Barometric altitude refers to the altitude of an aircraft as measured by a barometer, typically an altimeter located in the cockpit. This instrument determines altitude by precisely measuring the surrounding air pressure. Since air pressure naturally decreases with increasing height, an altimeter translates these pressure changes into a corresponding altitude reading.
How Barometric Altitude Works
The fundamental principle behind barometric altitude is the inverse relationship between air pressure and elevation. As you ascend, the column of air above you becomes shorter and less dense, resulting in lower atmospheric pressure.
- The Altimeter: This crucial flight instrument is essentially a calibrated barometer. Inside, a sealed aneroid capsule expands or contracts in response to changes in external air pressure. Mechanical linkages then convert these movements into a needle indication on a dial, displaying the current altitude.
- Standard Atmosphere: To provide a common reference, barometric altimeters are often calibrated based on a theoretical model known as the International Standard Atmosphere (ISA). This model defines standard pressure, temperature, and density at various altitudes, providing a baseline for calculations and consistent readings.
Factors Affecting Barometric Altitude Readings
While highly reliable, barometric altimeters are influenced by atmospheric conditions that can affect their accuracy. Pilots and other users must account for these factors.
1. Temperature Variations
Temperature significantly impacts air density, which in turn affects pressure.
- Warmer Air: Hot air is less dense and exerts less pressure. An altimeter will "read high" in warmer-than-standard conditions, meaning the aircraft is actually lower than indicated.
- Colder Air: Cold air is denser and exerts more pressure. An altimeter will "read low" in colder-than-standard conditions, meaning the aircraft is actually higher than indicated. Pilots often remember the phrase "high to low, look out below," meaning flying from an area of high pressure (or hot air) to low pressure (or cold air) without adjusting the altimeter will result in a lower actual altitude than indicated.
2. Changes in Atmospheric Pressure (Weather)
Weather systems create variations in surface pressure.
- High-Pressure Systems: Indicate denser air, causing the altimeter to read lower than actual altitude if not adjusted.
- Low-Pressure Systems: Indicate less dense air, causing the altimeter to read higher than actual altitude if not adjusted.
3. Altimeter Settings
To compensate for local pressure variations and provide accurate altitude readings relative to a specific reference, altimeters must be regularly adjusted. This is done by setting the altimeter setting (often called the "barometric setting" or "QNH" in aviation) provided by air traffic control or local weather stations.
- QNH (Query Nautical Height): When the altimeter is set to the local barometric pressure corrected to sea level, it will indicate the aircraft's altitude above Mean Sea Level (MSL). This is the most common setting used for en route flight and during approach to landing.
- Example: Before taking off from an airport, a pilot will tune their altimeter to the reported QNH of that airport. If the QNH is 29.98 inches of mercury (inHg), setting this value ensures the altimeter reads the airport's published elevation while on the ground.
- QFE (Query Field Elevation): When the altimeter is set so that it reads zero when on the ground at a specific airfield. This provides altitude above the airfield elevation. While used in some specific flight operations, it's less common for general flight than QNH.
Types of Barometric Altitude
Several related concepts of altitude stem from barometric measurements, each serving a specific purpose in aviation.
- Indicated Altitude: This is the raw altitude directly displayed on the altimeter's dial. It is the reading before any corrections for non-standard temperature or pressure settings are applied.
- Pressure Altitude: This is the altitude indicated when the altimeter's barometric setting is adjusted to the standard sea level pressure of 29.92 inches of mercury (or 1013.25 hectopascals). Pressure altitude is critical for:
- Aircraft performance calculations (e.g., takeoff distance, engine power).
- Maintaining vertical separation above the transition altitude (typically 18,000 feet in the U.S.), where all aircraft set their altimeters to 29.92 inHg, ensuring consistent vertical separation based purely on pressure, regardless of local weather.
- Density Altitude: This is essentially pressure altitude corrected for non-standard temperature. It represents the altitude in the standard atmosphere at which the air density would be the same as that actually existing at your location.
- Importance: Density altitude is paramount for aircraft performance. High density altitude (resulting from high temperatures, high humidity, and/or high elevation) means thinner air, which reduces engine power, propeller efficiency, and wing lift, requiring longer takeoff rolls and reducing climb performance.
- True Altitude (MSL): This is the actual vertical distance of the aircraft above Mean Sea Level. While the goal of barometric altimetry (with proper QNH setting) is to approximate true altitude, it can differ due to temperature variations.
Comparison of Altitude Types
Understanding the distinctions between these altitude measurements is crucial for safe and efficient flight operations.
| Altitude Type | Definition | Primary Use |
|---|---|---|
| Indicated Altitude | The direct reading shown on the altimeter. | Primary reference for pilots during flight, indicates pressure height relative to the altimeter setting. |
| Pressure Altitude | Altitude when altimeter is set to 29.92 inHg (1013.25 hPa). | Used for flight above the transition altitude, essential for aircraft performance calculations, and setting standard levels for air traffic control. |
| Density Altitude | Pressure altitude corrected for non-standard temperature. | Crucial for determining actual aircraft performance (e.g., takeoff/landing distance, climb rate) under prevailing atmospheric conditions. |
| True Altitude | The actual vertical distance above Mean Sea Level (MSL). | Used for navigation, terrain clearance, and determining the aircraft's actual height relative to sea level. It's the aim of barometric altimetry with correct QNH setting. |
Applications of Barometric Altitude
Beyond aviation, barometric altitude finds utility in various other fields:
- Aviation: It is the primary means for pilots to determine their height, maintain safe separation from other aircraft, and ensure adequate clearance from terrain and obstacles. Air traffic controllers use reported barometric altitudes to manage airspace.
- Outdoor Activities: Hikers and mountaineers use barometric altimeters in watches or handheld devices to track their elevation changes and gauge their progress.
- Meteorology: Weather stations use barometric pressure readings to forecast weather patterns, and meteorological balloons transmit pressure data to determine atmospheric profiles.
Barometric altitude, therefore, is a fundamental measurement based on air pressure, providing vital information for navigation, safety, and performance across various disciplines.
