The Driest Air Mass on Earth: Continental Polar (cP) and Its Antarctic Cousin

If you're asking which air mass has the lowest absolute humidity (water vapor per unit mass of air), the answer is Continental Polar (cP) — specifically, its most extreme manifestation: Continental Antarctic (cA).

Why cP/cA Air Is Bone-Dry

• Source regions: High-latitude continental interiors — Siberia, northern Canada, Greenland, Antarctica.
• Temperature: -40°C to -60°C (winter). Cold air has exponentially lower saturation vapor pressure (Clausius-Clapeyron). At -40°C, saturation mixing ratio is ~0.1 g/kg. At -60°C: ~0.01 g/kg.
• No moisture source: Frozen ground, ice sheets, snow cover — no evaporation. Any moisture precipitates as ice crystals (diamond dust).
• Subsidence: Polar highs feature sinking air → adiabatic warming → relative humidity plummets further (though absolute humidity stays near zero).

Air Mass Classification Refresher

cT (Continental Tropical) Is Hot-Dry, Not Cold-Dry

Saharan air (cT) has low relative humidity but higher absolute humidity than cP/cA because warm air holds more vapor. At 45°C, even 5% RH = ~4 g/kg — 400× more water vapor than -50°C air at saturation.

When cP Air Moves Out

• Cold air outbreaks: cP surges south → "Arctic blast," "polar vortex" events. Warms rapidly over snow-free ground → RH drops to <10% → extreme fire weather, static electricity, respiratory stress.
• Lake-effect snow: cP crosses Great Lakes → picks up massive moisture → dumps snow downwind.
• Modification: Within 2–3 days over warmer surfaces, cP loses its "driest air" title as it moistens.

Measurement Reality

Lowest recorded dewpoints: -60°C to -70°C at Antarctic stations (Vostok, Dome A). Equivalent to 0.001–0.01 g/kg water vapor. That's the practical floor for Earth's atmosphere.

Bottom Line

Continental Polar (cP) — and its ultimate form, Continental Antarctic (cA) — hold the title for driest air mass by absolute humidity. Not because they're "dry" in a relative sense, but because cold physics leaves almost no room for water vapor.