into an area of low pressure rises, making a counterclockwise rotation as it is spiraling in.
Now when you are watching a TV weather forecast, hopefully NewsChannel 9, you can have a better understanding of why the meteorologist or forecaster puts the big Blue “H’s”(high pressure) and the big Red “L’s”( low pressure) on our weather maps. Air pressure is a main factor in determining our daily weather, so check the barometric pressure to see if it is rising or falling before you head out the door.