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.
When it's compressed, air is said to be "under high pressure". Air at sea level is what we're used to, in fact, we're so used to it 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.
How much pressure are you under? Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!
Why doesn't all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.
Run for Cover! 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.
Control the weather! Use a special JavaScript barometer designed to demonstrate how air pressure and weather conditions are linked. You will also find instructions on how to build your own barometer!
Why do my ears pop? If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. 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.
Do you think a decrease in temperature could be explained in terms of air pressure? How?
.jpg)
Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable.
Air pressure can be increased (or decreased) one of two ways.
- First, simply adding molecules to any particular container will increase the pressure. A larger number of molecules in any particular container will increase the number of collisions with the container's boundary which is observed as an increase in pressure.
- A good example of this is adding (or subtracting) air in an automobile tire. By adding air, the number of molecules increase as well a the total of the collisions with the tire's inner boundary. The increased number of collisions forces the tire to expand and pressure increase.
- The second way of increasing (or decreasing) is by the addition (or subtraction) of heat. Adding heat to any particular container can transfer energy to air molecules. The molecules therefore move with increased velocity striking the container's boundary with greater force and is observed as an increase in pressure.
Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.
In the International Space Station, the density of the air is maintained so that it is similar to the density at the earth's surface. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch).
Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, meaning a decrease in air pressure. In fact, while the atmosphere extends more than 15 miles (24 km) up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).
Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we need to convert the station air pressures reading to a value with a common denominator.
The common denominator we use is the sea-level. At observation stations around the world, through a series of calculations, the air pressure reading, regardless of the station elevation, is converted to a value that would be observed if that instrument were located at sea level.
The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars".
- Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source.
- At sea level, standard air pressure in inches of mercury is 29.92.
- Millibars comes from the original term for pressure "bar". Bar is from the Greek "báros" meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.
Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.
- Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.
The Pascal
The scientific unit of pressure is the Pascal (Pa) named after after Blaise Pascal (1623-1662). One pascal equals 0.01 millibar or 0.00001 bar. Meteorology has used the millibar for air pressure since 1929.
When the change to scientific unit occurred in the 1960's many meteorologists preferred to keep using the magnitude they are used to and use a prefix "hecto" (h), meaning 100.
Therefore, 1 hectopascal (hPa) equals 100 Pa which equals 1 millibar. 100,000 Pa equals 1000 hPa which equals 1000 millibars. The end result is although the units we refer to in meteorology may be different, their numerical value remains the same. For example the standard pressure at sea-level is 1013.25 millibars and 1013.25 hPa.
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.
How are changes in weather related to changes in pressure?
The FALL of the barometer (decreasing pressure)
- In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind.
- In frosty weather, the fall of the barometer denotes thaw.
- If wet weather happens soon after the fall of the barometer, expect but little of it.
- In wet weather if the barometer falls expect much wet.
- In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.
- The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind).
The RISE of the barometer (increasing pressure)
- In winter, the rise of the barometer presages frost.
- In frosty weather, the rise of the barometer presages snow.
- If fair weather happens soon after the rise of the barometer, expect but little of it.
- In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.
- In wet weather, if the mercury rises suddenly very high, fine weather will not last long.
- The barometer rises highest of all for north and east winds; for all other winds it sinks.
The barometer UNSETTLED (unsteady pressure)
- If the motion of the mercury be unsettled, expect unsettled weather.
- If it stands at "MUCH RAIN" and rises to "CHANGEABLE" expect fair weather of short continuance.
- If it stands at "FAIR" and falls to "CHANGEABLE", expect foul weather.
- Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather.
These pressure observations hold true for many other locations as well but not all of them. Storms that occur in England, located near the end of the Gulf Stream, bring large pressure changes. In the United States, the largest pressure changes associated with storms will generally occur in Alaska and northern half of the continental U.S. In the tropics, except for tropical cyclones, there is very little day-to-day pressure change and none of the rules apply.
Why doesn't all that pressure squash me?
- Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.
The pressure underwater works the same as for atmospheric pressure. The pressure exerted underwater depends on the amount of water overhead. The density of water is about 1 gram per cubic centimeter, which is around 800 times that of air so that the pressure underwater changes much more quickly with depth than the atmospheric pressure changes with height. By a depth of 32.8 feet (around 10 meters), the water above the fish leads to a pressure of 1 atmosphere or 101,000 Pascals (101 kPa). At this depth, however, this is not the total pressure because we excluded the pressure exerted by the atmosphere. The total pressure at this depth is that due to the water, 1 atm, and that due to the atmosphere, 1 atm, for a total pressure of 2 atm, twice that at the surface of the ocean where the pressure felt is dominated by the atmospheric pressure.Water is nearly incompressible (cannot squeeze or make it expand easily [by changing the pressure] and so water's density does not change with depth). This means that for every change of 32.8 feet (or ~10 meters) in depth, the pressure increases by 1 atm (101 kPa).This is remarkable. The rate at which the pressure increases as one descends in the ocean, does not depend on the depth.
At a depth of 1,000 meters then, the pressure a fish feels is around 100 times the pressure that it feels at the surface of the ocean. The pressure at 1,000 meters is huge, it corresponds to a pressure of around 1,470 pounds per square inch squeezing the fish.
REVIEW:
Air Pressure helps forecastersNot only do differences in air pressure help determine wind speed and direction, they help forecast precipitation and clear weather. High pressure indicates sinking air where clouds cannot form. Low pressure indicates rising air, which allows clouds to form, bringing rain or snow.
Air Flows From High Pressure to Low Pressure
Rising and Sinking Air
Since warm air is less dense and creates less air pressure, it will rise; cold air is denser and creates greater air pressure, and so it will sink. When warm air rises, cooler air will often move in to replace it, so wind often moves from areas where it's colder to areas where it's warmer. The greater the difference between the high and low pressure or the shorter the distance between the high and low pressure areas, the faster the wind will blow. Wind also blows faster if there's nothing in its way, so winds are usually stronger over oceans or flat ground. Meteorologists can forecast the speed and direction of wind by measuring air pressure with a barometer.
| 1. | If you were on a mountain, would the weight of the air above you (air pressure) be greater than or less than it is now? |
| 2. | What do you think causes wind? |
| 3. | If you lived in Kansas and observed a sudden drop in air pressure, what kind of weather would you expect to see soon? |
| 4. | Why do hot air balloons rise? |
| 5. | Air weighs less than water, would you expect the pressure exerted by water to be greater or less than the pressure exerted by the same amount of air? |
BALLOON - AIR PRESSURE EXPERIMENT
- take notes with all senses
- give an explanation for the results of the experiment
