question: If barometric pressure changes with the weather, how can it still be accurate for altitude measurement?
comment: In airplanes that use a barometric altimeter, you calibrate the altimeter before you take off. The altitude of the airport is well known, and you set the altimeter to read that altitude while you are still on the ground. Even so, such altimeters are not that accurate. When it really matters, such as trying to land when visibility is low, airplanes use other techniques to ensure the reach the ground at the right time, while avoiding mountains or other terrain.
passages: As the air pressure decreases at altitudes above sea level (and increases below sea level) the uncorrected reading of the barometer will depend on its location. The reading is then adjusted to an equivalent sea-level pressure for purposes of reporting. For example, if a barometer located at sea level and under fair weather conditions is moved to an altitude of 1,000 feet (305 m), about 1 inch of mercury (~35 hPa) must be added on to the reading. The barometer readings at the two locations should be the same if there are negligible changes in time, horizontal distance, and temperature. If this were not done, there would be a false indication of an approaching storm at the higher elevation.
   Since gravity is not constant over large areas the orthometric height of a level surface other than the reference surface is not constant, and orthometric heights need to be corrected for that effect. For example, gravity is 0.1% stronger in the northern United States than in the southern, so a level surface that has an orthometric height of 1000 meters in Montana will be 1001 meters high in Texas.
   Barometric pressure and the pressure tendency (the change of pressure over time) have been used in weather forecasting since the late 19th century. When used in combination with wind observations, reasonably accurate short-term forecasts can be made. Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, give a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface brings clouds and sometimes precipitation. The larger the change in pressure, especially if more than 3.5 hPa (0.1 inHg), the greater the change in weather that can be expected. If the pressure drop is rapid, a low pressure system is approaching, and there is a greater chance of rain. Rapid pressure rises, such as in the wake of a cold front, are associated with improving weather conditions, such as clearing skies.
   Pressure on Earth varies with the altitude of the surface; so air pressure on mountains is usually lower than air pressure at sea level. 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. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude. Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.
   BULLET::::- Altitude sensing
  
  This is useful in aircraft, rockets, satellites, weather balloons, and many other applications. All these applications make use of the relationship between changes in pressure relative to the altitude. This relationship is governed by the following equation:
  
  This equation is calibrated for an altimeter, up to 36,090 feet (11,000 m). Outside that range, an error will be introduced which can be calculated differently for each different pressure sensor. These error calculations will factor in the error introduced by the change in temperature as we go up.
   The table below shows examples for three locations in the city of San Francisco, California. Note the corrected barometer readings are identical, and based on equivalent sea-level pressure. (Assume a temperature of 15 °C.)
   When atmospheric pressure is measured by a barometer, the pressure is also referred to as the "barometric pressure". Assume a barometer with a cross-sectional area "A", a height "h", filled with mercury from the bottom at Point B to the top at Point C. The pressure at the bottom of the barometer, Point B, is equal to the atmospheric pressure. The pressure at the very top, Point C, can be taken as zero because there is only mercury vapor above this point and its pressure is very low relative to the atmospheric pressure. Therefore, one can find the atmospheric pressure using the barometer and this equation:
   Practical applications must use a model rather than measurements to calculate the change in gravitational potential versus depth in the earth, since the geoid is below most of the land surface (e.g., the Helmert Orthometric heights of NAVD88).
  
  GPS measurements give earth-centered coordinates, usually displayed as height above the reference ellipsoid, which cannot be related accurately to orthometric height above the geoid without accurate gravity data for that location. In the US, NGS has undertaken the GRAV-D ten-year program to obtain such data with a goal of releasing a new definition in 2022.
   Average "sea-level pressure" is . In aviation, weather reports (METAR), 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. However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.
   Aneroid barometers have a mechanical adjustment that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude. Setting an aneroid barometer is similar to resetting an analog clock that is not at the correct time. Its dial is rotated so that the current atmospheric pressure from a known accurate and nearby barometer (such as the local weather station) is displayed. No calculation is needed, as the source barometer reading has already been converted to equivalent sea-level pressure, and this is transferred to the barometer being set—regardless of its altitude. Though somewhat rare, a few aneroid barometers intended for monitoring the weather are calibrated to manually adjust for altitude. In this case, knowing "either" the altitude or the current atmospheric pressure would be sufficient for future accurate readings.
   BULLET::::- Altimeter setting is a term and quantity used in aviation. The regional or local air pressure at mean sea level is called the altimeter setting, and the pressure which will calibrate the altimeter to show the height above ground at a given QNH airfield.
   Barometric formula
  
  The barometric formula, sometimes called the "exponential atmosphere" or "isothermal atmosphere", is a formula used to model how the pressure (or density) of the air changes with altitude. The pressure drops approximately by 11.3 Pa per meter in first 1000 meters above sea level.
  
  Section::::Pressure equations.
  
  There are two different equations for computing pressure at various height regimes below 86 km (or 278,400 feet). The first equation is used when the value of standard temperature lapse rate is not equal to zero:
  
  The second equation is used when standard temperature lapse rate equals zero:
  
  where:
   The time of the day must be considered, as there are two natural rises per day (locally around 10 am and 10 pm) and two natural falls per day (locally around 4 am and 4 pm). These daily changes in pressure can mask the movement of pressure systems and fronts past a location. The lowest natural dip in the pressure, in a stagnant weather pattern, occurs around 4 pm while the highest natural peak in pressure occurs around 10 am. Once plotted on a map, the analysis of isallobars (lines of equal pressure change) can be plotted on a map, which can indicate the direction of motion of high and low pressures systems across the map domain.
   The line between two of these marks is called the 'characteristic of barometric tendency' and is used by weather forecasters. The observer would first note if the pressure was lower or higher than three hours prior. Next, a code number would be chosen that best represents the three-hour trace. There are nine possible choices (0 to 8) and no single code has preference over another. In the case of the graph on the barograph, one of two codes could be picked. An 8 (steady then decreasing) or 6 (decreasing then steady). The observer should pick the 6 because it represents the last part of the trace and is thus most representative of the pressure change.
   Measurements of barometric pressure and the pressure tendency (the change of pressure over time) have been used in forecasting since the late 19th century. The larger the change in pressure, especially if more than , the larger the change in weather can be expected. If the pressure drop is rapid, a low pressure system is approaching, and there is a greater chance of rain. Rapid pressure rises are associated with improving weather conditions, such as clearing skies.
  
  Section::::Techniques.:Looking at the sky.
   At low altitudes above sea level, the pressure decreases by about for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure "p" to altitude "h":
  
  formula_1
  
  where the constant parameters are as described below:
  
  Section::::Local variation.
  
  Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.
   So cool air lying on top of warm air can be stable, as long as the temperature decrease with height is less than the adiabatic lapse rate; the dynamically important quantity is not the temperature, but the potential temperature—the temperature the air would have if it were brought adiabatically to a reference pressure. The air around the mountain is stable because the air at the top, due to its lower pressure, has a higher potential temperature than the warmer air below.
  
  Section::::Its use in estimating atmospheric stability.:Effects of water condensation: equivalent potential temperature.
   Older AWOS systems used a lithium chloride dew point sensor. Current AWOS systems use capacitive relative humidity sensors, from which dew point is calculated.
  
  Section::::Observing equipment.:Barometric pressure and altimeter setting.
  
  Data from a barometric pressure sensor are used to calculate QNH altimeter setting. Pilots rely on this value to determine their altitude. To ensure safe separation from terrain and other obstructions, high degree of accuracy and reliability is required from a pressure sensor.
   Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the (gauge) tire pressure goes up because atmospheric pressure goes down. The absolute pressure in the tire is essentially unchanged.
   Unlike the Minimum safe altitude used for IFR flight, the MEF does not include any margin for aircraft clearance above the terrain nor for altimeter error.
  
  Section::::United States.
   While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual height above ground. At low altitudes, the altimeter is commonly set to show the altitude above sea-level, which can be directly compared to the known elevation of the terrain. The pressure setting to achieve this varies with weather conditions. It is called QNH ("barometric pressure adjusted to sea level") or "altimeter setting" and the current local value is available from various sources, including air traffic control and the local METAR-issuing station.
   Thus the standard consists of a tabulation of values at various altitudes, plus some formulas by which those values were derived. To accommodate the lowest points on Earth, the model starts at a base geopotential altitude of "below sea level", with standard temperature set at 19 °C. With a temperature lapse rate of −6.5 °C (-11.7 °F) per km (roughly −2 °C (-3.6 °F) per 1,000 ft), the table interpolates to the standard mean sea level values of temperature, (1 atm) pressure, and a density of . The tropospheric tabulation continues to , where the temperature has fallen to , the pressure to , and the density to . Between 11 km and 20 km, the temperature remains constant.
   The watch features a barometer whose output is updated every hour in the form of a graphical display. This creates a barometric pressure chart which was used by Conrad Anker during his Himalayan expedition to predict the weather based on the atmospheric pressure trends; if the pressure decreased suddenly with no subsequent upward trend, it was a sign of an approaching storm, while a gradual pressure increase indicated improving weather. 
   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. While in 1774, Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion in Scotland (using plumb bob deviation to show the effect of gravity) and accurately measure elevation, William Roy using barometric pressure was able to confirm his 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. This early application of science gave people insight into how science could easily be put to practical use.
   BULLET::::- Sea level pressure (SLP) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature. This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. The reduction to sea level means that the "normal range of fluctuations" in pressure is the same for everyone. The pressures which are considered "high pressure" or "low pressure" do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools.
  
presuppositions: Barometric pressure is not accurate for altitude measurement due to changing weather.
corrections: Barometric pressure can be accurate for altitude measurement if it's calibrated before take off.
raw_presuppositions: Barometric pressure is not accurate for altitude measurement due to changing weather. Barometric pressure can't be accurate for altitude measurement if it changes with weather.
raw_corrections: Barometric pressure can be accurate for altitude measurement if it's calibrated before take off. Barometric pressure can be accurate for altitude measurement if it's calibrated.