Humidity is simply the presence of water vapor, the gaseous form of water, in the air. The answer to whether humidity can exist in freezing temperatures is definitively yes, as its existence does not rely on the temperature being above the freezing point of water.
This concept often seems contradictory because people associate high humidity with the sticky heat of summer. While warmer air can hold significantly more water vapor than cold air, the moisture does not vanish just because the temperature drops below 0°C (32°F). Humidity persists in the atmosphere regardless of temperature conditions.
Defining Humidity in Cold Air
Understanding humidity in cold environments requires distinguishing between the ways it is measured. Absolute humidity refers to the actual mass of water vapor present in a specific volume of air, such as grams of water per cubic meter. Specific humidity describes the mass of water vapor compared to the total mass of the moist air.
Cold air has a much lower capacity to hold water vapor than warm air, meaning absolute humidity is lower on a freezing winter day. The saturation point—the maximum amount of water vapor the air can contain before condensation—decreases substantially with temperature. Low temperatures cause water vapor molecules to cluster together more easily and transition into liquid or solid states.
The most commonly reported measurement, relative humidity, expresses the water vapor content as a percentage of the maximum amount the air can hold at that specific temperature. A small amount of water vapor in cold air can easily saturate it, leading to a high relative humidity value, sometimes reaching 100%. For example, air at -10°C (14°F) holding 2 grams of water vapor per cubic meter might be at 100% relative humidity, even though air at 30°C (86°F) could hold over 30 grams. This explains why weather reports can show high humidity percentages despite temperatures being well below freezing.
The Physics of Water Vapor Below Freezing
When water vapor is present in air below the freezing point, it undergoes unique phase changes that skip the liquid state. The process where water vapor transitions directly into ice, bypassing the liquid phase, is known as deposition. Deposition is responsible for the formation of frost, where water molecules in the air solidify instantly upon contact with a cold surface.
The reverse process, where ice or snow turns directly into water vapor without first melting, is called sublimation. Sublimation is observed when snow disappears from the ground when the temperature remains below freezing, or when ice cubes slowly shrink in a freezer.
Another element of cold humidity involves supercooled water, a temporary state where liquid water droplets remain unfrozen below 0°C. This occurs because the liquid water requires microscopic ice-nucleating particles to serve as a template for ice crystal formation. Without these particles, water can remain liquid in the atmosphere down to approximately -40°C.
Real-World Effects of Cold Humidity
The interaction of water vapor and freezing temperatures results in several weather phenomena. One common effect is hoar frost, which consists of delicate, feather-like ice crystals that form on surfaces like tree branches and wires. Hoar frost is the direct result of deposition, occurring when the air is humid and the surface temperature is lower than the air’s frost point.
Another manifestation of cold humidity is ice fog, which forms in extremely cold conditions, typically below -30°C. Unlike regular fog formed by liquid water droplets, ice fog consists of tiny ice crystals suspended in the air. This occurs when high relative humidity forces the water vapor to deposit directly into a solid form.
Rime ice is caused by the presence of supercooled water droplets in the air. Rime forms when these liquid droplets, suspended in freezing fog or clouds, strike an object. The impact causes the supercooled droplets to instantly freeze, building up a milky, opaque layer of ice on the windward side of the object.