Psychrometric Chart at 4,000 ft (87.5 kPa)
Moist-air properties drawn for 4,000 ft, where the atmosphere is 13.6% thinner than at sea level.
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The psychrometric chart at 4,000 ft
This chart is drawn for a barometric pressure of 87.5 kPa (12.69 psia, 25.84 in Hg, 87,511 Pa) — the standard atmosphere at 4,000 ft (1,219 m).
What changes at 4,000 ft
The same air state, solved at 87.5 kPa and at sea level. Relative humidity is referenced to saturation at the local pressure, so it is unchanged by definition — everything read from it moves.
| Air state | Wet bulb | Humidity ratio | Enthalpy | Specific volume |
|---|---|---|---|---|
| Summer design 35 °C, 40% RH |
23.5 °C 23.9 at sea level (-0.4 K) |
16.42 g/kg 14.13 at sea level (+16.2%) |
77.3 kJ/kg 71.5 at sea level |
1.037 m³/kg 0.893 at sea level (+16.2%) |
| Indoor comfort 24 °C, 50% RH |
16.7 °C 17.1 at sea level (-0.4 K) |
10.79 g/kg 9.30 at sea level (+16.1%) |
51.6 kJ/kg 47.8 at sea level |
0.992 m³/kg 0.854 at sea level (+16.1%) |
| Cool and humid 10 °C, 80% RH |
8.2 °C 8.3 at sea level (-0.1 K) |
7.06 g/kg 6.09 at sea level (+16%) |
27.9 kJ/kg 25.4 at sea level |
0.939 m³/kg 0.810 at sea level (+16%) |
Read the specific-volume column first: it is up 16.2% at 4,000 ft, so a fan moving a fixed volume moves that much less mass. Sensible capacity follows density, which is why a coil selected against a sea-level chart is oversold here. The wet bulb sits lower for the same dry-bulb and humidity, which is the other half of the story — it is why evaporative cooling gets more effective with altitude, not less.
Computed with PreciseFluids using ASHRAE equations, at 87,511 Pa. For the general theory — how to read the chart and plot HVAC processes — see the main psychrometric chart page.
Frequently asked
Why do I need a different psychrometric chart at 4,000 ft?
A psychrometric chart is only valid at the barometric pressure it was drawn for. At 4,000 ft standard pressure is 87.5 kPa (12.69 psia), about 13.6% below sea level. That shifts the saturation curve, widens wet-bulb depression and raises specific volume, so properties read from a sea-level chart are wrong here.
What is the barometric pressure at 4,000 ft?
The standard atmosphere gives 87,511 Pa at 4,000 ft (1,219 m) — 87.5 kPa, 12.69 psia, or 25.84 inches of mercury. This chart is drawn at that pressure rather than rescaled from sea level.
Does air hold more or less moisture at 4,000 ft?
More, per kilogram of dry air. At 35 °C and 40% relative humidity the humidity ratio is 16.42 g/kg at 4,000 ft against 14.13 g/kg at sea level, +16.2%. Relative humidity itself is unchanged — it is referenced to saturation at the local pressure.
Is evaporative cooling better at 4,000 ft?
Yes. Wet-bulb depression widens with altitude: air at 35 °C and 40% RH has a wet bulb of 23.5 °C at 4,000 ft against 23.9 °C at sea level, -0.4 K. That extra depression is what an evaporative cooler works against, which is why the technique suits high, dry places.
Why does cooling capacity drop at 4,000 ft?
Because a fan moves volume, not mass. Specific volume at 4,000 ft is 1.037 m³/kg against 0.893 at sea level, +16.2%, so the same airflow carries that much less dry air. Sensible capacity follows density, so a coil selected against a sea-level chart is oversold here.
Other elevations
Pick the pressure closest to your site — or read the full guide to the psychrometric chart on the main chart page, which covers how to read it and how to plot HVAC processes.
- Sea level — 101.3 kPa (14.70 psia)
- 2,000 ft — 94.2 kPa (13.66 psia)
- 6,000 ft — 81.2 kPa (11.78 psia)
- 8,000 ft — 75.3 kPa (10.92 psia)
- 10,000 ft — 69.7 kPa (10.11 psia)
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