Air pressure
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1) A planet's atmosphere is determined by its gravity - large planets with strong gravity can hold thicker atmospheres than low-mass planets with weak gravity. 2) Atmospheric pressure depends on the thickness of the atmosphere, which is influenced by a planet's gravity. Stronger gravity holds more gas and increases atmospheric pressure. 3) On Earth, atmospheric pressure is measured using barometers like mercury barometers invented by Torricelli, and aneroid barometers which indicate pressure changes using mechanical means without toxic materials. Atmospheric pressure decreases with increasing elevation and a falling barometer often signals approaching low pressure weather systems.
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Air pressure
- 1. Submitted To, Smt Linimol KS (lecture and my teacher in physical science) SNTC Sreekandewaram,poochakkal p.o., Cherthala - 688526 Submitted By, Syamaprasad.S physical science Rg.no:1821438302
- 2. • Atmosphere is kept by the planet’s gravity – Low mass (small) planets= low gravity =almost no atm. – High mass (large) planets = high gravity = thick atm. • Gravity and pressure – Air pressure depends on how much gas there is i.e. The atmospheric thickness.
- 3. Gravity and Atmospheric Pressure • The stronger the gravity, the more gas is held by the planet and also increases the weight of atmosphere on unit area
- 4. Earth’s Atmosphere • About 10 km thick • Consists mostly of molecular nitrogen (N2) and oxygen (O2)
- 5. The air is made up of molecules.
- 6. Gravity pulls the air moleculesGravity pulls the air molecules toward the earth, giving themtoward the earth, giving them weight. The weight of the airweight. The weight of the air molecules all around us ismolecules all around us is called the air pressure.called the air pressure.
- 7. High altitudes = lower pressureHigh altitudes = lower pressure Low altitudes = higher pressureLow altitudes = higher pressure
- 8. Atmospheric Pressure Gas pressureGas pressure depends on bothdepends on both density anddensity and temperature.temperature. Adding airAdding air moleculesmolecules increases theincreases the pressure in apressure in a balloon.balloon. Heating the airHeating the air also increasesalso increases the pressure.the pressure.
- 9. Air pressure isAir pressure is equal in allequal in all directions.directions. Pressure = force per unit areaPressure = force per unit area
- 10. AsAs elevationelevation goes upgoes up BarometricBarometric pressurepressure goesgoes down.down. This is an inverse relationship.This is an inverse relationship.
- 12. In 1643, Evangelista TorricelliIn 1643, Evangelista Torricelli invented the barometerinvented the barometer
- 13. Torricelli’s barometerTorricelli’s barometer used a glass columnused a glass column suspended in a bowl ofsuspended in a bowl of mercury. The pressuremercury. The pressure of the air moleculesof the air molecules pushed the mercury uppushed the mercury up into the glass tube.into the glass tube. The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.
- 14. AsAs atmosphericatmospheric pressurepressure increases…increases… The mercury inThe mercury in the tube rises.the tube rises. Sea level pressure used as a reference point
- 15. The Mercury BarometerThe Mercury Barometer Good:Good: Bad:Bad: •Simple to constructSimple to construct •Highly accurateHighly accurate •Glass tube is fragileGlass tube is fragile •Mercury isMercury is veryvery toxictoxic!!
- 16. TheThe AneroidAneroid BarometerBarometer •No fragile tubes!No fragile tubes! •No toxic chemicals!No toxic chemicals! •No batteries!No batteries! •Never needs winding!Never needs winding!
- 17. Most aneroidMost aneroid barometers have abarometers have a needle which can beneedle which can be set to remember theset to remember the previous reading.previous reading.
- 18. An aneroid barometerAn aneroid barometer uses a cell which hasuses a cell which has had most of the airhad most of the air removed.removed. As the air pressureAs the air pressure around the cellaround the cell increases, it pressesincreases, it presses on the cell, whichon the cell, which causes the needle tocauses the needle to move.move. Television weather forecasters usually give barometricTelevision weather forecasters usually give barometric pressure in inches of mercury. However, meteorologistspressure in inches of mercury. However, meteorologists measure atmospheric pressure inmeasure atmospheric pressure in millibars.millibars. MILLIBARS
- 22. Changing PressureChanging Pressure AA falling barometerfalling barometer = decreasing air pressure.= decreasing air pressure. This usually means:This usually means: Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.
Editor's Notes
- Your weight is the result of gravity pulling your mass down on the bathroom scales. Note that weight has units of a force, such as pounds.
- Air pressure can be thought of as the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up.
- Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.
- Torricelli didn’t actually build a barometer, but he gave detailed instructions on how to build one, so he is given credit for the invention. He was actually trying to prove the existence of a vacuum. Many scientists in his day didn’t believe that a vacuum could exist, hence the phrase, “nature abhors a vacuum.”
- The abbreviation “Hg” is the chemical symbol for mercury. Some kinds of pressure reading instruments, including some barometers, use the abbreviation “mmHg,” meaning “millimeters of mercury.” 760 mmHg is considered the standard “normal” atmospheric pressure at sea level. This unit is called a “torr,” after Torricelli. To construct a mercury barometer, fill a tube with a liquid. Invert then tube in a dish of liquid holding your thumb over the top of the tube until the the tube is immersed in the bowl of liquid, the atmospheric pressure will keep the liquid in the tube from emptying such that the weight of the liquid in the tube equalize with the atmospheric pressures. (Do not do this with mercury because of its toxicity) Mercury was used because it is a very heavy liquid, so the tube could be relatively short. The tube in a mercury barometer still has to be over a meter long. Students may want to try building a barometer using colored water. How high would the tube need to be? Merucy is about 11 times more dense than water. What if they used milk or some other liquid, would the height be the same?
- Although mercury has been used for hundreds of years, its toxic effects have only been fully realized in the last few decades. Students should NEVER handle mercury or broken mercury thermometers or barometers. Mercury should also never be thrown in the trash or washed down the drain, since it moves easily up the food chain from fish to humans. A local health department or environmental professional can assist with disposal of old or broken mercury instruments. GLOBE instrument specifications call for organic-fluid (non-mercury) or digital thermometers, except for the analog min/max thermometer, which is mounted in a shelter and is not handled by the students.
- Knowing how the air pressure is changing is as important as knowing the actual barometric reading. The set needle allows students to compare the current reading to the previous one. If the current reading is less than the previous one, the barometric pressure is falling. If the current reading is more than the previous one, the pressure is rising. If it is the same as the previous reading, the pressure is said to be steady. Weather forecasters often use the phrases “falling barometer,” “rising barometer,” or “steady barometer” as a way of referring to the change in atmospheric pressure.
- The word “aneroid” means “no air,” and refers to the partial vacuum inside the cell. The aneroid cell is shaped like a bellows, so that it can flex as air pressure changes. Increasing air pressure compresses the cell, causing the needle to register a change. Decreasing pressure allows the cell to expand, causing the needle to move in the opposite direction. The use of inches of mercury is a hold-over from the days of mercury barometers. It refers to the actual height of mercury in the glass tube. Millibars are metric system units, and as such are readily understood by scientists around the world.
- As we have noted earlier, higher elevations have fewer air molecules pressing downward, and so atmospheric pressure is lower. This means a barometer will read lower as it is carried to a higher elevation. Airplanes use a special type of barometer, called an altimeter, to measure altitude.
- Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.
- Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.