Update for CF Version:31

The surface temperature is the temperature at the interface, not the bulk temperature of the medium above or below. The surface called "surface" means the lower boundary of the atmosphere. A variable whose standard name has the form number_of_days_with_X_below|above_threshold is a count of the number of days on which the condition X_below|above_threshold is satisfied. It must have a coordinate variable or scalar coordinate variable with the a standard name of X to supply the threshold(s). It must have a climatological time variable, and a cell_methods entry for within days which describes the processing of quantity X before the threshold is applied. A number_of_days is an extensive quantity in time, and the cell_methods entry for over days should be "sum".

A velocity is a vector quantity. "Northward" indicates a vector component which is positive when directed northward (negative southward). Land ice velocity is defined as a two-dimensional vector, with no vertical component. "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves.

A velocity is a vector quantity. “Downward" indicates a vector component which is positive when directed downward (negative upward). Downward air velocity is the vertical component of the 3D air velocity vector. The standard name upward_air_velocity may be used for a vector component with the opposite sign convention.

Mole fraction is used in the construction "mole_fraction_of_X_in_Y", where X is a material constituent of Y. A chemical species denoted by X may be described by a single term such as "nitrogen" or a phrase such as "nox_expressed_as_nitrogen". The construction "dry_atmosphere_mole_fraction" means that the quantity refers to the whole atmospheric column and is calculated as the total number of particles of X in the column divided by the number of dry air particles in the same column, i.e. the effect of water vapor is excluded. For localized values within the atmospheric medium, standard names including "in_air" are used. The chemical formula for carbon dioxide is CO2.

"Single scattering albedo" is the fraction of radiation in an incident light beam scattered by the particles of an aerosol reference volume for a given wavelength. It is the ratio of the scattering and the extinction coefficients of the aerosol particles in the reference volume. A coordinate variable with a standard name of radiation_wavelength or radiation_frequency should be included to specify either the wavelength or frequency. "Aerosol" means the system of suspended liquid or solid particles in air (except cloud droplets) and their carrier gas, the air itself. "Ambient_aerosol" means that the aerosol is measured or modelled at the ambient state of pressure, temperature and relative humidity that exists in its immediate environment. "Ambient aerosol particles" are aerosol particles that have taken up ambient water through hygroscopic growth. The extent of hygroscopic growth depends on the relative humidity and the composition of the particles. To specify the relative humidity and temperature at which the quantity described by the standard name applies, provide scalar coordinate variables with standard names of "relative_humidity" and "air_temperature". The specification of a physical process by the phrase "due_to_" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.

Mole fraction is used in the construction "mole_fraction_of_X_in_Y", where X is a material constituent of Y. A chemical species denoted by X may be described by a single term such as "nitrogen" or a phrase such as "nox_expressed_as_nitrogen". The construction "dry_atmosphere_mole_fraction" means that the quantity refers to the whole atmospheric column and is calculated as the total number of particles of X in the column divided by the number of dry air particles in the same column, i.e. the effect of water vapor is excluded. For localized values within the atmospheric medium, standard names including "in_air" are used. Methane is a member of the group of hydrocarbons known as alkanes. There are standard names for the alkane group as well as for some of the individual species. The chemical formula for methane is CH4.

A velocity is a vector quantity. "Upward" indicates a vector component which is positive when directed upward (negative downward). Upward air velocity is the vertical component of the 3D air velocity vector. The standard name downward_air_velocity may be used for a vector component with the opposite sign convention.

Specific humidity is the mass fraction of water vapor in (moist) air. In an atmosphere model, stratiform cloud is that produced by large-scale convergence (not the convection schemes). The specification of a physical process by the phrase due_to_process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase. "tendency_of_X" means derivative of X with respect to time. "specific" means per unit mass.

"Amount" means mass per unit area. Zero change in land ice amount is an arbitrary level. "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves.

The quantity with standard name acoustic_signal_roundtrip_travel_time_in_sea_water is the time taken for an acoustic signal to propagate from the emitting instrument to a reflecting surface and back again to the instrument. In the case of an instrument based on the sea floor and measuring the roundtrip time to the sea surface, the data are commonly used as a measure of ocean heat content.

A velocity is a vector quantity. "Eastward" indicates a vector component which is positive when directed eastward (negative westward). Land ice velocity is defined as a two-dimensional vector, with no vertical component. "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves.