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A particle's tendency to take up water vapor, which depends on composition, solubility and ambient humidity. In a common parameterization, a dimensionless $\kappa$ summarizes soluble-water uptake for use with dry diameter in Köhler theory. A single composition parameter does not remove the importance of particle size or parcel supersaturation.

Anne Willem Omta coauthored research on liquid water.^[ref]

The liquid phase of water. Its density is much greater than that of dilute water vapor, allowing the liquid's specific volume to be neglected in the usual integrated Clausius-Clapeyron relation.

Changes in temperature, water vapor and radical chemistry alter methane removal. Warming can increase its hydroxyl radical reaction coefficient and the hydroxyl radical source from oxygen singlet D atom plus water, tending to reduce chemical lifetime at fixed precursor conditions. Changed emissions and competing radical sinks can modify or reverse that conditional response.

An aerosol particle becomes a freely growing cloud droplet after crossing its Köhler theory growth barrier under the parcel's atmospheric supersaturation history. Counting particles whose critical supersaturation is below a specified peak gives an equilibrium-threshold estimate; calculating that peak requires cooling, water vapor depletion and condensational-growth kinetics. Neither total aerosol number nor dry size alone determines activation.

The temperature at which an air parcel with a specified water vapor partial pressure first becomes saturated on cooling at fixed pressure. It solves $E(T_D)=e_v$ with saturation pressure $E$. A falling dew point during ascent does not by itself imply loss of water vapor.

Fixed water vapor mole fraction gives $d\ln e_v=d\ln p$. Combining hydrostatic balance with the Clausius-Clapeyron relation yields $d\ln T_D/dz=-(g{R}_v/L_v{R}_d)(T_D/T)$. If $T_D/T\simeq 1$, then $T_D(z)\simeq T_D(0)e^{-z/H}$. All ratios use absolute temperature; constant latent heat, fixed composition and approximate parcel–environment temperature equality are required.

Dry air is the atmospheric gas mixture considered with its water vapor removed; reference descriptions also exclude contaminants such as suspended smoke. It is still a mixture, principally nitrogen and oxygen, with argon and smaller constituents. Dryness does not mean chemical purity or absence of oxygen.^[ref]

The distinction is useful for keeping a fixed basis while water vapor changes. If a sample contains $1.00\mathrm{kg}$ of dry air and $0.010\mathrm{kg}$ of water vapor, its moist total mass is $1.010\mathrm{kg}$. The humidity ratio is $0.010$ kilograms of vapor per kilogram of dry air, whereas the specific humidity is $0.010/1.010\approx 0.00990$ kilograms per kilogram of moist air. These differ because their denominators describe different physical samples.^[ref]

Frost is ice deposited or formed on a surface under sufficiently cold conditions. Hoar frost grows directly from water vapor, whereas frozen dew begins as liquid droplets that subsequently freeze. The frost point is the saturation condition relative to ice; it is distinct from the dew point defined relative to liquid water.

At night a surface can lose thermal radiation faster than surrounding air replaces the energy, so the surface temperature can fall below the air temperature measured at standard height. Frost can therefore occur even when that air observation remains above freezing.^[ref] Clouds and stronger mixing alter this energy exchange and hence the likelihood of a surface deposit.

Frozen dew is a surface ice deposit made by first condensing liquid water and then freezing those droplets. It differs in its sequence of phase changes from hoar frost, which grows directly from water vapor. A surface can first cool to its dew point while still above freezing and then cool further.

The distinction matters when interpreting visible deposits: the presence of ice alone does not establish direct vapour deposition. The relevant temperature is that of the surface, which may differ from a nearby sheltered air measurement because of thermal radiation and local heat exchange.^[ref]

Huib J. Bakker coauthored research on liquid water.^[ref]

The liquid phase of water. Its density is much greater than that of dilute water vapor, allowing the liquid's specific volume to be neglected in the usual integrated Clausius-Clapeyron relation.

The humidity ratio $w$ is the mass of water vapor divided by the mass of dry air in a sample:It is commonly expressed as kilograms of water vapor per kilogram of dry air, or grams per kilogram. The denominator excludes the water vapor itself; the related specific humidity is a mass fraction of the entire moist sample.^[ref]

Using the ideal gas law for both components giveswhere $M_v,M_d$ are molar masses, $p_v$ is the water-vapor partial pressure, and $p$ is total pressure. At $p=101.325\mathrm{kPa}$ and $p_v=1.6\mathrm{kPa}$, this yields $w\approx 0.00998$, or about $9.98g/kg$ of dry air.^[ref]

Unlike relative humidity, the humidity ratio does not compare vapor pressure with saturation vapor pressure. Warming air without adding or removing water vapor can change its relative humidity while leaving $w$ unchanged. This distinction matters when comparing incoming and outgoing air in measurements of evaporation or biological heat loss.

The liquid phase of water. Its density is much greater than that of dilute water vapor, allowing the liquid's specific volume to be neglected in the usual integrated Clausius-Clapeyron relation.

Meteorology studies atmospheric structure, motion and the processes producing weather. Uneven heating generates pressure gradients, air motion transports heat and water vapor, and condensation or freezing releases latent heat. Rotation and surface friction help determine how that motion develops.

A weather station measures local conditions, while comparing observations over space reveals evolving atmospheric systems. Wind, temperature, relative humidity and pressure constrain different parts of the physical state; no one sensor completely describes it. Forecasts combine these observations with equations for fluid motion, energy and moisture. Climatology instead emphasizes statistics over longer periods, using many of the same physical mechanisms and observing systems.

Michel F. Kropman coauthored research on liquid water.^[ref]

The liquid phase of water. Its density is much greater than that of dilute water vapor, allowing the liquid's specific volume to be neglected in the usual integrated Clausius-Clapeyron relation.

A saturated ascending air parcel releases latent heat as water vapor condenses, offsetting part of its expansion cooling. Its temperature decreases less steeply than the dry adiabatic lapse rate, with the gradient depending on temperature, pressure, moisture and whether condensate remains in the parcel. It is not a universal constant equal to the typical environmental lapse rate.

Methane is an infrared-active greenhouse gas with absorption in the terrestrial thermal spectrum. Its direct forcing is supplemented by chemically induced changes in tropospheric ozone and stratospheric water vapor. Its finite atmospheric lifetime makes sustained emissions and emission pulses different experiments: stopping emissions permits the methane burden to decline much faster than the long tail of a carbon-dioxide perturbation. Quantitative comparisons require a specified emission mass, time horizon and treatment of indirect effects, as formalized by global warming potential.

Methane reaching the stratosphere is oxidized and is an important source of stratospheric water vapor. That water can feed HOx radical family chemistry and alter radiative cooling and polar-cloud conditions. Methane also converts chlorine atoms into hydrogen chloride, opposing some chlorine-driven ozone loss. These effects compete: increasing methane does not have one universal ozone sign at every altitude and latitude.

The lost ozone-column photolysis shielding is $\Delta N=20(2.69\times 10^{16})=5.38\times 10^{17}\mathrm{molecules}\mathrm{c}{\mathrm{m}}^{-2}$, so its optical-depth decrement is $\sigma \Delta N=0.0538$. Under the stipulated effective-cross-section Beer-Lambert model, the transmitted photodissociation flux increases by $\exp (0.0538)$.

The short-lived oxygen singlet D atom satisfies $[\mathrm{O}({}^{1}D)]\simeq J_{{\mathrm{O}}_3}[{\mathrm{O}}_3]/(k_q[M])$ when quenching dominates. At fixed local ozone, water and air density, hydroxyl radical production is proportional to $J_{{\mathrm{O}}_3}$. ConsequentlyA linear optical-depth approximation would give $5.38\%$. A realistic calculation integrates the wavelength-dependent absorption cross section, quantum yield and actinic flux; clouds and multiple scattering modify the simple transmitted-beam model.

Ozone change can also alter the radiative temperature structure, water vapor and circulation, thereby changing the water reaction channel and the supply of ozone/precursors to the troposphere. If the held-fixed tropospheric ozone assumption is relaxed, altered ozone itself changes the photochemical source of oxygen singlet D atom. These are additional feedbacks, distinct from the isolated UV-transmission calculation above.

At fixed precursor emissions, more tropospheric water vapor in a warmer climate promotes the ozone-photolysis/O(${}^{1}D$)-water loss route, especially in clean tropical lower air. Warmer conditions also accelerate some precursor chemistry; lightning nitrogen oxides, stagnation and stronger stratosphere-to-troposphere ozone transport can increase ozone elsewhere. Thus near-surface regional ozone and the integrated tropospheric burden can respond differently. There is no universal decrease of the global burden from the humidity effect alone; competing chemistry and transport must be evaluated consistently.

For the chemical sink, ${\tau }_{\mathrm{C}{\mathrm{H}}_4}\sim 1/(k[\mathrm{OH}])$. Warming increases the methane–hydroxyl radical rate constant and, with increased water vapor, often increases hydroxyl radical production, tending to shorten the chemical lifetime. Changes in ozone photodissociation, radical recycling, lightning and competing carbon monoxide/VOC/methane consumption can reinforce or oppose that trend. Increasing methane itself consumes hydroxyl radicals and lengthens its methane perturbation lifetime. The physical/chemical warming tendency should therefore be separated from scenarios with changed emissions; shorter chemical lifetime at fixed precursor conditions is a tendency, not an unconditional prediction for the future atmosphere.^[ref]