Barometric Pressure, Altitude and Weather

This page goes past unit conversion: it turns a reading into an altitude, and a three-hour trend into a forecast. For a straight conversion with every unit live in one view, the barometric pressure converter is quicker. Both use the same constants.

Barometric pressure (atmospheric pressure) is the weight of the air column above a given point. It is measured in hectopascals (hPa), millibars (mb), millimeters of mercury (mmHg), or inches of mercury (inHg). Pressure changes predict weather; altitude affects pressure at a known rate.

Key formulas:

Pressure change rate for weather prediction: Trend = (Current Pressure − Pressure 3 Hours Ago) ÷ 3 hours

Barometric Altitude Formula: Altitude = 44,330 × (1 − (P ÷ P₀)^0.1903) meters where P₀ = sea level pressure (1013.25 hPa standard)

Pressure at altitude (simplified): P = P₀ × (1 − 0.0000226 × h)^5.256 where h = altitude in meters

Unit conversions:

• 1 hPa = 1 mb = 0.02953 inHg = 0.75006 mmHg
• Standard sea level: 1013.25 hPa = 29.92 inHg = 760 mmHg

Weather interpretation of pressure trends (per 3 hours):

• Rising > 1 hPa: clearing weather likely
• Steady (±1 hPa): stable conditions
• Falling 1–2 hPa: rain possible
• Falling > 2 hPa: storm approaching
• Falling > 5 hPa: severe storm warning

Worked example, altitude from pressure: Your barometer reads 900 hPa. What is your approximate altitude?

900 ÷ 1013.25 = 0.8882 0.8882^0.1903 = 0.9777 Altitude = 44,330 × (1 − 0.9777) = 44,330 × 0.0223 ≈ 988 meters (about 3,240 feet)

The middle two lines used to read 0.9879 and 0.0121, which are what you get with an exponent of 0.1 rather than 0.1903. The final answer was right and the working underneath it was not, so anyone checking the arithmetic on a calculator arrived at 536 metres and concluded the page was broken.

Reference pressures:

• Dead Sea (lowest land): ~1,065 hPa | Sea level: 1,013.25 hPa | Denver (5,280 ft): ~843 hPa | Everest summit: ~337 hPa

The trend matters far more than the reading. A single pressure value tells you very little on its own, which is why the forecast bands above are the weakest part of this page. What forecasters actually watch is the direction and the rate of change over the last three hours. A steady 1,005 hPa is an ordinary damp day; 1,005 and falling three hectopascals an hour is a different afternoon entirely. If your barometer only shows one number, note it down twice a few hours apart and the trend will tell you more than the absolute figure ever will.

Station pressure versus sea-level pressure. This trips people up with home weather stations. Raw station pressure is what the sensor genuinely measures where it sits, and it drops roughly 1 hPa for every 8 metres of altitude. Sea-level pressure is that reading mathematically corrected back down to sea level so that stations at different heights can be compared. Airports and forecasts quote the corrected figure. If you live at any real elevation and your device reports raw station pressure, every value will look alarmingly low and the fair-or-stormy bands above will be useless. Denver sits near 843 hPa on a perfectly pleasant day.