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What is atmospheric pressure? What is the average air pressure at the surface of the Earth? What is barometric pressure? How is it affected by weather and altitude? Cabin air pressure Your Vehicle’s Tire Pressure Air pressure is the weight of the Earth's atmosphere pressing down on everything on the surface. 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! Higher Altitudes and Outer Space At 31 km or about 19 miles above the earth's surface, in the stratosphere, the air pressure is only 1/100th what it is at sea level. From this level on, the atmosphere quickly deteriorates into nothingness. Above 100 km or just over 62 miles above the surface, the international definition for outer space, the pressure approaches zero and nearly becomes a vacuum. Humans cannot exist unprotected in such a low-pressure environment. Other Uses for Pressure Air pressure is used in a number of _______ applications and consumer products. Pressurized air can be utilized in hydraulic machinery that uses the expansion of air to move different parts. Aerosol canisters use pressurized air to make different chemicals spray out, as the high pressure naturally rushes out into the lower atmospheric pressure when used. Basketballs and similar objects, like tires, are inflated so that they bounce or support other objects. Under-inflation can leave them weakened or less effective, while excess pressure can cause them to burst or more easily rupture during use. Changes in Pressure and Weather Atmospheric pressure varies slightly over the earth's surface, and variations in pressure are responsible for various types of weather. Low pressure systems are associated with storms, tornadoes, and hurricanes. Sometimes the air pressure at sea level can drop as low as 870 millibars, which is about 85% of average air pressure. This only happens during the most severe storms. Pressure variations on the earth's surface cause wind: as high pressure air moves toward low pressure areas, creating gusts. What Happens if Air Pressure Changes? 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. Boiling point of water Clean fresh water boils at about 100 °C (212 °F) at standard atmospheric pressure. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water.[14] Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. This is why cooking at elevations more than 1,100 m (3,600 ft) above sea level requires adjustments to recipes. What is the average air pressure at the surface of the Earth? The standard, or near-average, atmospheric pressure at sea level on the Earth is 1013.25 millibars, or about 14.7 pounds per square inch. The gauge pressure in my automobile tires is a little more than twice that value. If you live at a higher altitude, the pressure will be lower since there is less air above weighing down upon you. The pressure also varies by relatively small amounts as high and low pressure systems move past. Definition of atmospheric pressure:Equals 14.70 pounds per square inch (PSI), 101330 pascals (newtons per square meter), 29.92 inches (760 millimeters) of mercury, 1013.25 millibars (mb), and 1.03 technical atmosphere. Also called barometric pressure. Cabin air pressure Although aircraft cabins are pressurized, cabin air pressure at cruising altitude is lower than air pressure at sea level. At typical cruising altitudes in the range 11 000–12 200 m (36 000–40 000 feet), air pressure in the cabin is equivalent to the outside air pressure at 1800–2400 m (6000–8000 feet) above sea level. As a consequence, less oxygen is taken up by the blood (hypoxia) and gases within the body expand. The effects of reduced cabin air pressure are usually well tolerated by healthy passengers. Oxygen and hypoxia Cabin air contains ample oxygen for healthy passengers and crew. However, because cabin air pressure is relatively low, the amount of oxygen carried in the blood is reduced compared with that at sea level. Passengers with certain medical conditions, particularly heart and lung diseases and blood disorders such as anaemia (in particular sickle-cell anaemia), may not tolerate this reduced oxygen level (hypoxia) very well. Some of these passengers are able to travel safely if arrangements are made with the airline for the provision of an additional oxygen supply during flight. However, because regulations and practices differ from country to country and between airlines, it is strongly recommended that these travellers, especially those wishing to carry their own oxygen, contact the airline early in their travel plans. An additional charge is often levied on passengers who require supplemental oxygen to be provided by the airline. Gas expansion As the aircraft climbs in altitude after take-off, the decreasing cabin air pressure causes gases to expand. Similarly, as the aircraft descends in altitude before landing, the increasing pressure in the cabin causes gases to contract. These changes may have effects where air is trapped in the body. Passengers often experience a “popping” sensation in the ears caused by air escaping from the middle ear and the sinuses during the aircraft’s climb. This is not usually considered a problem. As the aircraft descends in altitude prior to landing, air must flow back into the middle ear and sinuses in order to equalize pressure. If this does not happen, the ears or sinuses may feel as if they are blocked and pain can result. Swallowing, chewing or yawning (“clearing the ears”) will usually relieve any discomfort. As soon as it is recognized that the problem will not resolve itself using these methods, a short forceful expiration against a pinched nose and closed mouth (Valsalva manoeuvre) should be tried and will usually help. For infants, feeding or giving a pacifier (dummy) to stimulate swallowing may reduce the symptoms. Individuals with ear, nose and sinus infections should avoid flying because pain and injury may result from the inability to equalize pressure differences. If travel cannot be avoided, the use of decongestant nasal drops shortly before the flight and again before descent may be helpful. As the aircraft climbs, expansion of gas in the abdomen can cause discomfort, although this is usually mild. Some forms of surgery (e.g. abdominal surgery) and other medical treatments or tests (e.g. treatment for a detached retina) may introduce air or other gases into a body cavity. Travellers who have recently undergone such procedures should ask a travel medicine physician or their treating physician how long they should wait before undertaking air travel. What is barometric pressure? How is it affected by weather and altitude? AnswerBarometric pressure is air or atmospheric pressure as measured by a barometer. Because gases in the earth’s atmosphere have mass, they have weight and exert pressure on things inside the atmosphere. The entire column of air above a place has a weight and creates pressure on the surface corresponding to this weight. The atmosphere is approximately 100 miles (160 km) thick and ranges from near vacuum at the top to a pressure defined as one atmosphere (or about one bar) at sea level. One atmosphere is 14.7 lbs per square inch of pressure (101 kilopascals in metric units). Many early barometers used the weight of a counterbalanced column of liquid mercury metal to measure the pressure. In these terms, the pressure at sea level is 29.9 in of mercury. This is equal to 760 mm, as millimeters of mercury is another widely used measurement for atmospheric pressure. A millibar is often used in weather reports and forecasts. It is 1/1000 of a bar, which is approximately one atmosphere of pressure. Instead of mercury, many modern barometers use other, more convenient and safer mechanical (aneroid) or electronic means to measure barometric pressure. Temperature and humidity can affect barometric readings because they affect the density of the air. Air density affects the weight of a given volume of air and thus the air pressure. The weight of air above a place also varies because of atmospheric circulation. A weather high is an region where air pressure is higher than normal, often corresponding to clear, dry conditions. A low is a region of lessened air pressure often meaning storms. If barometric pressure is changing to lower pressure, a storm system or front may be near. Changes to higher pressures may mean improving weather. This is how barometric pressure is related to weather and weather forecasts. As noted above, air pressure changes with altitude, so a barometer can be used to measure altitudes as well. The highest pressure is at sea level, while at 18,000 ft the air pressure is only half as much, at 27,500 ft only a third as much, at 53,000 ft only a tenth as much pressure, and so on. Of course there are inaccuracies in using barometers to determine altitude due to the weather and air density, which also affect air pressure. Your Vehicle’s Tire Pressure What Is The Correct Pressure? The first place to look for the correct air pressure for your specific tires is your vehicle manual. Correct air pressure should also be listed on the tire placard attached to the vehicle door edge, doorpost, glove box door or fuel door. You may use the tool above to view the manufacturer recommended inflations for standard sized tires on your vehicle. Under (Cabin) Pressure Sure, humans evolved to thrive in Earth's atmosphere, but it's important to realize that we only evolved to thrive in a thin layer of the planet's gaseous outer layer. Air pressure changes depending on altitude. In the same way that the water pressure in the ocean is greater on the seafloor than it is just below the surface, air pressure decreases the higher you ascend through the atmosphere. When humans breathe thinner, high-altitude air, they have a harder time taking in enough oxygen. And when we hang out at heights higher than 9,800 feet (3,000 meters), our bodies become susceptible to a host of unpleasant or even deadly illnesses, like these: Altitude sickness: Also the bane of high-altitude mountain climbers, reduced air pressure and lower oxygen concentration levels can cause extreme shortness of breath due to fluid buildup in the lungs. In extreme cases, this can lead to brain swelling, resulting in confusion, coma or death. Ear barotrauma: The Eustachian tube connects your middle ear to the outside world. If this tube becomes blocked, changes in atmospheric pressure can cause a pressure differential that can result in dizziness, discomfort, hearing loss, ear pain and nose bleeds. Decompression sickness: Divers know this condition as the bends, and it can occur in the air, as well as in the water. Exposure to low barometric pressures can cause dissolved nitrogen in the blood stream to form harmful bubbles that can cause everything from drowsiness to stroke. Hypoxia: As low pressure means less oxygen in every breath you breathe, the brain receives less oxygen at high altitudes. The physiological results often include cognitive impairment or light-headedness, which can seriously impair a pilot's ability to fly the plane. Pressurized cabins enable pilots, crew and passengers to avoid these pitfalls of flying at high altitude. While the air outside the cabin thins out the higher a plane climbs, compressed air inside the cabin maintains more surface-level air pressure and oxygen-rich air. In the event of accidental loss of cabin pressure, emergency oxygen masks provide the necessary air quality. Pressurized flight suits achieve the same effect as pressurized cabins, only on an individual basis. Characterized by enclosed helmets, these suits typically see use in military and high-performance aircraft. Cabin Air Systems Here are further guidelines. Pressure What is pressure? |