The Detroit Post
Friday, 26 February, 2021

How Is Weather Pressure

Maria Johnson
• Saturday, 21 November, 2020
• 9 min read

An important characteristic of the Earth's atmosphere is its air pressure, which determines wind and weather patterns across the globe. This gravitational force causes the atmosphere to push against everything it surrounds, the pressure rising and falling as Earth turns.

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One atmosphere is equal to 1,013 millibars (MB) at sea level, which translates into 760 millimeters of quicksilver when measured on a mercury barometer. The highest barometric pressure on record was 1,083.8 MB (adjusted to sea level), measured in Agatha, Siberia, on December 31, 1968.The lowest pressure ever measured was 870 MB, recorded as Typhoon Tip struck the western Pacific Ocean on October 12, 1979.

Under these conditions, lows normally produce clouds, precipitation, and other turbulent weather, such as tropical storms and cyclones. Areas prone to low pressure do not have extreme diurnal (day versus night) nor extreme seasonal temperatures because the clouds present over such areas reflect incoming solar radiation back into the atmosphere.

As a result, they cannot warm as much during the day (or in the summer), and at night, they act as a blanket, trapping heat below. High- pressure areas are normally caused by a phenomenon called subsidence, meaning that as the air in the highs cools, it becomes denser and moves toward the ground.

Subsidence also evaporates most of the atmosphere's water vapor, so high- pressure systems are usually associated with clear skies and calm weather. Unlike areas of low pressure, the absence of clouds means that areas prone to high- pressure experience extremes in diurnal and seasonal temperatures since there are no clouds to block incoming solar radiation or trap outgoing long wave radiation at night.

Because the converging air is wet and full of excess energy, it expands and cools as it rises, creating the clouds and heavy rainfall that are prominent throughout the area. Sub polar low- pressure cells: This area is at 60 degrees north/south latitude and features cool, wet weather.

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In the Northern Hemisphere, their meeting forms the polar front, which produces the low- pressure cyclonic storms responsible for precipitation in the Pacific Northwest and much of Europe. In the Southern Hemisphere, severe storms develop along these fronts and cause high winds and snowfall in Antarctica.

With these systems, winds move away from the poles in an anticyclone, which descends and diverges to form the polar easterlies. By studying these highs and lows, scientists are better able to understand the Earth's circulation patterns and predict the weather for use in daily life, navigation, shipping, and other important activities, making air pressure an important component to meteorology and other atmospheric science.

The standard atmosphere (symbol: ATM) is a unit of pressure defined as 101,325 Pa (1,013.25 HPA ; 1,013.25 bar), which is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The ATM unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 ATM.

On average, a column of air with a cross-sectional area of 1 square centimeter (cm 2), measured from mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or “weight” of about 10.1 newtons, resulting in a pressure of 10.1 N/cm 2 or 101/m 2 (101 kilopascals, PA). A column of air with a cross-sectional area of 1in 2 would have a weight of about 14.7lb f, resulting in a pressure of 14.7lb f /in 2.

Map showing atmospheric pressure in bar or hPa15-year average mean sea-level pressure for June, July, and August (top) and December, January, and February (bottom). This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet.

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When barometers in the home are set to match the local weather reports, they display pressure adjusted to sea level, not the actual local atmospheric pressure. Average sea-level pressure is 1013.25 bar (101.325 PA; 29.921 ING; 760.00 MMH).

In aviation, weather reports (Metal), QNH is transmitted around the world in millibars or hectopascals (1 hectopascal = 1 millibar), except in the United States, Canada, and Colombia where it is reported in inches of mercury (to two decimal places). The United States and Canada also report sea-level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars.

The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1050 bar (105 PA; 31 ING), with record highs close to 1085 bar (108.5 PA; 32.0 ING). The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 870 bar (87 PA; 26 ING).

It is directly proportional to the mass of air over that location. Pressure (p), mass (m), and the acceleration due to gravity (g), are related by P = F/A = (m×g)/A, where A is surface area.

Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round.

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Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right above was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about 1.2 PA (12 HPA) for every 100 meters. ParameterDescriptionValue p 0 Sea level standard atmospheric pressure 101325Pa L Temperature lapse rate, = g / c p for dry are 0.00976K/m c p Constant- pressure specific heat1004.68506J/(kg·K) T 0 Sea level standard temperature288.16K g Earth-surface gravitational acceleration9.80665 m/s 2 M Molar mass of dry air0.02896968 kg/mol R 0 Universal gas constant 8.314462618J/(MOL·K) Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate.

This effect is strongest in tropical zones, with an amplitude of a few millibars, and almost zero in polar areas. The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1084.8 HPA (32.03 ING) measured in Tosontsengel, Mongolia on 19 December 2001.

The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agatha in Even Autonomous Drug, Russia (66°53'N, 93°28'E, elevation: 261 m, 856 ft) on 31 December 1968 of 1083.8 HPA (32.005 ING). The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.

A below-sea-level surface pressure record of 1081.8 HPA (31.95 ING) was set on 21 February 1961. The lowest non-tornadic atmospheric pressure ever measured was 870 HPA (0.858 ATM; 25.69 ING), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean.

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The measurement was based on an instrumental observation made from a reconnaissance aircraft. One atmosphere (101.325 PA or 14.7 psi) is also the pressure caused by the weight of a column of fresh water of approximately 10.3 m (33.8 ft).

Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions. A typical gas-using residential appliance in the US is rated for a maximum of 1/2 psi, which is approximately 14 w.g.

Similar metric units with a wide variety of names and notation based on millimeters, centimeters or meters are now less commonly used. Pure water boils at 100 °C (212 °F) at earth's standard atmospheric pressure.

A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers. An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains thanks to the availability of reliable pressure measurement devices.

William Roy, using barometric pressure, was able to confirm Masculine's height determinations, the agreement being to within one meter (3.28 feet). This method became and continues to be useful for survey work and map making.

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Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993. ^ Sample Metal of CVR Nav Canada ^ Montreal Current Weather, CBC Montreal, Canada, retrieved 2014-03-30 ^ Jacob, Daniel J..

^ A quick derivation relating altitude to air pressure Archived 2011-09-28 at the Payback Machine by Portland State Aerospace Society, 2004, accessed 05032011 ^ a b World: Highest Sea Level Air Pressure Above 750 m,, 2001-12-19, archived from the original on 2012-10-17, retrieved 2013-04-15 ^ World: Highest Sea Level Air Pressure Below 750 m,, 1968-12-31, archived from the original on 2013-05-14, retrieved 2013-04-15 ^ Kramer, MR; Springer C; Bergman N; Glazer M; Public M; Bar-Yishay E; Godfrey S (March 1998). “Rehabilitation of hypoxemic patients with COPD at low altitude at the Dead Sea, the lowest place on earth” (PDF).

Consequently then, in general, for every thousand feet of elevation gain, the pressure drops about 1 inch of mercury. For example, locations near 5000 feet (about 1500 meters) above mean sea level normally have pressures on the order of 24 inches of mercury.

ALTIMETER SETTING: This is the pressure reading most commonly heard in radio and television broadcasts. Instead, it is the pressure “reduced” to mean sea level using the temperature profile of the “standard” atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude.

The altimeter setting is an attempt to remove elevation effects from pressure readings using “standard” conditions. Each molecule is too small to feel and only exerts a tiny bit of force.

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First, simply adding molecules to any particular container will increase the pressure. A good example of this is adding (or subtracting) air in an automobile tire.

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.

Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch).

Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at ground level from 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 convert the station air pressures reading to a value with a common denominator.

At sea level, standard air pressure is 29.92 inches of mercury. A millibar is 1/1000th of a bar and is approximately equal to 1000 dunes (one done 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).

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At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.

The most basic change in pressure is the twice daily rise and fall in due to the heating from the sun. The magnitude of the daily cycle is the greatest near the equator decreasing toward the poles.

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. In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.

In frosty weather, the rise of the barometer presages snow. If it stands at “MUCH RAIN” and rises to “CHANGEABLE” expect fair weather of short continuance.

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If it stands at “FAIR” and falls to “CHANGEABLE”, expect foul weather. 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. When the change to scientific unit occurred in the 1960s many meteorologists preferred to keep using the magnitude they are used to and use a prefix “Hector” (h), meaning 100.

The end result is although the units we refer to in meteorology may be different, their numerical value remains the same.

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