Atmospheric pressure is the pressure exerted by the weight of the air column above a point. At sea level, it averages 1013.25 hPa (hectopascals) or 1 atm. It decreases with altitude and varies according to weather conditions.
Discover FizziQ
How to measure it in class
With the FizziQ app, the smartphone barometer allows measurement of atmospheric pressure.
Steps:
- Open FizziQ and select the “Barometer” or “Pressure” instrument
- Note the pressure at ground level
- Go up a few floors and observe the pressure decrease
- Calculate the pressure variation per floor (approximately 0.1 hPa/m)
- Track pressure variations over several days and relate to weather
- Compare with weather forecasts
Scientific activities on this topic
- Measuring altitude with the barometer: https://www.fizziq.org/en/activities/altimeter/
- Weather monitoring
- Pressure variation in an elevator
Learn more
Origin of pressure:
Earth’s atmosphere has a mass of approximately 5 x 10 to the 18th kg. This mass of air exerts pressure on the Earth’s surface. The higher we go, the less air above us, so the pressure decreases.
Pressure units:
| Unit | Value at 0 m |
|---|---|
| Pascal (Pa) | 101,325 |
| Hectopascal (hPa) | 1013.25 |
| Millibar (mbar) | 1013.25 |
| Atmosphere (atm) | 1 |
| mmHg (Torr) | 760 |
Variation with altitude:
Pressure decreases by approximately half every 5500 m.
| Altitude | Pressure (hPa) |
|---|---|
| 0 m (sea) | 1013 |
| 1000 m | 900 |
| 3000 m | 700 |
| 5500 m | 500 |
| Mont Blanc (4810 m) | 540 |
Formula
Barometric formula (isothermal atmosphere):
P(h) = P0 x exp(-h/H)
where:
- P(h): pressure at altitude h (Pa)
- P0: pressure at sea level (101325 Pa)
- h: altitude (m)
- H: scale height approximately equals 8500 m
Local approximation: Delta P approximately equals -12 Pa/m or -0.12 hPa/m
Application examples
- A weather depression corresponds to P < 1013 hPa (bad weather likely)
- An anticyclone corresponds to P > 1013 hPa (good weather likely)
- Ears pop on airplanes due to pressure variation
- Crushed water bottles at altitude illustrate pressure difference
FAQ
Q: How does the smartphone barometer work? A: It is a piezoelectric sensor that measures the deformation of a membrane under pressure. It is very accurate (plus or minus 1 hPa).
Q: Can altitude be measured with the barometer? A: Yes, but you need to know the reference pressure at sea level (which varies with weather). It is less accurate than GPS for significant movements.
Q: Why does pressure drop before rain? A: Depressions (low pressure) are associated with rising air movements that create clouds. A pressure drop often announces bad weather.
Q: What accuracy can be expected with FizziQ? A: Approximately plus or minus 1 hPa, which is plus or minus 8 m in altitude. Sufficient to measure the height of a building.
Related concepts
Barometer - Altitude - Meteorology - Pascal - Force and Pressure - Atmosphere