What Altimeter Setting represents
An altimeter setting reduces station pressure using the ISA tropospheric temperature profile rather than a measured mean column temperature. It is intended to make an altimeter indicate field elevation under the specified reference convention.
Altimeter Setting begins with station pressure, station elevation. Label every entry as observed, reduced, modeled, or assumed, and keep pressure type, vertical datum, timestamp, and reference atmosphere attached to the result.
Domain and boundary checks
At zero elevation, altimeter setting equals station pressure. The standard-atmosphere base term must remain positive.
Before accepting Altimeter Setting, vary one input while holding the others fixed and predict the sign of change. This catches inverted pressure ratios, Celsius–kelvin errors, metres–feet mistakes, and endpoint reversal.
Assumptions and stopping point
This simplified relationship does not reproduce every official instrument, rounding, gravity, or high-elevation procedure. It provides educational arithmetic and carries no operational aviation authority.
Altimeter Setting supplies transparent atmospheric arithmetic. It does not issue a forecast, identify hazardous weather, certify an instrument, define an official observing product, or authorize aviation, marine, field, or laboratory operations.
To extend the Altimeter Setting record, use the related Equivalent Potential Temperature Calculator with compatible inputs and conventions.
Uncertainty and sensitivity
Test Altimeter Setting with a credible low and high value for the least certain input, keeping the other entries unchanged. Report the resulting range in approximate altimeter setting rather than treating interface precision as measurement accuracy.
This Altimeter Setting sensitivity range is not automatically a confidence interval. It omits correlations, representativeness error, vertical interpolation, sensor bias, and structural uncertainty in the chosen atmospheric model.
Pressure and height conventions
Pressure must be identified as absolute station pressure, reduced sea-level pressure, or altimeter setting before it enters Altimeter Setting. Geometric elevation, geopotential height, pressure altitude, and density altitude likewise answer different questions.
For Altimeter Setting, preserve the source datum and reference surface. Converting one vertical coordinate to another requires a stated gravity or standard-atmosphere model; renaming a column does not perform that conversion.
Temperature and moisture conventions
Absolute-temperature formulas inside Altimeter Setting require kelvins even when the interface accepts degrees Celsius. Layer-mean virtual temperature is not interchangeable with a single surface thermometer reading.
When moisture affects Altimeter Setting, distinguish specific humidity, mixing ratio, dew point, and virtual temperature. Each has a separate definition, and entering the same number under another label changes the physics.
Retaining an auditable record
Save raw Altimeter Setting inputs, conversions, constants, formula version, unrounded output, rounded result, and any quality flags. Another reader should be able to repeat the calculation without guessing pressure type or altitude datum.
If an observation or assumption changes, make a dated Altimeter Setting revision and preserve the earlier value. State whether the change corrects an error, updates a measurement, or explores a separate scenario.
Using Approximate altimeter setting downstream
Transfer unrounded approximate altimeter setting with its unit and full metadata when feeding another calculation. Round only the reported Altimeter Setting result, and never strip a sign from a tendency or direction-dependent quantity.
A single Altimeter Setting value describes one parcel, layer, column, point pair, or interval. Spatial fields and trends require comparable repeated data, documented interpolation, and treatment of missing or flagged values.
Formula and unit route
The working relationship is A = Pₛ ÷ (1 − 2.25577×10⁻⁵z)^5.25588. Altimeter Setting performs only the displayed conversion and does not retrieve a sounding, station report, forecast, or hidden atmospheric field.
Carry SI conversions explicitly through Altimeter Setting: Celsius becomes kelvins where required, hectopascals become pascals for force equations, and kilometre scaling must be reconciled before interpreting a gradient.
To extend the Altimeter Setting record, use the related Air Density Calculator with compatible inputs and conventions.
Checked numerical example
At 900 hPa station pressure and 1,000 m elevation, the approximate altimeter setting is 1,014.66 hPa.
Reset restores this Altimeter Setting example. Reproduce it independently with the stated constants before substituting measurements, and investigate discrepancies before trusting additional digits.
Selecting compatible observations
Enter calibrated station pressure and the station or field elevation for the same location. Preserve whether elevation refers to sensor height, airport reference point, or another datum because those are not automatically interchangeable.
For Altimeter Setting, record location, sensor height, elevation datum, valid time, averaging interval, pressure reduction convention, temperature definition, moisture variable, and instrument resolution as applicable.
To extend the Altimeter Setting record, use the related Atmospheric Scale Height Calculator with compatible inputs and conventions.
Interpreting Approximate altimeter setting
The computed setting is a reference pressure in hPa, not local station pressure and not necessarily the meteorological sea-level pressure produced by a weather network.
Compare Altimeter Setting outputs only when the source quantities share physical meaning and reference conventions. A numerically similar station pressure, sea-level pressure, and altimeter setting can still represent different quantities.
Common atmospheric calculation errors
Frequent Altimeter Setting mistakes include treating hPa as Pa, using Celsius in a gas-law denominator, reversing upper and lower pressures, mixing feet with metres, or applying sea-level pressure to a local-density calculation.
Reject impossible Altimeter Setting combinations rather than forcing an answer. Preserve genuine zero, missing data, trace values, and below-detection readings as different states. Check signs, logarithm arguments, denominator size, and the applicable vertical layer before using the output.
Checking the atmospheric result
Can forecast or scenario values be entered?
Yes. Mark the Altimeter Setting result as a scenario; the page does not fetch, verify, or issue a weather forecast.
How can I check Altimeter Setting?
Repeat A = Pₛ ÷ (1 − 2.25577×10⁻⁵z)^5.25588 with the recorded conversions, then test the checked example and a physical boundary.
Why could an official source differ?
An official product may use a different pressure reduction, datum, constants, sounding, moisture correction, instrument procedure, or rounding than Altimeter Setting uses.
Does the result carry operational authority?
No. Altimeter Setting is educational calculation support, not an aviation, marine, forecasting, laboratory, or safety authority.
How should I round the answer?
Keep full precision inside Altimeter Setting, then round no more finely than the least certain observation or model assumption supports.
When should I recalculate?
Recalculate Altimeter Setting when the time, site, pressure type, elevation datum, parcel, layer, instrument value, or model assumption changes.