Meteorology Class 3: Understanding Atmospheric Pressure

Introduction to Pressure

• Pressure affects our ears during altitude changes
• Surface pressure patterns help predict weather

What is Pressure?

• Formula: Pressure (P) = Force (F) / Area (A)
• Column of air: As altitude increases, air particles decrease
• Force of air reduces with altitude, leading to lower pressure

Pressure and Altitude

• Pressure decreases with altitude
• Standard rate: 1 hectopascal decrease per 27 to 30 feet
• Pressure measured in hectopascals (equal to millibars)
• Sea level pressure: 1013.25 hPa or 29.92 inches of mercury

Altimetry

• Altimeters measure pressure difference from a datum point
• Key settings: QFE, QNH, and Standard
• QFF: Adjusted for daily conditions, includes temperature correction
  • Temperature correction: 4 feet per 1000 feet per degree of iso deviation
  • Alternative: 1% altitude error for every 2.5 degrees iso deviation

Example Calculation

• Aircraft at flight level 200, temperature -40°C
• QNH at nearby airfield: 998 hPa
• Calculate obstacle clearance with temperature corrections

Isobar Charts

• Isobars: Lines of equal pressure used in weather prediction
  • High pressure marked as 'H' and low pressure as 'L'
• Close isobars indicate stronger winds
• Used for weather prediction

Summary

• Pressure decreases with altitude
• Measured in hectopascals/millibars
• QNH and QFF used for altitude and weather prediction
• Isobars show areas of equal pressure; closer lines mean stronger winds

Note

• For more details on altimetry and pressure settings, refer to additional resources on general navigation series.