Vertical variation of pressure
Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"
Meaning
Atmospheric pressure falls as you go higher, because less air is left above you. Near the ground, pressure falls by about 1 mb for every 10 m of height. The rate of fall slows higher up because the air gets thinner. About half of all the air lies below 5.5 km. Air does not rush upwards, even though pressure is much higher below than above. This is because gravity pulls the air down just as strongly. This balance is called hydrostatic balance.
| Level | Pressure (mb) |
|---|---|
| Sea level | 1,013.25 |
| 1 km | 898.76 |
| 5 km | 540.48 |
| 10 km | 265.00 |
Example
At Khardung La in Ladakh, pressure is about 650 mb. That is far below the 1,013 mb at sea level, so the air has much less oxygen.
Don't confuse with
- Horizontal distribution of pressure: this is the change in pressure from place to place at the same level, shown by isobars. It drives winds. The change with height is much larger, but gravity balances it.