Humidity & dew point

Why is moist air less dense at the same pressure and temperature?

Compare the gases under the same conditions before discussing cloud.

Vapour is a gas, not a suspended droplet

Water vapour is invisible gaseous water. The visible white parts of cloud consist of tiny liquid droplets, ice particles or both. Atmospheric vapour is important because it supplies cloud and precipitation processes, transports latent energy and absorbs and emits radiation. The amount varies greatly with source, temperature and air-mass history rather than being a fixed percentage of the atmosphere.

At the same total pressure and temperature, a moist gas mixture is less dense than dry air. Water has a molecular mass of about 18 compared with about 29 for the dry-air mixture. For the same pressure and temperature, replacing some heavier dry-air molecules with lighter water-vapour molecules reduces the mass in a given volume. This comparison concerns water in the gas phase; adding suspended liquid water is a different mass-loading effect.

Most atmospheric moisture comes from evaporation from oceans, lakes, wet ground and vegetation, including plant transpiration. Ice and snow can supply vapour by sublimation. Winds and vertical mixing transport it away from the source. An inland location can receive very moist maritime air, and a coastal location can receive dry continental air: proximity alone does not establish the moisture of the arriving air mass.

What does 100% relative humidity mean?

Use vapour pressure rather than imagining that air is a container with a fixed number of water slots.

Actual and saturation vapour pressure

Saturation with respect to liquid water is the condition in which the water-vapour pressure equals the saturation vapour pressure at the current temperature. At a water surface, evaporation and condensation can then balance. The saturation value rises strongly with temperature, which is why the same vapour amount can be close to saturation in cold air and far from it after warming.

Relative humidity is actual vapour pressure divided by saturation vapour pressure at the same air temperature, expressed as a percentage: RH = 100e/es(T). Near 100%, the air is close to saturation. The reference surface matters below freezing because saturation vapour pressure over ice is lower than over supercooled liquid water. An ice-saturated parcel need not be saturated with respect to liquid water.

At constant total pressure, warming with no moisture change raises the saturation vapour pressure and lowers RH. Cooling raises RH until saturation is approached. Adding water vapour raises actual vapour pressure and RH if temperature is held fixed; removing vapour lowers it. A change in RH alone therefore does not prove that moisture was added or removed: temperature and water-vapour changes must be separated.

How close is the air to saturation?

Compare the actual temperature with the temperature at which the existing vapour would saturate.

A temperature linked to moisture

Dew point is the temperature to which air must be cooled at constant pressure and unchanged water-vapour content to reach saturation with respect to liquid water. A higher dew point at the same pressure indicates greater vapour content. It is not the temperature of falling dew, and it does not mean condensation has already occurred throughout the air. Below freezing, distinguish dew point over liquid water from frost point over ice when the product requires that distinction.

The temperature–dew-point spread is T − Td. A small spread means high RH and little cooling is required to reach saturation. When T and Td coincide, RH is approximately 100% relative to the stated reference. In ordinary unsaturated conditions Td is below T. Cloud, fog and precipitation processes can then alter both values, so a near-zero spread does not prove that every surface is already covered in fog.

For ordinary near-surface temperatures, a spread of about 5 °C suggests RH roughly 70–75%, and about 10 °C suggests roughly 50–55%. These are estimates, not a universal linear conversion: the precise ratio depends on temperature. For example, +20 °C with Td +15 °C gives about 73%, while +20 °C with Td +10 °C gives about 53%. Use the product or an appropriate calculation when a precise humidity value is needed.

Why can afternoon RH fall while the dew point stays almost unchanged?

Hold moisture approximately constant, then follow the heating and cooling cycle.

A percentage changes with the reference

On a fair-weather day with little moisture advection or evaporation change, temperature rises after sunrise while dew point can remain approximately steady. The saturation vapour pressure rises and RH falls, often reaching its minimum near the afternoon temperature maximum. This does not mean the warming air has lost all its water vapour; its reference saturation value has changed.

During night-time cooling, the saturation value falls and RH rises, often towards its maximum near the daily temperature minimum. If surfaces cool below the dew point, dew or frost can deposit there before the air at observing height becomes foggy. Fog formation additionally depends on whether a sufficient depth of air reaches saturation and how turbulence and moisture supply evolve.

Real dew points also change: sea-breeze arrival, moisture advection, evaporation, entrainment of dry air or precipitation can modify the vapour content. Therefore the typical inverse temperature–RH daily cycle is most useful when its constant-moisture assumption is stated. A sudden RH rise during a warm afternoon may indicate added moisture rather than local radiative cooling.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.