Atmospheric Pressure –
Atmospheric pressure is the force exerted on a unit of surface in the Earth’s atmosphere by the weight of the air above it. In most circumstances, atmospheric pressure is approximately accurately estimated by the weight of the air above the measurement point. Areas of low pressure have a less atmospheric mass above those locations, while areas of high pressure have more atmospheric mass above those locations.
Similarly, as altitude increases, there is less atmospheric mass above that level, so pressure decreases with increasing altitude. A column of air with a cross-section of one square inch from sea level to the top of the atmosphere weighs 6.3 kilograms (and a column of air with a cross-section of one square centimetre weighs a little over a kilogram).
Standard atmospheric pressure The standard atmosphere (symbol atm) is a unit of pressure and is defined as equal to 101,325 Pa or 101.325 a. The following units are equivalent, but only up to the decimal places shown: 760 mmHg (torr), 29.92 inHg, 14.696 psi, 1013.25 millibars. A standard atmosphere is the standard pressure used in pneumatic fluid power (ISO R554), aerospace technology (ISO 2533) and the petroleum industry (ISO 5024).
In 1999, the International Union of Pure and Applied Chemistry (UPAC) stated that the standard pressure for the purpose of specifying the properties of substances should be defined as 100 kPa (≈750.01 kPa) instead of the value of 101.325 kPa for “one standard atmosphere”. Torr) or 29.53 inches. This value is used as the standard pressure for the compressor and pneumatic tool industry (ISO 2787). (Also see Standard temperature and pressure) In the United States, compressed air flow is often measured in “standard cubic feet” per unit of time, where “standard” refers to the equivalent amount of moisture at standard temperature and pressure. Atmospheric pressure decreases by 4% for every 1,000 feet you climb.
However, this standard atmosphere is defined slightly differently: temperature = 20 °C. (68 °F), air density = 1.225 / kg/m³ (0.0765 lb / cu ft), altitude = sea level and relative humidity = 20%. Standard in the air conditioning industry instead of which temperature is often = 0 °C. (32 °F). For natural gas, the Gas Processors Association (GPA) specifies a standard temperature of 60 °F (15.6 °C), but varied, including 14.65 pounds per square inch (101.0 kPa), 14.656 pounds per square inch (101.05 kPa), 14.73 pounds per square inch (101.6 kPa) and 15.025 pounds per square inch (103.59 kPa). A variety of “base” temperatures may also be used.
Mean Sea Level Pressure –
Mean sea level pressure (MSLP) is the pressure at sea level or (when measured at a given height over the ground) the centre pressure reduced to sea level, assuming an isothermal layer at the centre temperature. There is pressure.
It Is this pressure that is commonly given in weather reports on radio, television, newspapers and the Internet. When home barometers are set to match local weather reports, they measure sea level pressure, not the actual local atmospheric pressure. See Altimeter (barometer vs. absolute).
The altimeter adjustment In the aviation industry set QNH or QFE, is another atmospheric pressure reduction to sea level, but the method of marking this reduction is somewhat different.
QNH (QNH)
Due to barometric altimeter adjustments, the altimeter will read altitude when it is in the airspace. Under ISA temperature conditions the altimeter will read the height above mean sea level in the surrounding airspace.
QFE
Barometric altimeter adjustments will cause the altimeter to read zero at the reference base (usually the threshold of an airstrip) of a specific airspace. Under ISA temperature conditions the altimeter will read the height from a base in the surrounding airspace.
Variation in atmospheric pressure with altitude –
Pressure changes smoothly from the Earth’s surface to the top of the mesosphere. Although pressure changes with the seasons, NASA has averaged conditions for all parts of Earth throughout the year. The following is a list of air pressure (as a fraction of one atmosphere) with the associated average altitude. The table gives a rough estimate of the air pressure at various altitudes.
Variation in local atmospheric pressure –
Atmospheric pressure varies widely over Earth and these changes are important for the study of weather and climate. For the effects of changes in air pressure on weather, see the Pressure system.
Atmospheric pressure refers to a diurnal or semi-diurnal (twice-a-day) cycle caused by global atmospheric currents. This effect is strongest in tropical regions with amplitudes of a few millibars and almost zero in polar regions. These changes have two superimposed cycles, a diurnal (24-hour) cycle and a semi-diurnal (12-hour) cycle.
Atmospheric pressure records –
The highest barometric pressure ever recorded on Earth was 1,085.7 hectopascals (32.06 inches of mercury) at Tonsontsengel, Mongolia, on 19 December 2001. The lowest non-illusory atmospheric pressure ever measured was 870 hPa (25.69 inHg) on October 12, 1979, during a storm in the western Pacific Ocean. This measurement was based on instrumental observations taken by a reconnaissance aircraft. The lowest atmospheric pressure ever measured at Earth’s surface, adjusted for sea level, was 850 hPa (25.10 in), recorded during an F4 hurricane on June 24, 2003, in Manchester, South Dakota. This measurement was recorded using a native rod.
Atmospheric pressure based on the height of water –
Atmospheric pressure is often measured with a mercury barometer and a height of about 760 millimetres (30 in) of mercury is used to represent (and measure) atmospheric pressure. Since mercury is not a substance that humans commonly come into contact with, water often provides a more intuitive way of estimating the pressure of one atmosphere.
One atmosphere (100 kPa or 14.7 psi) is the amount of pressure that can lift water approximately 10.3 m (34 ft). Thus, a diver experiences a pressure of about 2 atmospheres (1 atm of air and 1 atm of water) 10.3 meters below water. This is the maximum height to which a column of water can be lifted by suction.
The boiling point of water –
Water is about 100 °C. (212 °F) boils at standard atmospheric pressure. The boiling point is the temperature at which the vapour and atmospheric pressure around water equalise. Because of this, the boiling point of water decreases at low pressure and increases at high pressure. This is why cooking at altitudes above 3,500 feet (1,100 m) above sea level requires adjustments in recipes. A rough estimate of altitude can be obtained by measuring the temperature at which water boils; This method was used by explorers in the middle of the 19th century.
By Chanchal Sailani | January 04, 2023, | Editor at Gurugrah_Blogs.
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