Understanding air pressure

Understanding air pressure

The weight of the air pressing down on the Earth, the ocean and on the air below causes air pressure. Earth's gravity, of course, causes the downward force that we know as "weight." Since the pressure depends upon the amount of air above the point where you're measuring the pressure, the pressure decreases as you go higher.

Air pressure is related to its density, which is related to the air's temperature and height above the Earth's surface.

Air pressure changes with the weather. In fact, it's one of the most important factors that determines what the weather is like. You can do some basic weather forecasting by using the wind and barometric pressure.

Air pressure is also called barometric pressure because barometers are used to measure it.

The National Weather Service reports surface air pressure in inches of mercury, while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists generally use hectopascals to measure air pressure.

In the rest of the world, measurements are usually given in hectopascals, although you'll sometimes see them in centimeters of mercury, especially on older barometers.

The term hectopascals is replacing the term millibars. The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large of a unit to be a convenient measure of Earth's air pressure.

Inches of mercury and centimeters of mercury measure how high the pressure pushes the mercury in a barometer.

The use of direct pressure measurements goes back to the late 19th century. This was when the Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather, unlike measures of the height of the mercury in a barometer.

Air pressure and your body

Changes in air pressure, especially rather quick changes, can affect your body. The most obvious of these are the discomfort or even pain you feel in your ears when your gain or lose altitude rather quickly, such as in an aircraft, or even a fast elevator that goes up or down several floors.

Air pressure changes can also be felt in other ways. The links below have more on what happens:

Air pressure corrections

When you read a barometer the reading directly from it is the "station pressure."

Two things affect the barometer's reading, the high or low air pressure caused by weather, and the air pressure caused by the station's elevation, or how high it is above sea level.

No matter what weather systems are doing, the air's pressure decreases with height. If you're trying to draw a weather map of air pressure patterns, you need a way to remove the effects of the station's elevation. That is, you want to see what the pressure would be at the station if it were at sea level. Otherwise, all high-elevation locations would be mapped as having low pressure.

You need to calculate, sea-level pressure, which is defined as: "A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station."

To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature.

Another kind of barometric reading is the altimeter setting, which aircraft use. It's defined as: "The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined." For it, all you need is the station pressure and the elevation, you can ignore the temperature.

How pressure decreases with altitude

As you go higher in the air, the atmospheric pressure decreases.

The exact pressure at a particular altitude depends of weather conditions, but a couple of approximations and a formula can give you a general idea of how pressure decreases with altitude.

A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1,000 foot altitude gain. If you're using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules work quite well for elevations or altitudes of less than two or three thousand feet.

The standard atmosphere is a table giving values of air pressure, temperature and air density for various altitudes from the ground up. You can think of these values as averages for the entire Earth over the course of a year.

Understanding air density and its effects

Understanding air density and its effects

By Jack Williams, USATODAY.com

In simple terms, density is the mass of anything - including air - divided by the volume it occupies.

In the metric system, which scientists use, we usually measure density in terms of kilograms per cubic meter.

The air's density depends on its temperature, its pressure and how much water vapor is in the air. We'll talk about dry air first, which means we'll be concerned only with temperature and pressure.

In addition to a basic discussion of air density, we will also describe the effects of lower air density – such as caused by going to high altitudes – on humans, how humidity affects air density – you might be surprised – and the affects of air density of aircraft, baseballs, and even racing cars.

The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects. The higher the temperature, the faster the molecules are moving. As the air is heated, the molecules speed up, which means they push harder against their surroundings.

If the air is in a balloon, heating it will expand the balloon, cooling it will cause the balloon to shrink as the molecules slow down. If the heated air is surrounded by nothing but air, it will push the surrounding air aside. As a result, the amount of air in a particular "box" decreases when the air is heated if the air is free to escape from the box. In the free atmosphere, the air's density decreases as the air is heated.

Pressure has the opposite effect on air density.

Increasing the pressure increases the density. Think of what happens when you press down the handle of a bicycle pump. The air is compressed. The density increases as pressure increases.

Altitude and weather systems can change the air's pressure. As you go higher, the air's pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 18,000 feet. At 100,000 feet above sea level the air's pressure is only about 10 millibars. Weather systems that bring higher or lower air pressure also affect the air's density, but not nearly as much as altitude.

We see that the air's density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air's density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter's day in Alaska. (Related: Understanding air pressure)

Effects of lower density on humans

If you go high enough, either by climbing a mountain or going up in an airplane that does not have a pressurized cabin, you will begin feeling the effects of lower air pressure and density.

As air pressure decreases oxygen continues to account for about 21% of the gasses in the air as it does at sea level. But, there is less oxygen because there is less of all of the air's gasses. For instance, by the time you go to 12,000 feet the air's pressure is about 40% lower than at sea level. This means that with each breath you are getting about 40% less oxygen than at the lower altitude.

These effects aren't felt in airliners because the cabins are pressurized to keep the air density inside about the same as it would be about 6,000 or 7,000 feet above sea level.

The year 2016 witnessed hectic activity at the Ministry of Environment, Forest and Climate Change – on the international and national fronts.

Ministry of Environment and Forests30-December, 2016 18:14 IST

2016: At A Glance

               While at the international level, India steered the negotiations on amendment in Montreal Protocol for amendment for phase down of HFCs at Kigali, Rwanda, at COP-22 in Morocco, the International Solar Alliance was signed.   On the national front, the Ministry released new categorisation of industries, revamped Waste Management Rules and revised standards for Common Effluent Treatment Plants across industrial clusters.  The Ministry granted general approval for creation of public utility infrastructure in Left-Wing Extremism-affected districts.   ENVIS - a portal was launched to reach out to people and popularise science, as well as sustainable practices.  MoEFCC also continued to make efforts to control pollution.

A snapshot of prominent decisions and events that unfolded in the Ministry of Environment, Forest and Climate Change in 2016 is being given below.  It encapsulates the major happenings of this year: 

 A loan agreement was signed between Government of India and Japan International Cooperation Agency (JICA) for the cleaning of Mula-Mutha river in Pune.  The project will have a significant, direct beneficial impact in terms of reduction of pollution load in the river and improvement of the quality of its water.

 The Ministry notified stricter environment standards for sugar industries operating in various states in the country. The primary aim of these standards is to minimise water pollution. The Ministry notified the standards on January 14, 2016.  The revised standards will lead to improved operational performance of sugar industries through implementation of wastewater discharge standards and waste water conservation and pollution control management protocol. 

 The Environment Ministry held the first National Stakeholder Consultation on the Biodiversity Finance Initiative. The objective of the meeting is to introduce the BIOFIN project to the various stakeholders, enumerate the programmes/activities being undertaken by different organizations in the context of India’s 12 National Biodiversity Targets developed in line with 20 global Aichi biodiversity targets, and then assess the expenditure being made by different organizations for activities related to biodiversity conservation.  The Ministry partnered with the United Nations Development Programme (UNDP) in a global project on Biodiversity Finance Initiative (BIOFIN).

 "Solid Waste Management Rules, Construction Waste Management Rules, Plastic Waste Management Rules, Bio-medical Waste Management Rules and Hazardous Waste Management Rules were revamped."

 The Ministry notified revised standards for Common Effluent Treatment Plants across industrial clusters.   The primary aim of the revised standards is to minimise water pollution.  The revised standards were notified on January 1, 2016.

 MoEF has decentralised the process of granting environment clearance for sustainable sand mining and mining of minor minerals. The Ministry has constituted District Environment Appraisal Committee (DEAC) and District Environment Impact Assessment Authority (DEIAA) for appraisal and approval of mining of minor minerals upto 5 hectares and 25 hectares in case of cluster respectively.

 The Ministry also granted general approval for creation of public utility infrastructure in Left-Wing Extremism-affected districts.  The general approval was granted, keeping in view the importance of creation of public utility infrastructure.

 A portal on Environment Information System (ENVIS) was launched to find out new ways to reach out to people and popularise science, as well as sustainable practices. Environmental Information System (ENVIS) Portal - http://envis.nic.in, a new initiative, runs parallel with the Digital India Objective, which works on improving the digital literacy in the environment sector and deliver services digitally all over the country.  Environmental Information System (ENVIS), a Central Sector Scheme of the Ministry has been implemented since 1982. The purpose of the scheme is to integrate country-wide efforts in environmental information collection, collation, storage, retrieval and dissemination through ENVIS websites, which are dedicated to different interesting themes.  The Ministry also held an exhibition of ENVIS knowledge products at the Ministry premises. 

The ninth meeting of the National Steering Committee on Climate Change (NSCCC) was held. The Committee approved the Detailed Project Reports (DPRs) on adaptation submitted by governments of Telangana, Mizoram, Jammu & Kashmir, Meghalaya and Chhattisgarh for funding, under the National Adaptation Fund on Climate Change (NAFCC) at a total cost of approximately Rs. 108 crore for implementation in these states.

 India became the 56th signatory State to sign the ‘Raptor MoU’ that was concluded on October 22, 2008 and came into effect on November 1, 2008 on conservation of birds of prey in Africa and Eurasia. The ‘Raptor MoU’ extends its coverage to 76 species of birds of prey, out of which 46 species, including vultures, falcons, eagles, owls, hawks, kites, harriers, etc. also occur in India.

 Environment Ministry released new categorisation of industries.  Based on pollution scores, the re-categorisation was finalised as - Red, Orange, Green and White.         

                                                                                                                                                   

24x7 real-time monitoring of pollution from industries done continuously.

 On March 22, 2016, the Ministry notified the new environment standards for Gensets running on Liquid Petroleum Gas (LPG) / Natural Gas (NG), Diesel with LPG / NG and Petrol with LPG / NG operated in various cities / towns in the country. The primary aim of the new standards for Gensets is to control air and noise pollution emanating from the operation of Gensets.

 On March 18, 2016, Government notified Plastic Waste Management Rules, 2016. Earlier, the draft rules, namely the Plastic Waste Management Rules, 2015 were published by the Government of India on 25th May, 2015.   

 On March 23, 2016, the E-Waste Management Rules, 2016 were notified.  For the first time, the Rules brought the producers under Extended Producer Responsibility (EPR), along with targets.  The draft E-Waste (Management) Rules, 2015 had been notified for public consultation vide GSR No. 472 (E) dated 10th June, 2015.

 New Bio-medical Waste Management Rules were notified on March 27, 2016.  The rules mandated bar code system for proper control.

 Construction & Demolition Waste Management Rules notified for the first time.   Under the Rules, duties have been separately earmarked for waste generators, service providers and contractors, state government and local authorities, CPCB and SPCBs and duties of concerned Central ministries. 

Revised Hazardous Waste Management Rules were notified on April 3, 2016 to ensure resource recovery and disposal of hazardous waste in an environment-friendly way.  For the first time, rules made to distinguish between hazardous waste and other wastes.

Solid Waste Management Rules were notified on April 5, 2016 after 16 years. States have also agreed to implement the new Waste Management Rules notified by the Ministry in March-April 2016.

India and 171 other countries signed Paris Agreement at the High-level signing ceremony at UN General Assembly on April 22, 2016.   India reiterated that it would reduce Emission Intensity by 35%.;  Build 40% capacity for Power Generation from non-fossil fuels;  Create additional carbon sink of 2.5 billion tonnes and invest in reducing our vulnerability and enhancing adaptation to Climate Change.

The Prime Minister inaugurated the international conference on rule of law for supporting the 2030 development agenda/sustainable development goals.  The Prime Minister emphasised that sustainable development is our priority. 

Centre extended assistance to the tune of Rs. 800 crore for cleaning up the lakes in Bengaluru.

Union Cabinet gave its approval to move official amendments in Compensatory Afforestation Fund (CAF) Bill, 2015.   Utilization of these amounts will facilitate timely execution of appropriate measures to mitigate impact of diversion of forest land.

Environment Minister chaired fifth meeting on mitigation of air pollution in Delhi and NCR on April 27, 2016.

Government allotted Rs. 5 crore for fighting Uttarakhand forest fires. 

 A web-based application – www.iwms.nic.in was launched for integrated waste management system. The objective of the application is to enhance transparency in the working of the Ministry and other Centre/State level regulatory bodies and also to reduce the time taken for processing applications under the recently notified waste management regulations. 

 Government approved a series of measures for conservation and preservation of lakes in Bengaluru.   Corporate sector also involved in the effort to conserve and preserve the lakes in Bengaluru.

 Environment Ministry to create ‘urban forests’ in 200 cities to increase green cover.   ‘Urban Forestry Scheme’ launched in Pune, to create an ‘urban jungle’ on about 80 acres of land.

 Government sanctioned Rs. 164 Crore for Rajasthan, Rs. 156 Crore for Telangana for Compensatory Afforestation.

 Asia’s first ‘Gyps Vulture Reintroduction Programme’ launched at Pinjore in Haryana. 

 Environment Ministry to sustain cleanliness campaign in National Parks, Wildlife Sanctuaries and Zoos throughout the year to achieve total sanitation and cleanliness by 2nd October 2019.

 Web portal launched for mining of sand and minor minerals.  Tracking of proposals at all stages has been entirely automated. 

 MoU signed between JNU and National Institute of Animal Welfare to conduct courses on development of animal welfare sciences.

  Compensatory Afforestation Fund Bill 2016 passed in Rajya Sabha on July 28, 2016.   The passing of the Bill has ended the long era of ad-hocism and will help the Centre and State Governments to utilise these amounts in a planned manner. It will facilitate make available more than Rs. 6,000 crores per annum to the States/UTs for conservation, protection, improvement and expansion of forest and wildlife resources of the country.

 Khangchendzonga National Park (KNP), Sikkim inscribed as India's First 'Mixed' Site on UNESCO World Heritage List.

 Environment Minister sought a detailed report on elephant deaths, directed authorities to take steps for conservation of elephants.  Ministry initiated steps to prepare a Regional Landscape Plan for the conservation of elephants in the East Central Elephant Landscape. 

 India gave a call for establishing an international clean energy data-grid.

 BRICS nations joined hands to save environment.   Second meeting of BRICS Environment Ministers held in Goa on September 16-17, 2016.  The areas agreed for mutual cooperation are - abatement and control of air and water pollution, efficient management of liquid and solid waste, climate change and conservation of biodiversity.

 Genetic Engineering Appraisal Committee (GEAC) invited comments for the proposal on authorisation of Environmental release of Genetically Engineered Mustard on September 6, 2016.

 Thirteen nations joined hands to discuss Biotechnology, Environmental Risk Assessment and Biosafety Regulation; South Asia Biosafety Conference inaugurated at Hyderabad. 

 Environment Ministry announced major initiative for R&D into next generation HFC refrigerant as alternatives to HFCs.

 Environment Ministry ordered incineration of High GWP HFC-23 on October 13, 2016; India steered the negotiations on amendment in Montreal Protocol for amendment for phase down of HFCs at Kigali, Rwanda.  Shri Anil Madhav Dave gave the go-ahead for releasing the order for incinerating the HFC–23 by producers of HCFC–22 gas.  Environment Minister emphasised relevance of Indian lifestyle and its low carbon footprint at negotiations for phasing down of HFCs in Kigali. 

 India welcomed landmark HFC agreement at Kigali on October 15, 2016.  The Kigali Agreement is a reaffirmation of the global intent to mitigate climate change and exemplifies international co-operation in this regard. The Kigali Amendment to the Montreal Protocol is legally binding and will come into force from January 1, 2019.

 Cabinet approved ratification of Paris Agreement on September 28, 2016. 

 Environment Minister constituted monitoring committee to oversee outbreak of H5 avian influenza.

 Environment Ministry reviewed ambient air quality status of Delhi and directed neighbouring states to effectively enforce ban on stubble burning on October 31, 2016. 

 Environment Ministry constituted four-member team for conservation and management of Loktak Lake.   The Committee visited Loktak Lake and submitted a report to the Ministry. 

 On November 4, 2016, Environment Ministry held a meeting with NCR states on measures to control air pollution.   Chairman, CPCB to review the progress of action taken on measures to control pollution with the concerned State Governments/NCT of Delhi and SPCBs/PCC on a monthly basis.Ministry to undertake such a review on a quarterly basis.

 On November 30, 2016, Cabinet approved the negotiating position adopted by the Government at the Meeting of Parties to the Montreal Protocol of the Vienna Convention for Protection of Ozone Layer held at Kigali, Rwanda.

 Paris Climate Agreement came into force on November 4, 2016.  Environment Ministry Building displayed ‘Smiley’ With Lights, with Slogan – “We Did It”.  

 India participated in COP-22 meeting in Marrakech, Morocco.  Signing ceremony of Framework Agreement on International Solar Alliance took place at COP-22.   COP-22 concluded on November 20, 2016.   

 On December 6, 2016, Environment Ministry issued the final notification on Eco-Sensitive Zone in Sanjay Gandhi National Park, Mumbai.  The Eco-Sensitive Zone has a minimum extent of 100 metres and maximum extent of up to 4 km from the Park boundary.   The objective of notifying Eco-Sensitive Zones is to create a buffer as further protection around Protected Areas (PAs) such as National Parks and Wildlife sanctuaries.   The Notification also provides that in areas around Housing Societies and with high human habitation, and in view of the fact that the National Park has a large population of leopards, a high wall with fencing may be erected to ensure that man-animal conflict is avoided.  Draft notification S.O (229 (E) was published on 22.01.2016 around the Park as Eco-Sensitive Zone from ecological and environmental point of view and for regulating development around the National Park.

 Environment Minister visited forestry institutes in Dehradun, Uttarakhand and exhorted forest officers to be sincere and committed towards the forestry sector.

 India participated in the United Nations Biodiversity Conference held from December 2-17, 2016 at Cancun, Mexico placed India's country position during discussion on substantial, financial and operational issues at Plenary and Working Groups. Indian delegation also participated in several side meetings, organized 11 side events and participated in CEPA fair for showcasing India's achievements and activities in different thematic and technical areas related to biodiversity and biosafety.

 EIA Notification, 2006 on integration of environmental conditions with building permissions amended.   The notification will come into effect, if States do make the required changes in their building bye-laws.  These changes have to be done in consultation with the Environment Ministry. 

 Centre announced notification of Prevention of Cruelty to Animals (Pet Shops) Rules, 2016.  The objective of these rules is to make pet shops accountable and to prevent cruelty inflicted on animals kept in such pet shops.

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HK

(Release ID :156016)