Hectopascal (hPa) - Unit Information & Conversion
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What is a Hectopascal?
Hectopascal (hPa) = 100 pascals. Standard unit for atmospheric pressure in meteorology. 1 hPa = 1 millibar exactly. Sea level pressure: ~1013 hPa. Used worldwide for weather reports, aviation altimetry.
History of the Hectopascal
Adopted by meteorologists in 1980s to replace millibar while transitioning to SI units. Convenient because 1 hPa = 1 mbar exactly, allowing seamless transition. Now standard for weather services globally (WMO recommendation).
Quick Answer
What is a Hectopascal? Hectopascal (hPa) = 100 pascals = 1 millibar exactly. Standard unit for atmospheric pressure in weather forecasts. Sea level: ~1013 hPa. High pressure: >1020 hPa (fair weather). Low pressure: <1000 hPa (storms). Use our pressure converter for conversions.
Key Facts: Hectopascal
| Property | Value |
|---|---|
| Symbol | hPa |
| Quantity | Pressure |
| System | Metric/SI Derived |
| Derived from | Pascal |
| Category | Pressure |
| Standard Body | NIST / ISO |
Definition
1 hPa = 100 Pa = 0.1 kPa = 1 mbar (exactly) = 0.001 bar = 0.0145 PSI = 0.000987 atm = 0.75 mmHg
Why Hectopascal?
The hectopascal perfectly bridges SI and traditional meteorology:
- SI compliance: Uses SI base unit (Pascal)
- Backward compatibility: 1 hPa = 1 mbar exactly (no conversion needed)
- Convenient scale: Typical atmospheric pressure ~1,000 hPa (manageable numbers)
- Global adoption: WMO (World Meteorological Organization) standard since 1980s
History: The Transition from Millibar
The Evolution of Pressure Units
- Pre-1900s: Mercury column (mmHg) standard for all pressure
- 1900s-1980s: Meteorologists adopted millibar (mbar = 1/1000 bar)
- Why: Sea level pressure ~1,013 mbar (convenient round number)
- Advantage: Easy to work with 3-4 digit numbers
- 1980s: International metric shift pushed for SI units
- Problem: 1 mbar = 100 Pa (awkward multiplier)
- Solution: 1 hectopascal = 100 Pa = 1 mbar (exact equivalence!)
- 1980s-present: Smooth transition using hPa = mbar equivalence
- No conversion confusion (same number, new name)
- Allows decades of data to be "relabeled" without recalculation
Why This Worked
The brilliance: WMO chose hectopascal specifically because:
- 100 Pa = 1 hPa (clean, easy power of 10)
- 1 hPa = 1 mbar (exact, no confusion)
- Sea level pressure stayed ~1,013 hPa (same numbers as before)
- All existing meteorology infrastructure could transition seamlessly
Result: Weather services worldwide switched from mbar to hPa with zero data loss.
Common Uses
Meteorology: Weather forecasts, atmospheric pressure reporting (worldwide standard). Aviation: Altimeter settings, flight level pressure (ICAO standard). Oceanography: Atmospheric pressure corrections for sea level measurements. Climatology: Historical pressure data, pressure trends. Medical: Altitude acclimatization, pressure chambers, hyperbaric medicine.
Real-World Examples
Weather System Pressures
- Very high pressure: 1,040-1,050 hPa (anticyclone, clear weather)
- Record high: 1,084.8 hPa (Agata, Russia, December 2010) - exceptional cold, high density air
- High pressure system: 1,020-1,035 hPa (fair weather)
- Characterized by: Clear skies, low wind, stable conditions
- Typical duration: 3-7 days per system
- Average sea level: 1,013.25 hPa (standard atmosphere)
- By definition (exactly)
- Corresponds to: 760 mmHg, 14.696 PSI, 29.92 inHg
- Low pressure system: 980-1,000 hPa (cloudy, rain)
- Characterized by: Cloud formation, winds converge, precipitation likely
- Typical duration: 3-5 days passage
- Storm: 960-980 hPa (strong low pressure)
- Wind speed: 25-40 knots typical
- Precipitation: Significant to heavy
- Hurricane (Category 1): 980-990 hPa (75-95 mph winds)
- Hurricane (Category 3): 945-965 hPa (111-129 mph winds)
- Hurricane (Category 5): <920 hPa (157+ mph winds)
- Strongest hurricane: 882 hPa (Typhoon Tip, 1979, Western Pacific)
- Strongest tornado: Associated with even lower pressures (~900 hPa in immediate vortex)
Pressure and Weather Prediction
Rising pressure trend (→ improving weather):
- Faster rise (>5 hPa/3 hours): Clear weather coming soon
- Slow rise (2-5 hPa/3 hours): Gradual improvement
- Plateau high: Stable, anticyclonic conditions
Falling pressure trend (→ deteriorating weather):
- Faster fall (>5 hPa/3 hours): Storm approaching rapidly
- Steady fall (2-5 hPa/3 hours): Weather deterioration expected
- Rapid drop: Severe storm, possible hurricane formation
Rate of pressure change is more predictive than absolute pressure value!
Altitude Pressure (ISA Standard)
- Sea level: 1,013.25 hPa
- 1,000 ft (305m): 977 hPa (~96% of sea level)
- 5,000 ft (1,524m): 843 hPa (~83%)
- 10,000 ft (3,048m): 697 hPa (~69%, cabin pressure minimum)
- 18,000 ft (5,486m): 500 hPa (FL180, half sea level)
- Mount Everest (8,848m): 313 hPa (~31%)
- Concorde cruise (18,000m): ~78 hPa
- Space station orbit (400km): <<0.0001 hPa (vacuum)
Aviation Flight Levels
- FL000 (sea level): 1,013 hPa
- FL100 (10,000 ft): 697 hPa
- FL180 (18,000 ft): 500 hPa (transition altitude)
- FL250 (25,000 ft): 378 hPa (commercial cruise)
- FL300 (30,000 ft): 301 hPa (typical long-haul cruise)
- FL350 (35,000 ft): 238 hPa (long-range wide-body)
- FL390 (39,000 ft): 188 hPa (high efficiency altitude)
Note: Flight levels are pressure-referenced, not altitude-referenced!
Geographic Variations
- Dead Sea (-430m, Earth's lowest point): ~1,063 hPa
- Amsterdam (sea level): ~1,013 hPa
- Denver (1,609m, "Mile High City"): ~835 hPa
- Mexico City (2,240m): ~760 hPa
- La Paz, Bolivia (3,640m, highest capital): ~640 hPa
- Lhasa, Tibet (3,656m): ~640 hPa
- Mount Kilimanjaro (5,895m): ~540 hPa
- Mount Everest (8,848m): 313 hPa
Seasonal and Regional Patterns
- Tropical regions: Relatively stable ~1,010-1,015 hPa year-round
- Temperate winter: Can drop to 970-980 hPa during major storms
- Temperate summer: Typically 1,015-1,025 hPa
- Polar regions: Highly variable, extreme lows possible (<960 hPa)
- Monsoonal regions: Dramatic seasonal shifts (1,000+ hPa to 960 hPa)
Pressure Measurement and Instruments
Barometers
-
Mercury barometer: Gold standard for precision
- Accuracy: ±1 hPa
- Traditional measurement: 760 mmHg = 1,013 hPa
- Advantages: Direct, no calibration needed
- Disadvantages: Fragile, contains toxic mercury
-
Aneroid barometer: Mechanical, portable
- Accuracy: ±2-3 hPa
- Uses: Weather stations, portable instruments
- Advantages: Robust, no mercury
- Disadvantages: Requires periodic calibration
-
Electronic barometer: Modern standard
- Accuracy: ±0.5-1 hPa
- Uses: Smartphones, weather stations, scientific instruments
- Advantages: Digital, networkable, precise
- Disadvantages: Requires power, can drift
Weather Station Calibration
- Reference standard: Official meteorological barometer at sea level
- Calibration interval: Annual for professional stations
- Accuracy requirement: ±1 hPa for weather reporting
- Corrections applied:
- Altitude correction (station height above sea level)
- Temperature correction (mercury density changes)
- Latitude correction (gravity varies)
Altimetry in Aviation
Pilots use pressure-altitude conversion for safe flight:
- Altimeter setting: Adjusted to local station pressure
- QFE (Field Elevation): Altimeter reads 0 at runway
- QNH (sea level): Altimeter corrected to sea level equivalent
- QFF: Sea level pressure with theoretical ISA adjustment
- Standard setting: FL180 and above use 1,013.25 hPa universally (prevents mid-air collisions)
Practical Applications
Weather Forecasting
Professional meteorologists use pressure patterns to predict weather:
- Pressure gradient: Wind speed depends on pressure gradient (close isobars = strong winds)
- Pressure tendency: Rate of change more predictive than absolute value
- Pressure anomaly: Deviation from seasonal normal indicates unusual systems
- Pressure oscillations: Diurnal (daily) pressure cycles inform convective activity
Health and Altitude
- Sea level: Normal atmospheric pressure, optimal oxygen availability
- 2,000m (Denver, Mexico City): ~80% pressure, most people acclimate
- 3,000m: Altitude sickness threshold (~65% pressure)
- 4,000m: Significant acclimatization needed (~60% pressure)
- 5,500m: Extreme altitude, limited oxygen (~50% pressure)
- 8,848m (Everest): "Death zone", only 31% pressure, human survival limits
Industrial Applications
- Pressure cooker: Increased pressure raises boiling point (120-130 hPa absolute)
- Vacuum packaging: Reduced pressure removes oxygen (~10 hPa)
- Hyperbaric chambers: Increased pressure (240+ hPa) for medical treatment
- Clean rooms: Positive pressure (1.25 hPa above ambient) prevents contamination
- Pneumatic systems: Compressed air systems often 600-800 hPa (gauge pressure)
Comparison with Other Pressure Units
Quick Conversion Reference
| Unit | 1,000 hPa Equivalent |
|---|---|
| Pascal (Pa) | 100,000 Pa |
| Kilopascal (kPa) | 100 kPa |
| Millibar (mbar) | 1,000 mbar |
| Bar | 1 bar |
| PSI | 14.5 PSI |
| mmHg | 750 mmHg |
| inHg | 29.53 inHg |
| Atmosphere (atm) | 0.987 atm |
Why hPa Over Alternatives?
| Aspect | hPa | kPa | mbar | PSI |
|---|---|---|---|---|
| SI compliant | ✅ Yes | ✅ Yes | ❌ No | ❌ No |
| Meteorology standard | ✅✅✅ | ✅ | ✅✅ (legacy) | ❌ |
| Familiar numbers | ✅ (1,000s) | ❌ (100s) | ✅ (1,000s) | ❌ (10s-100s) |
| Backward compatible | ✅ (= mbar) | ✅ | ✅ (= hPa) | ❌ |
| Global adoption | ✅ (WMO standard) | ✅ | Legacy | Limited |
Hectopascal Conversion Formulas
To Pascal:
To Kilopascal:
To Megapascal:
To Bar:
To Millibar:
To Atmosphere:
To Technical Atmosphere:
To Torr:
To Millimeter of Mercury:
To Inch of Mercury:
To Pound per Square Inch:
To Kilopound per Square Inch:
To Kilogram-force per Square Centimeter:
To Kilogram-force per Square Meter:
To Millimeter of Water Column:
To Inch of Water Column:
Frequently Asked Questions
Formula: PSI = hPa × 0.0145 Examples:
- 1,013 hPa = 14.7 PSI (sea level)
- 1,000 hPa = 14.5 PSI
- 900 hPa = 13.05 PSI
- 800 hPa = 11.6 PSI hPa to PSI converter →
Convert Hectopascal
Need to convert Hectopascal to other pressure units? Use our conversion tool.