Hygrometer
The Core Question: Does Barometric Pressure Change Dew Point?
When weather enthusiasts, hikers, or HVAC technicians observe a barometric pressure drop on their hygrometer or barometer, a common question arises:
Does dropping air pressure change the dew point temperature?
The answer depends on whether the pressure change is caused by an incoming weather front at a fixed location or by changing physical altitude.
At sea level, an incoming low-pressure storm system might cause barometric pressure to drop from 1013 hPa to 990 hPa (a ~2% shift). That pressure difference alone changes the dew point by only a fraction of a degree.
However, when you move vertically through the atmosphere—such as driving from Denver (5,280 ft) up to a mountain pass at 10,000 ft—the massive decrease in atmospheric pressure causes the dew point to drop substantially.
The Physics: Dalton's Law of Partial Pressures
To understand why pressure connects to dew point, we look at Dalton's Law of Partial Pressures.
The total atmospheric pressure ($P_{\text{total}}$) measured by a barometer is the sum of the pressures exerted by every individual gas in the atmosphere: $$P_{\text{total}} = P_{\text{nitrogen}} + P_{\text{oxygen}} + P_{\text{argon}} + e$$
Where $e$ is the partial vapor pressure of water.
The dew point is mathematically defined as the temperature to which air must be cooled at constant pressure for the saturation vapor pressure ($e_s$) to equal the actual vapor pressure ($e$): $$e_s(T_d) = e$$
Notice what is missing from this equation: total atmospheric pressure does not dictate saturation vapor pressure. Saturation vapor pressure ($e_s$) depends almost exclusively on temperature, governed by the Clausius-Clapeyron relation.
Therefore, if the absolute number of water molecules in a room remains unchanged and the volume is constant, shifting the barometric pressure slightly will not significantly alter the dew point.
What Happens When Air Climbs in Elevation?
While changing pressure in a rigid container has little effect on dew point, the real atmosphere is fluid and unconfined.
When wind forces moist air up a mountain slope (or when warm air rises through convection):
- Atmospheric pressure drops: At 10,000 feet (3,000 m), atmospheric pressure is only about 700 hPa—nearly 30% lower than at sea level.
- The air parcel expands: Because the surrounding air is thinner, the rising parcel expands in volume.
- Molecules spread out: Expansion dilutes the concentration of water molecules per cubic meter, directly lowering the partial vapor pressure ($e$).
- The dew point drops: With lower partial vapor pressure, the air needs to cool to a lower temperature to reach saturation.
| Elevation | Typical Barometric Pressure | Temperature Lapse Rate (Dry) | Dew Point Lapse Rate |
|---|---|---|---|
| Sea Level (0 ft) | 1013.25 hPa (29.92 inHg) | Base Temperature | Base Dew Point |
| 2,000 ft (610 m) | 942 hPa (27.82 inHg) | −10.8°F (−6.0°C) | −3.0°F (−1.7°C) |
| 5,000 ft (1,524 m) | 843 hPa (24.90 inHg) | −27.0°F (−15.0°C) | −7.5°F (−4.2°C) |
| 10,000 ft (3,048 m) | 697 hPa (20.58 inHg) | −54.0°F (−30.0°C) | −15.0°F (−8.3°C) |
The Cloud Base: The Lifting Condensation Level (LCL)
Because temperature cools faster with altitude (~5.4°F per 1,000 ft) than the dew point drops (~1.5°F per 1,000 ft), the temperature and dew point converge as air ascends.
The exact altitude where the temperature drops to meet the dew point is the Lifting Condensation Level (LCL)—the flat bottom base of cumulus clouds:
$$\text{Cloud Base Height (feet)} \approx \frac{\text{Surface Temperature (°F)} - \text{Surface Dew Point (°F)}}{4.4} \times 1,000$$
If the ground temperature is 80°F and the dew point is 58°F: $$\text{Cloud Base} = \frac{80 - 58}{4.4} \times 1,000 = 5,000 \text{ feet}$$
At 5,000 feet, the pressure drop and temperature cooling bring the air to 100% relative humidity, and liquid cloud droplets condense out of thin air.
Why Mountain Climates Feel So Dry
Skiers and mountain hikers frequently suffer from chapped lips, dry skin, and nosebleeds. Even if a mountain resort reports 60% relative humidity on a cold winter afternoon, the dew point is exceptionally low (often below 10°F / −12°C).
The reduced barometric pressure at high altitude limits the maximum absolute quantity of water vapor the atmosphere can physically sustain per unit volume. The air is chemically and physically drier, accelerating moisture evaporation from human skin and respiratory passages.
Frequently Asked Questions
Does high barometric pressure make humidity feel worse?
High barometric pressure often brings calm winds and clear skies. If the air mass is moist (as in a summer Bermuda High over the US East Coast), high temperatures and solar radiation combine with high dew points to create stifling heat index values. The discomfort is caused by high water vapor concentration, not the barometric pressure itself.
Can you calculate dew point from barometric pressure and temperature alone?
No. Barometric pressure and temperature tell you the air's density, but they say nothing about how much moisture is present. You must also measure relative humidity, wet-bulb temperature, or partial vapor pressure.
Does a falling barometer mean rain is guaranteed?
A falling barometer indicates an approaching low-pressure trough, which promotes upward vertical air motion. While rising air cools and condenses water vapor into clouds, rain only occurs if the air mass contains sufficient moisture (a high enough dew point) to produce precipitation.
How do digital hygrometers adjust for pressure?
Most consumer hygrometers calculate dew point using standard sea-level approximations (such as the Magnus-Tetens formula at 1013.25 hPa). High-precision weather instruments and psychrometers incorporate an onboard barometric pressure sensor to adjust the psychrometric constant for true elevation.