✈ FAA PHAK + NOAA ISA Standard Atmosphere
Barometric Pressure Altitude Correction Calculator for US Aviation and Meteorology
The only free web calculator that simultaneously outputs station pressure, sea level pressure (MSLP), altimeter setting (QNH), pressure altitude, density altitude, air density, oxygen partial pressure, and water boiling point from a single entry. Three modes: altitude-to-pressure, station pressure analysis, and METAR altimeter decoder. Built on the NOAA Standard Atmosphere (1966), ICAO Doc 7488/3, and FAA PHAK (2024) formulas. Pre-loaded with five high-elevation US airports.
✈ Pressure + Density Altitude
🎁 Altimeter Setting (QNH)
🌊 METAR Decoder
🩺 Air Density + O2 Level
☕ Water Boiling Point
📄 PDF Report
7 Units
hPa, inHg, mmHg, kPa, Pa, psi, atm
29.92
inHg standard ISA sea-level pressure
NOAA
1966 Standard Atmosphere formula
FAA
PHAK 2024 density altitude formula
Barometric Pressure and Altitude: What Every US Pilot and Meteorologist Needs to Know
When you look at a weather map showing a high-pressure system over the Intermountain West or a deep Gulf low approaching the Texas coast, every pressure reading you see has been corrected to the same reference level: mean sea level. Without this correction, a weather station in Denver at 5,431 feet and a weather station in Miami at 8 feet would never report comparable pressures, because the higher you go, the less atmosphere there is above you pressing down. Denver’s actual station pressure on a standard day is around 843 hPa, while Miami’s is about 1013 hPa, a difference driven entirely by elevation rather than weather. Sea-level reduction makes both stations comparable, which is why your local news reports a barometric pressure of around 1013 to 1020 hPa regardless of whether you live in the mountains or on the coast.
For aviation, the pressure reduction problem becomes even more critical. A pilot flying from Miami into Denver needs to know that their altimeter reads correctly throughout the flight. The altimeter is simply a very sensitive aneroid barometer connected to a calibration dial called the Kollsman window. When the pilot sets their local altimeter setting (QNH) into the Kollsman window, the instrument reads their height above mean sea level. If the pilot forgets to update the altimeter setting when crossing from a high-pressure region to a low-pressure region during flight, the altimeter will over-read the actual altitude, a condition described by the old aviation saying “high to low, look out below.” This calculator converts between every pressure type so pilots, weather enthusiasts, mountain athletes, and food safety professionals can work with the exact pressure value they need.
The Five Pressure Types Used in US Aviation and Meteorology
Understanding the difference between station pressure, sea level pressure, altimeter setting, pressure altitude, and density altitude is fundamental to aviation safety and meteorological analysis. Station pressure is the actual measured atmospheric pressure at the location of the sensor, uncorrected for elevation. Sea level pressure, or MSLP, is the theoretical pressure that would exist if the station were at sea level, computed using the hypsometric equation with a virtual temperature correction that accounts for the actual air mass properties. The altimeter setting, also called QNH, uses the International Standard Atmosphere temperature profile rather than the actual air temperature in its reduction formula, which is why the altimeter setting and sea level pressure are not identical values. Pressure altitude is the altitude in the standard atmosphere corresponding to the current station pressure, which is what an aircraft altimeter reads when set to 29.92 inHg regardless of the actual altimeter setting. And density altitude, the most operationally important of all for aircraft performance, is the pressure altitude corrected for non-standard temperature, telling the pilot what altitude the aircraft’s aerodynamics and engine will experience based on current air density.
Who Uses This Calculator and Why
Pilots at high-elevation US airports such as Denver International (5,431 ft), Salt Lake City International (4,228 ft), Albuquerque International (5,355 ft), and Flagstaff Pulliam (7,014 ft) routinely encounter density altitudes that significantly exceed field elevation during summer heat. On a hot summer afternoon at Denver with an outside air temperature of 95°F (35°C), the density altitude can easily reach 9,000 to 10,000 feet, meaning a 172 Skyhawk sitting on the tarmac at 5,431 feet MSL will perform as if it is at 10,000 feet, requiring a substantially longer takeoff roll and reduced climb rate. NWS meteorologists at offices covering the Intermountain West compute station-to-sea-level pressure corrections hundreds of times per day. Mountaineers planning climbs of Colorado fourteeners or Cascade peaks use pressure altitude to assess equipment performance and acclimatization needs. And high-altitude home cooks in Denver need to know that water boils at about 202°F rather than 212°F, which affects canning safety and recipe timing, as documented by the USDA National Center for Home Food Preservation.
How This Barometric Pressure Altitude Calculator Works: The ISA Formulas Explained
The NOAA/ICAO International Standard Atmosphere Formula
The International Standard Atmosphere is a mathematical model of the atmosphere established jointly by NOAA, NASA, the US Air Force, and the International Civil Aviation Organization. It defines how pressure, temperature, and density change with altitude under idealized standard conditions. The tropospheric barometric formula, valid from sea level to the tropopause at approximately 11,000 meters (36,089 feet), is the foundation of every pressure-altitude conversion tool used in aviation and meteorology worldwide.
P = 1013.25 * (1 – 0.0065 * h_m / 288.15) ^ 5.25588 [hPa]
P = 226.32 * exp(-0.0001577 * (h_m – 11000)) [hPa]
AS_hPa = P_station / (1 – 0.0065 * z_m / 288.15) ^ 5.25588
AS_inHg = AS_hPa / 33.8639
MSLP = P_station * exp(g * z / (Rd * T_mean))
T_mean = T_station_K + 0.0065 * z_m / 2
PA_ft = (29.92 – AS_inHg) * 1000 + field_elev_ft
ISA_T = 15 – 2 * (PA_ft / 1000) [°C]
ISA_deviation = OAT_C – ISA_T
DA_ft = PA_ft + 120 * ISA_deviation
Why the Altimeter Setting and MSLP Are Different Numbers
This is one of the most frequently misunderstood distinctions in US aviation weather, and this calculator is one of the few tools that computes both simultaneously so you can see the difference. The MSLP (sea level pressure) uses the actual hypsometric equation with a mean virtual temperature computed from the observed surface temperature and the environmental lapse rate. The altimeter setting uses the International Standard Atmosphere temperature profile, assuming temperature decreases at exactly 6.5°C per kilometer regardless of the actual temperature profile. Because the standard atmosphere temperature assumption rarely matches reality exactly, the two values will almost always differ slightly. NWS Lead Forecaster Craig Sanders, in a weather.gov publication, specifically cautions pilots that altimeter settings and sea level pressure values from METAR reports are not interchangeable and that pilots should always use the altimeter setting from the ATIS or ASOS broadcast because it is computed using the same ISA formula as the aircraft altimeter calibration.
Three Real US Examples Using the Barometric Pressure Altitude Calculator
✈ Example 1 – Denver International Airport KDEN (Hot Summer Day)
High-Elevation Density Altitude: The “Mile High” Performance Trap
A private pilot flying a Cessna 172 Skyhawk is planning a Saturday afternoon departure from Denver International on a July afternoon. The ATIS reports altimeter 29.78 inHg, temperature 35°C (95°F), dew point 12°C. KDEN field elevation is 5,431 feet. This is a classic hot summer afternoon at one of the busiest high-elevation airports in the United States, and correctly computing density altitude is a safety-critical calculation before every takeoff.
METAR: KDEN 152153Z 18012KT 10SM FEW070 35/12 A2978
Altimeter Setting (QNH): 29.78 inHg = 1008.2 hPa
Field Elevation: 5,431 ft / 1,655 m
OAT: 35°C (95°F)
Pressure Altitude: (29.92 – 29.78) * 1000 + 5431 = 5,571 ft
ISA Temperature at PA: 15 – 2*(5571/1000) = 3.9°C
ISA Deviation: 35 – 3.9 = +31.1°C (HOT)
Density Altitude = 5571 + (120 * 31.1) = 9,303 ft
Air Density: P/(Rd*T) = 1008.2*100 / (287.05 * 308.15) = 1.140 kg/m3
Density as % of SL: 1.140/1.225 = 93.1%… wait, that’s before temp
Actual: 100841/(287.05*308.15) = 1.139 kg/m3 = 92.9% of sea level
Density Altitude: 9,303 ft at a 5,431 ft airport. The aircraft performs as if it is at 9,303 ft despite sitting on the ground at 5,431 ft. Per FAA-P-8740-02, takeoff roll for a normally aspirated aircraft may be 50-70% longer than standard. Air density at 92.9% of sea level. Water boiling point 94.5°C / 202.1°F. Pilots must consult the C172 POH density altitude performance charts before departure.
⛅ Example 2 – Standard Day at Sea Level, Miami KMIA
Baseline Sea-Level Reference: Understanding Standard Atmosphere Conditions
An NWS Miami forecaster is calibrating a new personal weather station installed at the KMIA perimeter at an elevation of 8 feet above sea level. The station reads a station pressure of 1012.9 hPa on a morning when the air temperature is 22°C. The forecaster wants to verify the sea level pressure reduction and altimeter setting the station should report, to compare against the official ASOS reading for quality control.
Station Elevation: 8 ft / 2.4 m
Station Pressure (observed): 1012.9 hPa
OAT: 22°C
Altimeter Setting (NWS ASOS formula):
AS = 1012.9 / (1 – 0.0065*2.4/288.15)^5.25588
AS = 1012.9 / 0.999946 = 1012.95 hPa = 29.92 inHg
Sea Level Pressure (hypsometric):
T_mean = 22 + 273.15 + 0.0065*2.4/2 = 295.16 K
MSLP = 1012.9 * exp(9.80665*2.4/(287.05*295.16)) = 1013.02 hPa
Pressure Altitude: (29.92 – 29.92)*1000 + 8 = 8 ft (essentially 0)
Density Altitude: PA + 120*(22-14.98) = 8 + 842 = 850 ft
Air Density: 1012.9*100/(287.05*295.15) = 1.195 kg/m3 = 97.6% of SL
Water Boiling Point: 100.0°C / 212.0°F
Near-perfect standard day at KMIA. Station pressure 1012.9 hPa matches MSLP 1013.0 hPa almost exactly due to near-zero elevation correction. Altimeter setting 29.92 inHg (ISA standard). Density altitude 850 ft above sea level due to the warm temperature (22°C vs 15°C ISA standard at sea level, ISA dev +7°C). Air density 97.6% of sea level standard. Normal cooking and canning procedures apply at this elevation.
🏔 Example 3 – Colorado Fourteener Climb Planning (14,000 ft MSL)
High-Altitude Mountaineering: Oxygen, Cooking, and Acclimatization Planning
A team of hikers is planning to summit Mount Elbert (14,440 ft / 4,401 m), the highest peak in Colorado and the second-highest in the contiguous United States. The team leader is using the barometric pressure altitude calculator to assess the oxygen environment they will encounter at the summit and to understand why their camp stove water takes so long to boil at their 11,500-foot base camp.
Summit Elevation: 14,440 ft / 4,401 m
Summit ISA Pressure: 1013.25*(1 – 0.0065*4401/288.15)^5.25588
= 1013.25 * 0.586 = 594.0 hPa
ISA Temperature at summit: 15 – 6.5*4.401 = -13.6°C
Air Density: 59400/(287.05*259.5) = 0.797 kg/m3 = 65.1% of SL
O2 Partial Pressure: 594.0 * 0.2095 = 124.4 hPa = 58.6% of SL O2
Base camp (11,500 ft / 3,505 m):
ISA Pressure: 1013.25*(1-0.0065*3505/288.15)^5.25588 = 650.3 hPa
Water Boiling Point: 100-(1013.25-650.3)*0.037 = 86.6°C / 187.9°F
Summit Boiling Point: 100-(1013.25-594.0)*0.037 = 84.5°C / 184.1°F
At 14,440 ft (Mt Elbert summit): Air density 65.1% of sea level. O2 partial pressure 124.4 hPa (58.6% of sea-level oxygen). Above FAA 14 CFR 91.211 mandatory O2 threshold of 14,000 ft. Per FAA PHAK Chapter 16, supplemental oxygen is required for flight at this altitude. Water boils at 84.5°C (184.1°F) at the summit, 87°C at base camp. Pasta and rice require significantly longer cooking times. Team should acclimatize with standard ascent-rate protocols and never dismiss altitude sickness symptoms.
16 Frequently Asked Questions About Barometric Pressure and Altitude Correction
What is the difference between altimeter setting, sea level pressure, and station pressure?+
These three pressure values are related but computed differently, and mixing them up is one of the most common errors in both aviation and home weather station setup. Station pressure (Ps) is the actual observed atmospheric pressure at the station’s elevation, uncorrected for altitude. Sea level pressure (MSLP) reduces the station pressure to what it would theoretically be at sea level using the hypsometric equation with a virtual temperature correction that accounts for the actual temperature and moisture of the local air mass. The altimeter setting (QNH) also reduces station pressure to sea level, but uses the International Standard Atmosphere temperature profile (6.5°C/km lapse rate, 15°C at sea level) rather than the actual temperature profile. Because the standard atmosphere rarely matches actual conditions exactly, the altimeter setting and MSLP will almost always be slightly different numbers. NWS Lead Forecaster Craig Sanders specifically notes in a weather.gov publication that pilots should always use the altimeter setting (not the sea level pressure) from the ATIS broadcast because it uses the same ISA formula as the aircraft altimeter calibration, while the MSLP uses the actual temperature structure and will not match the altimeter reading on non-standard temperature days.
How do I find the altimeter setting from a METAR report?+
In US METAR format, the altimeter setting appears as the letter A followed by four digits representing the setting in hundredths of an inch of mercury, without a decimal point. For example, A2992 means 29.92 inHg. In international ICAO METAR format used outside the United States, the altimeter setting appears as Q followed by the setting in whole hectopascals, such as Q1013 for 1013 hPa. You can find METARs at
aviationweather.gov/metar (FAA/NWS Aviation Weather Center),
weather.gov/asos, or by tuning to the ATIS/AWOS frequency for your local airport. In a typical US METAR such as “KDEN 152153Z 18012KT 10SM FEW070 35/12 A2978 RMK AO2 SLP120,” the altimeter setting is A2978 (29.78 inHg) and the remarks include SLP120 which means the sea level pressure is 1012.0 hPa (add 1000 when first digit is 0-5, add 900 when first digit is 6-9). The METAR decoder tab of this calculator converts any QNH reading into all pressure types simultaneously.
What is density altitude and why does it matter for aircraft performance?+
Density altitude is pressure altitude corrected for non-standard temperature, and it is the most operationally important altitude for aircraft performance. The FAA-P-8740-02 Density Altitude pamphlet defines it as “the altitude in the standard atmosphere at which the current air density exists.” Aircraft engines, propellers, and wings all respond to air density rather than geometric altitude. A thinner atmosphere (higher density altitude) reduces engine power output (less oxygen to burn fuel), reduces propeller and rotor efficiency (less air mass to move), and increases the speed required to generate adequate lift (more stall speed). The rule of thumb from FAA-P-8740-02: for every 1,000 feet of density altitude above sea level, a normally aspirated engine loses approximately 3 percent of its sea-level power output. At a density altitude of 8,000 feet, an aircraft has about 76 percent of its sea-level performance available. The AOPA Air Safety Institute documents that density altitude accidents typically occur because pilots visually assess a runway that “looks fine” without computing the actual density altitude performance numbers. The FAA formula for density altitude is: DA = PA + 120 x (OAT in Celsius minus ISA temperature at that pressure altitude), where ISA temperature at any pressure altitude equals 15 minus 2 times the pressure altitude in thousands of feet.
What is the standard sea-level pressure in the United States and why is it 29.92 inHg?+
The standard sea-level atmospheric pressure is 1013.25 hectopascals (hPa), which equals 29.9213 inches of mercury (inHg), conventionally rounded to 29.92 inHg in aviation. This value was established by the International Standard Atmosphere model, first formalized in the NOAA/NASA/US Air Force U.S. Standard Atmosphere (1966) and adopted by the International Civil Aviation Organization in ICAO Doc 7488/3. It represents the average sea-level pressure of the Earth’s atmosphere, not a typical or most-common observed pressure, and rarely occurs exactly in nature. The choice of inHg units for US aviation altimeter settings is historical: mercury barometers were the standard atmospheric pressure instrument from the 17th century through the early 20th century, and aviation altimeters were calibrated in inches of mercury from their earliest development in the 1920s. Most of the rest of the world uses hectopascals (equivalent to millibars) for aviation, displayed as QNH in the Q1013 format in international METAR. Both units are in active use in the US: METAR reports include altimeter settings in inHg while NWS surface analysis charts and model data use hPa/mb. This calculator converts between all seven pressure units simultaneously.
Why does water boil at a lower temperature in Denver than at sea level, and does it matter for cooking?+
Water boils when its vapor pressure equals the surrounding atmospheric pressure. At sea level with 1013.25 hPa of atmospheric pressure, water’s vapor pressure equals atmospheric pressure at 100°C (212°F), which is why we consider 100°C the boiling point. At Denver’s 5,431-foot elevation where atmospheric pressure is approximately 832 hPa, water’s vapor pressure equals atmospheric pressure at a lower temperature, approximately 94.5°C (202°F). The reduced boiling temperature has real practical consequences. For home food preservation, the USDA National Center for Home Food Preservation at the University of Georgia explicitly states that boiling water canners are not recommended for use above 6,000 feet for low-acid foods, and that high-acid foods like fruits and tomatoes require longer processing times at altitude because the lower boiling temperature means the thermal kill step is less effective. For cooking pasta, eggs, and other foods, Denver residents routinely notice that recipes developed for sea level require more time at altitude because the lower boiling temperature transfers heat to food less effectively. The practical adjustment guideline from the USDA: for every 1,000 feet above sea level, increase recipe cooking times in boiling water by approximately 5 percent to compensate for the lower temperature. This calculator’s water boiling point output gives you the precise temperature at your altitude.
How do I calibrate my home weather station’s sea level pressure reading?+
Most personal weather stations sold in the US (Ambient Weather, Acurite, Davis, Ecowitt, and others) have a calibration setting for “relative pressure” or “sea level pressure” that requires a fixed offset from the observed station pressure. The correct value to enter is the difference between the ISA-based altimeter setting and the raw station pressure, not the difference between MSLP and station pressure, because the altimeter setting formula is what most US weather apps and NWS products display as “barometric pressure.” The simplest calibration procedure is: find the current official altimeter setting from the nearest ASOS station (available at
weather.gov/asos), use this calculator’s Station Pressure Analysis mode with your station’s raw pressure reading and elevation to compute the theoretical altimeter setting, compare the two values, and apply the difference as a calibration offset in your station’s settings. Alternatively, if you know your precise elevation, use the Altitude to Pressure mode to find the ISA standard pressure at your elevation, then compare it to your station’s raw reading on a standard-ish day to determine a typical calibration offset. The Citizen Weather Observer Program (CWOP) at NOAA provides automated quality control for personal weather station data submitted to the Weather Underground and AWEKAS networks.
At what altitude does supplemental oxygen become required under FAA regulations?+
FAA 14 CFR Part 91.211 (verified 2025) establishes the supplemental oxygen requirements for unpressurized aircraft. Required flight crewmembers must use supplemental oxygen when at cabin altitudes above 12,500 feet MSL for more than 30 consecutive minutes, and at all times when at cabin pressure altitudes above 14,000 feet MSL. All aircraft occupants must be provided supplemental oxygen when at cabin pressure altitudes above 15,000 feet MSL, though they are not legally required to use it. For pressurized aircraft, FAR 91.211(b) requires that the aircraft maintain a cabin pressure altitude of 15,000 feet MSL or lower unless all occupants are provided with supplemental oxygen equipment. In unpressurized aircraft above 25,000 feet, the regulations become extremely restrictive because at that altitude the partial pressure of oxygen is insufficient to maintain consciousness even with oxygen concentrators. From a physiological standpoint, the FAA PHAK Chapter 16 notes that subtle impairment of cognitive function and reaction time can begin as low as 8,000 to 10,000 feet in individuals who are not acclimatized, which is why oxygen is recommended for extended flights above 10,000 feet even though not legally required at that altitude. This calculator displays the hypoxia risk assessment for any altitude input based on these FAR 91.211 thresholds.
What is the International Standard Atmosphere (ISA) and who created it?+
The International Standard Atmosphere is a mathematical model of how pressure, temperature, and density change with altitude in a globally averaged idealized atmosphere. It was developed cooperatively by NOAA, NASA, and the US Air Force, published as the U.S. Standard Atmosphere (1966), and adopted internationally by the International Civil Aviation Organization as ICAO Doc 7488/3 Manual of the ICAO Standard Atmosphere (2nd edition 1964, 3rd edition 2002). The ISA defines sea-level conditions as 1013.25 hPa pressure, 15°C temperature, and 1.225 kg/m3 air density. In the troposphere (from sea level to 11,000 meters or 36,089 feet), the ISA assumes temperature decreases at a constant lapse rate of 6.5°C per kilometer. At the tropopause (11,000 m) the temperature becomes constant at -56.5°C and remains constant through the lower stratosphere up to 20,000 meters in a layer called the isothermal layer. Above 20,000 meters, temperature increases again in higher atmospheric layers, but these are not relevant for most aviation or meteorological calculations. The ISA is a standard reference, not a typical or expected atmospheric state: actual atmospheric conditions deviate from ISA continuously in both temperature and pressure, which is why density altitude calculations require inputting the actual outside air temperature rather than assuming ISA temperature at any given pressure altitude.
What is the formula for pressure altitude used by the FAA?+
The FAA Pilot’s Handbook of Aeronautical Knowledge (2024, Chapter 11) provides two equivalent methods for computing pressure altitude. The practical rule-of-thumb formula is: pressure altitude equals field elevation plus (29.92 minus the current altimeter setting in inHg) times 1,000 feet. So if the altimeter setting is 29.78 inHg at a 5,000-foot airport, the pressure altitude is 5,000 plus (29.92 minus 29.78) times 1,000, which equals 5,000 plus 140, giving 5,140 feet. The more precise formula based on the ISA barometric equation is: pressure altitude in meters equals 44,307.7 times (1 minus (P/1013.25)^0.190263), where P is the station pressure in hPa. This calculator uses the precise formula in its computations and the rule-of-thumb for display in the METAR decoder mode. Both formulas give very similar results within the range of atmospheric pressures encountered in normal operations. Pressure altitude can also be obtained directly by setting 29.92 inHg in the Kollsman window of an aircraft altimeter and reading the indicated altitude, which is why it is sometimes called “standard pressure altitude” or “indicated altitude with altimeter set to standard.”
How does cold weather affect altimeter accuracy and when should cold temperature corrections be applied?+
The standard aircraft altimeter assumes ISA temperature at all altitudes above the station. When actual temperatures are significantly colder than ISA standard, the aircraft is actually lower than the altimeter indicates because cold dense air compresses the lower atmosphere, making the pressure surfaces lower than their standard altitudes. The FAA PHAK notes that in very cold conditions, altimeter errors can accumulate to several hundred feet. The FAA has implemented Cold Temperature Required Altitude Corrections (CTRACs) for published instrument approaches at airports where this effect is significant. For general aviation, the approximate correction formula is: altitude error in feet equals 4 times the temperature deviation below ISA in Celsius times the indicated altitude above the station in thousands of feet. For example, at -30°C deviation from ISA (a common winter temperature in the northern Plains states) at 4,000 feet above the station, the altimeter error would be approximately 4 times 30 times 4, which equals 480 feet: you would actually be 480 feet lower than indicated. The FAA issued updated guidance on Cold Temperature Altitude Corrections in AC 91-116 and through Notices to Airmen (NOTAMs) at affected airports. This barometric pressure calculator does not compute cold temperature altitude corrections, but understanding that the altimeter setting formula uses ISA temperature is the foundation for understanding why the correction is necessary in cold weather.
What is air density and why does it decrease with altitude?+
Air density is the mass of air per unit volume, measured in kilograms per cubic meter. At sea level under ISA standard conditions, air density is 1.225 kg/m3. Air density decreases with altitude for two compounding reasons: as you ascend, there is less atmosphere above you pressing down, so the pressure decreases; and simultaneously, temperature decreases (in the troposphere), further affecting density. The relationship between pressure, density, and temperature follows the ideal gas law: density equals pressure divided by the gas constant times temperature in Kelvin (rho = P / (Rd * T)). At Denver’s 5,431 feet on a standard day, the pressure is about 82 percent of sea level and the temperature is lower, giving a density of about 85 percent of sea level. On a hot summer afternoon at Denver with temperature 10 degrees Celsius above ISA standard, that density drops further to around 80 to 83 percent of sea level, which is what the density altitude calculation captures. For aircraft engines, lower air density means fewer oxygen molecules per cubic meter entering the intake, directly reducing the maximum power output by roughly the same percentage as the density reduction. For propellers and wings, lower density means each cubic meter of air accelerated by the prop or deflected by the wing has less mass, requiring higher airspeeds to generate the same thrust and lift, which is why both stall speed and takeoff speed increase with density altitude.
What are Flight Levels and when do US pilots switch from altimeter setting to standard pressure?+
Flight Levels are altitude designations expressed in hundreds of feet based on the International Standard Atmosphere pressure setting of 29.92 inHg (1013.25 hPa). When a pilot sets 29.92 inHg in the altimeter Kollsman window and reads FL180, the altimeter indicates approximately 18,000 feet above the standard pressure datum, regardless of the actual sea level pressure at that location. In the United States, the Transition Altitude is 18,000 feet MSL, meaning that aircraft operating at or above 18,000 feet MSL (FL180 and above) must set their altimeters to 29.92 inHg and fly Flight Levels. Below 18,000 feet, pilots set QNH (the local altimeter setting from ATIS/AWOS) to read altitude above mean sea level. The rationale for switching to standard pressure above the Transition Altitude is coordination: at high altitudes where multiple aircraft from different geographic regions may be sharing airspace, it is simpler for all aircraft to reference the same pressure datum (29.92 inHg) rather than trying to coordinate between different local altimeter settings across different pressure systems. This is why, above FL180, two aircraft separated by 1,000 feet of pressure altitude are guaranteed to have at least 1,000 feet of real separation (plus or minus cold temperature errors), because both are using the same pressure reference.
How accurate is the water boiling point calculation at altitude?+
The water boiling point calculation in this calculator uses an empirical linear approximation: boiling point in Celsius equals 100 minus (1013.25 minus current pressure in hPa) times 0.0370. This approximation is accurate to within about 0.5°C across the altitudes relevant to US cooking and outdoor recreation (sea level to 14,000 feet). The more precise relationship between pressure and boiling point is described by the Clausius-Clapeyron equation from classical thermodynamics, which gives the exact temperature at which water’s vapor pressure equals the surrounding atmospheric pressure. For comparison, at Denver’s 832 hPa station pressure, the linear approximation gives 93.6°C while the precise Clausius-Clapeyron equation gives approximately 94.4°C, a difference of less than 1°C that is negligible for practical cooking purposes. The USDA National Center for Home Food Preservation uses similar approximations in its altitude cooking and canning guidance tables, which are calibrated in 500 to 1,000-foot altitude increments rather than precise pressures, giving an inherent uncertainty of about 0.5°C in their own boiling point references. For food safety purposes, what matters is that water boiling at altitude is measurably less hot than at sea level, and that processing times for home canning must be adjusted upward according to USDA altitude correction tables available at
nchfp.uga.edu.
Where do I get official real-time pressure altitude data for US airports?+
The authoritative source for real-time altimeter settings, station pressure, and sea level pressure for US airports is the NWS Automated Surface Observing System (ASOS) network. ASOS data is available in near real-time at
weather.gov/asos, through the METAR browser at
aviationweather.gov/metar operated by the FAA/NWS Aviation Weather Center, and via any ATIS or AWOS broadcast at the airport. For pilots, the ATIS frequency for your destination airport is available in the FAA Chart Supplement and on sectional charts. Standard METAR reports are issued hourly (approximately 55 minutes past the hour) from ASOS stations and as Special METARs (SPECI) when conditions change significantly. For weather stations without ASOS access, the NOAA NWS observation page at
weather.gov shows current pressure observations from thousands of stations across the US. The NWS Surface Analysis at weather.gov displays mean sea level pressure in hPa on weather maps updated hourly, and the SPC mesoanalysis at spc.noaa.gov provides gridded pressure and pressure altitude data derived from the RAP model analysis at hourly intervals for the continental United States.
Can this calculator be used for mountain weather forecasting?+
This calculator provides the foundational pressure and altitude relationships that are essential for understanding mountain weather, but it is not a weather forecasting tool itself and should not be used as the sole resource for mountain trip planning. What it does provide for mountain applications includes the ISA pressure at any summit altitude (for equipment and acclimatization planning), oxygen partial pressure at altitude (for assessing hypoxia risk and supplemental oxygen needs above 10,000 feet), water boiling point (for backcountry cooking and camp stove fuel planning), air density at altitude (for understanding how camping stoves and automotive engines perform differently at high altitude), and the relationship between observed barometric pressure and the equivalent altitude (useful for understanding how a falling barometer at a mountain summit relates to approaching weather systems). For actual mountain weather forecasting and thunderstorm avoidance in the Colorado Rockies, the Cascades, or the Sierra Nevada, the definitive resources are the NWS Mountain Weather Forecast at weather.gov, the NWS Denver office forecasts at weather.gov/bou for the Colorado mountains, and the Colorado Avalanche Information Center forecasts which include mountain weather as part of their snowpack and hazard analysis at avalanche.state.co.us.
What is the exponent 5.25588 in the ISA barometric formula and where does it come from?+
The exponent 5.25588 in the ISA barometric formula P equals P0 times (1 minus L*h/T0)^5.25588 is derived from the ratio g*M/(R*L), where g is gravitational acceleration (9.80665 m/s2), M is the molar mass of dry air (0.0289644 kg/mol), R is the universal gas constant (8.31432 J/(mol*K)), and L is the temperature lapse rate (0.0065 K/m). Computing this: 9.80665 times 0.0289644 divided by 8.31432 times 0.0065 equals approximately 5.25588. This exponent appears because the ISA barometric formula is derived from combining the hydrostatic equilibrium equation (dP/dz = -rho*g), the ideal gas law (P = rho*R*T/M), and the assumed constant temperature lapse rate (T = T0 – L*z). Integrating this system gives a power-law relationship between pressure and altitude in the troposphere, rather than the simple exponential form that would apply in an isothermal atmosphere. The exponent appears in several equivalent forms in the literature: 5.25588 (NOAA 1966), 5.2561 (some engineering references), and 5.25578 (computed from more precise constants). The small differences between these values reflect rounding in the physical constants used and have negligible practical effect in the altitude range from sea level to the tropopause. This calculator uses 5.25588 per the NOAA Standard Atmosphere Technical Note (1966) and ICAO Doc 7488/3.