Altitude and Pressure
How Elevation Changes Air Pressure
Learn the RelationshipAs you climb a mountain or fly in an aircraft, atmospheric pressure drops. This relationship between altitude and pressure affects everything from aircraft altimeters to cooking times. 理解する this connection explains why your ears pop, why water boils at lower temperatures at high altitude, and how altimeters work.
Pressure at Various Altitudes
| Altitude (m) | Altitude (ft) | Pressure (hPa) | Pressure (inHg) | % of Sea Level |
|---|---|---|---|---|
| 0 (sea level) | 0 | 1013 | 29.92 | 100% |
| 500 | 1,640 | 955 | 28.21 | 94% |
| 1,000 | 3,281 | 899 | 26.55 | 89% |
| 1,600 (Denver) | 5,280 | 840 | 24.81 | 83% |
| 2,000 | 6,562 | 795 | 23.48 | 78% |
| 3,000 | 9,843 | 701 | 20.70 | 69% |
| 4,000 | 13,123 | 616 | 18.19 | 61% |
| 5,500 | 18,045 | 505 | 14.92 | 50% |
| 8,848 (Everest) | 29,029 | 330 | 9.75 | 33% |
| 10,668 (cruise alt) | 35,000 | 240 | 7.09 | 24% |
Why Pressure Decreases with Altitude
Atmospheric pressure is caused by the weight of air above you. At sea level, you're under the entire atmosphere. As you ascend:
- Less air is above you, so less weight presses down
- Air density decreases (fewer molecules per volume)
- The decrease is exponential, not linear
The Barometric Formula
Pressure decreases roughly 12% per 1000 meters in the lower atmosphere. More precisely:
- At 5,500m: 50% of sea level pressure
- At 11,000m: 25% of sea level pressure
- At 16,000m: 12.5% of sea level pressure
Aviation and Altimeters
How Altimeters Work
Aircraft altimeters are barometers calibrated to show altitude. By measuring pressure and using the standard pressure-altitude relationship, they display height above sea level.
Altimeter Settings
- QNH: Altitude above sea level (standard setting)
- QFE: Height above airfield
- Standard (29.92/1013): Flight levels above 18,000 ft
Why Settings Matter
If actual pressure differs from standard, altimeters read incorrectly:
- Lower pressure = altimeter reads HIGH (dangerous)
- Higher pressure = altimeter reads LOW (conservative)
The saying: "High to low, look out below" warns pilots that flying into lower pressure areas makes their altimeter overread.
Cooking at High Altitude
Lower pressure means water boils at lower temperatures:
| Altitude | Boiling Point | Cooking Impact |
|---|---|---|
| Sea level | 100°C (212°F) | Standard times |
| 1,000m (3,300 ft) | 96.7°C (206°F) | Slight increase |
| 1,600m (5,280 ft) | 94.4°C (202°F) | 10-20% longer |
| 2,000m (6,600 ft) | 93.3°C (200°F) | 20-30% longer |
| 3,000m (10,000 ft) | 90°C (194°F) | 30-40% longer |
High-Altitude Cooking Adjustments
- Boiling: Add more time, not more heat
- Baking: Reduce leavening, increase liquid
- Pressure cookers: More effective at altitude
Other Altitude-Pressure Effects
Vehicle Performance
Engines produce less power at altitude due to lower air density—approximately 3% less power per 1000 feet. Turbochargers compensate by compressing intake air.
Weather Forecasting
Weather maps show sea-level corrected pressure so different elevations can be compared. Denver's actual pressure is always lower than what appears on weather maps.
Sports Performance
High-altitude training (lower oxygen) stimulates red blood cell production. Athletes return to sea level with enhanced oxygen-carrying capacity.
まとめ
Atmospheric pressure decreases predictably with altitude—roughly halving every 5,500 meters. This relationship is fundamental to aviation (altimeters), cooking (boiling points), human physiology (altitude sickness), and vehicle performance. 理解する that pressure drops about 1 hPa per 8 meters (or 1 inHg per 1000 feet) helps explain many altitude-related phenomena.