Detailed Study Notes on Atmospheric Pressure

Overview of Atmospheric Pressure

  • Discussion about the importance of atmospheric pressure after exploring air mass and its movements due to the sun.

Introduction to Atmospheric Pressure

  • Atmospheric pressure pertains to the weight of air, which, despite being invisible, has measurable weight.

  • Different measurements can be utilized to quantify atmospheric pressure.

Measurement of Atmospheric Pressure

Units of Pressure Measurement

  • Atmospheric pressure is commonly expressed in various units, including:

    • Pound per Square Inch (psi): A standard unit used to measure air mass weight in aviation; a one-inch by one-inch column from sea level to the top of the atmosphere.

      • At sea level, atmospheric pressure is 14.7 psi.

    • Inches of Mercury: A historic unit derived from mercury barometers, frequently utilized in aviation in the U.S. and Canada.

      • The standard pressure at sea level is 29.92 inches of mercury.

    • Ectopascals (hPa): The metric unit equivalent; standard pressure is 1013.2 hPa.

      • Commonly used in Europe for altimeters and weather reports.

    • Millibars (mb): Another metric measurement; identical value conversion as ectopascals at 1013.2 mb.

    • Millimeters of Mercury (mmHg): An additional, less common unit; one standard unit is 760 mmHg.

Explanation of Barometers

  • A barometer measures atmospheric pressure using a column of mercury contained in a tube.

  • The column rises or falls depending on the changes in pressure acting on it:

    • More atmospheric pressure results in higher mercury levels in the barometer tube.

    • This linkage illustrates how pressure affects volume measurement within a contained space.

Change in Pressure with Altitude

  • Pressure decreases as altitude increases; specifically, it drops about one inch of mercury per 1,000 feet of ascent.

    • At 18,000 feet, the weight of the air mass drops to approximately half that at sea level, showing a clear relationship between altitude and pressure.

Summary of Key Takeaways

  • Standard atmospheric pressure values:

    • Sea level: 14.7 psi, 29.92 inches of mercury, 1013.2 hPa, 1013.2 mb, and 760 mmHg.

    • The decrease of pressure with altitude demonstrates a vital application for aviation, impacting flight safety and altimeter readings.

  • Remember: The pressure decreases at a rate of approximately one inch of mercury for every 1,000 feet gained in altitude, a crucial concept for navigating airspace effectively.