The weight of the atmosphere above a location, measured in hPa (hectopascals) or millibars. Sea-level standard is 1013.25 hPa. Falling pressure warns of storms; rising pressure brings clear skies.
basicWhat is Atmospheric Pressure?
Atmospheric pressure is the weight of the atmosphere pressing down on a given point. It is measured in hectopascals (hPa) — equivalent to millibars (mb) — or in inches of mercury (inHg) in older systems.
The standard sea-level atmospheric pressure is 1013.25 hPa (or 29.92 inHg). This is the worldwide reference baseline. Actual pressure at any place and time varies by:
- Altitude — pressure decreases ~1 hPa per 8 metres near the surface
- Weather — low-pressure systems can drop pressure 30-40 hPa below standard; high-pressure systems lift it 10-20 hPa above
- Temperature — warmer air is less dense → lower pressure
- Latitude — slight variation due to Earth’s rotation
For weather forecasting, the direction and rate of pressure change is often more informative than the absolute value. Falling pressure means a low-pressure system is approaching — and with it, clouds, wind, rain. Rising pressure means high pressure is taking over, bringing clear and calm conditions.
How atmospheric pressure works
Air has mass. At sea level, a 1 cm² column of air extending all the way up to the top of the atmosphere weighs about 1.03 kg. We don’t feel this crushing weight because:
- The pressure is balanced — pushing equally from all sides
- Internal body fluids and gases match the external pressure
When we ascend a mountain or fly in an aircraft, the column of air above us is shorter, so the pressure decreases. At 5,500 m (~Everest base camp), pressure is about half of sea level. At 8,800 m (Everest summit), pressure is about one-third of sea level.
Pressure ranges in South Asia
Sea-level pressure in different conditions:
| Condition | Pressure (hPa) |
|---|---|
| Super Cyclonic Storm core (e.g., Amphan, Phailin) | 900-940 |
| Severe cyclone core | 940-980 |
| Tropical depression | 990-1005 |
| Monsoon trough | 994-1004 |
| Standard sea-level | 1013.25 |
| Strong winter high pressure | 1020-1030 |
| Cold dry winter morning | 1025-1035 |
Notable South Asian pressure events:
- 1999 Odisha Super Cyclone: 912 hPa core pressure — one of the lowest ever recorded in the Bay of Bengal
- 2020 Cyclone Amphan: 925 hPa — Category 5 equivalent
- 2007 Cyclone Sidr: 944 hPa — devastating Bangladesh
- Delhi winter mornings: regularly 1020-1025 hPa
High and low pressure
High pressure (anticyclones):
- Air sinks toward the surface, suppressing cloud formation
- Winds blow clockwise (in Northern Hemisphere) around a high-pressure center
- Brings clear skies, calm winds, stable conditions
- Winter examples: Siberian High influences north India
- Summer examples: Subtropical High over Tibet supports the monsoon
Low pressure (cyclones):
- Air rises at the center, cooling and condensing into clouds
- Winds blow counterclockwise (NH) around the low
- Brings clouds, rain, wind, unsettled weather
- Tropical cyclones (Bay of Bengal, Arabian Sea) are extreme low-pressure events
- Monsoon depressions are weaker low-pressure systems
How pressure changes affect weather
Falling pressure (e.g., 1015 → 1005 hPa over 6 hours):
- Storm approaching
- Wind picking up
- Rain or thunderstorms likely within 6-12 hours
- Possible cyclone if pressure continues to fall
Rising pressure:
- Storm passing or dissipating
- Skies clearing
- Wind dying down
- Cooler, drier conditions ahead
Steady high pressure:
- Sustained fair weather
- May trap pollutants near surface (winter inversion in IGP)
Steady low pressure:
- Persistent unsettled weather
- Active monsoon trough position
For tropical cyclone tracking, pressure drops below 990 hPa in the Bay of Bengal during cyclone season is an immediate warning signal.
How pressure is measured
Barometer: instrument for measuring atmospheric pressure.
- Mercury barometer (traditional): measures the height of a column of mercury supported by atmospheric pressure. Sea level = 760 mm Hg = 29.92 inches = 1013.25 hPa.
- Aneroid barometer: a sealed metal capsule that flexes with pressure changes — common in home weather stations.
- Digital pressure sensor: piezoelectric or capacitive sensors used in smartphones, smart watches, automatic weather stations.
- Radiosondes: balloon-borne sensors providing pressure profiles aloft.
Mausam Online pressure data
Mausam Online displays surface atmospheric pressure alongside temperature, humidity, wind and weather conditions on every city page. Pressure values come from Open-Meteo’s ECMWF model output.
How to interpret what you see:
- 990-1005 hPa: low pressure — expect unsettled weather
- 1006-1015 hPa: normal range
- 1016-1025 hPa: high pressure — expect stable conditions
- Falling rapidly (-5 hPa in 3 hours): severe weather risk
- Rising rapidly: improving weather
Frequently asked questions
Why does air pressure drop before a storm? Low-pressure systems are regions where air is rising rather than sinking. As air rises, it cools and condenses into clouds and rain. The rising motion reduces the weight of the air column at the surface, lowering measured pressure. The deeper the rising air (and the more intense the system), the lower the pressure.
What’s the lowest pressure ever recorded? 870 hPa — Typhoon Tip in the Western Pacific, 1979. The lowest in the Bay of Bengal was around 912 hPa during the 1999 Odisha Super Cyclone. Cyclone Amphan (2020) reached 925 hPa.
Can I predict weather using just a barometer? Yes, to a limited extent — barometric tendency (rising/falling) is one of the oldest forecasting tools. Falling pressure = storm coming. Rising pressure = clearing. Old sailing-ship captains relied on it for weather avoidance. Modern numerical forecasts are far more accurate, but barometric tendency remains a useful short-term cue.
Why does pressure decrease with altitude? Because the column of air above you gets shorter as you go higher. The weight of the air column decreases proportionally to the remaining air mass overhead. Pressure halves approximately every 5,500 m of altitude gain.
Where can I see live pressure for my city? Mausam Online displays surface pressure on every city page. See Delhi, Mumbai, Chennai, Kolkata, Dhaka, Karachi.