What Are the Standard Sea-Level Values for Pressure and Temperature in the International Standard Atmosphere?

Why the Standard Atmosphere Exists

Aviation runs on consistency. Pilots fly different aircraft, in different countries, at different times of year — yet they all need their instruments to speak the same language. That is exactly why the International Standard Atmosphere (ISA) was created. It defines a fixed set of reference conditions that allow engineers to design aircraft, manufacturers to publish performance charts, and pilots to interpret their instruments with confidence.

The Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25) introduces the standard atmosphere in the Weather Theory chapter, and for good reason. Understanding what the atmosphere is supposed to look like helps you recognize — and correct for — the times when it behaves differently. On your checkride, your Designated Pilot Examiner will almost certainly ask you to recite these baseline values. More importantly, they will want to know that you understand what those numbers actually mean.

The Two Numbers You Must Know Cold

At sea level, the International Standard Atmosphere defines two fundamental conditions. Standard pressure is 29.92 inches of mercury (inHg), and standard temperature is 15 degrees Celsius, or 59 degrees Fahrenheit. That is it. Two values, and both need to roll off your tongue without hesitation.

One place students stumble is the pressure side of that equation. You may see standard pressure expressed in millibars on weather charts or in avionics menus — 1013.2 mb (or equivalently, 1013.25 hectopascals) is the same pressure as 29.92 inHg, just stated in different units. Knowing this equivalence is not just trivia. If an examiner asks about setting your altimeter and you only know one unit, you will look underprepared. Know both, and know that they are the same physical pressure expressed two different ways.

The temperature value trips up candidates who only memorize one version. Both representations matter: 15°C for technical conversations about aircraft performance and density altitude, and 59°F for the everyday context your examiner might use to check whether you truly understand the concept. If you can only produce one of the two, practice the other until it is equally automatic.

The Standard Lapse Rate and Why It Matters

The standard atmosphere does not just describe conditions at sea level — it describes how those conditions change with altitude. Temperature decreases as you climb, and the ISA defines that rate of change as approximately 2 degrees Celsius per 1,000 feet of altitude gain. This is called the standard lapse rate.

Here is one of the most common errors on oral exams: stating the lapse rate as 2 degrees Fahrenheit per 1,000 feet. That answer is wrong, and it signals to your examiner that you memorized a number without attaching meaning to it. The lapse rate is 2°C per 1,000 feet. If you want a rough Fahrenheit equivalent, it is closer to 3.5°F per 1,000 feet — but the FAA standard is defined in Celsius, and that is the version you should use.

Why does the lapse rate matter practically? Because it feeds directly into density altitude calculations, which drive every performance number in your Pilot's Operating Handbook. On a hot day, the temperature at a given elevation is higher than standard, meaning the air is less dense than the ISA predicts. Your engine makes less power, your wings generate less lift, and your takeoff roll gets longer. The standard atmosphere gives you the baseline; real-world deviations from that baseline tell you how much your performance will suffer.

How Standard Values Connect to Your Altimeter

Your altimeter is essentially a barometer that has been calibrated to report altitude instead of pressure. It does this by assuming the atmosphere conforms to ISA standards. When you set 29.92 inHg in the Kollsman window, you are telling your altimeter to report pressure altitude — your altitude above the standard datum plane, regardless of what the actual sea-level pressure is at your location that day.

This is the setting used above 18,000 feet in the United States (Flight Level airspace), where all aircraft use 29.92 to maintain a common reference. Below 18,000 feet, you dial in the local altimeter setting from an ATIS or ATC to get indicated altitude corrected for local pressure conditions. Understanding the difference between pressure altitude and indicated altitude — and how 29.92 bridges those concepts — shows your examiner that you grasp altimetry at a functional level, not just a rote one.

When conditions deviate from standard in both pressure and temperature, the difference between indicated altitude and true altitude can be significant, especially in cold weather. That is a deeper conversation for density altitude and true altitude questions, but it all roots back to the same ISA baseline you are establishing right now.

Putting It All Together for Your Oral Exam

When your examiner asks about standard sea-level conditions, give a complete, confident answer: pressure is 29.92 inHg (equivalent to 1013.2 mb), temperature is 15°C or 59°F, and temperature decreases at a standard lapse rate of 2°C per 1,000 feet as you climb. Then briefly connect those values to why they matter — altimeter calibration, pressure altitude, and aircraft performance calculations.

That depth of answer demonstrates exactly what examiners are looking for: not just memorized facts, but a pilot who understands how the pieces fit together. The PHAK's Weather Theory chapter on the Standard Atmosphere section is worth a careful read before your checkride, because the concepts there underpin questions across meteorology, performance, and systems.

If you want to practice questions like this in a realistic oral exam format, try SimulatedCheckride.com.