Chapter 3.  Description of the Data

Table of Contents

3.1. Units
3.2. Long Name
3.3. Standard Name
3.4. Ancillary Data
3.5. Flags

The attributes described in this section are used to provide a description of the content and the units of measurement for each variable. We continue to support the use of the units and long_name attributes as defined in COARDS. We extend COARDS by adding the optional standard_name attribute which is used to provide unique identifiers for variables. This is important for data exchange since one cannot necessarily identify a particular variable based on the name assigned to it by the institution that provided the data.

The standard_name attribute can be used to identify variables that contain coordinate data. But since it is an optional attribute, applications that implement these standards must continue to be able to identify coordinate types based on the COARDS conventions.

3.1. Units

The units attribute is required for all variables that represent dimensional quantities (except for boundary variables defined in Section 7.1, “Cell Boundaries” and climatology variables defined in Section 7.4, “Climatological Statistics”). The value of the units attribute is a string that can be recognized by UNIDATA"s Udunits package [UDUNITS], with a few exceptions that are given below. The Udunits package includes a file udunits.dat, which lists its supported unit names. Note that case is significant in the units strings.

The COARDS convention prohibits the unit degrees altogether, but this unit is not forbidden by the CF convention because it may in fact be appropriate for a variable containing, say, solar zenith angle. The unit degrees is also allowed on coordinate variables such as the latitude and longitude coordinates of a transformed grid. In this case the coordinate values are not true latitudes and longitudes which must always be identified using the more specific forms of degrees as described in Section 4.1, “Latitude Coordinate” and Section 4.2, “Longitude Coordinate”.

Units are not required for dimensionless quantities. A variable with no units attribute is assumed to be dimensionless. However, a units attribute specifying a dimensionless unit may optionally be included. The Udunits package defines a few dimensionless units, such as percent, but is lacking commonly used units such as ppm (parts per million). This convention does not support the addition of new dimensionless units that are not udunits compatible. The conforming unit for quantities that represent fractions, or parts of a whole, is "1". The conforming unit for parts per million is "1e-6". Descriptive information about dimensionless quantities, such as sea-ice concentration, cloud fraction, probability, etc., should be given in the long_name or standard_name attributes (see below) rather than the units.

The units level, layer, and sigma_level are allowed for dimensionless vertical coordinates to maintain backwards compatibility with COARDS. These units are not compatible with Udunits and are deprecated by this standard because conventions for more precisely identifying dimensionless vertical coordinates are introduced (see Section 4.3.2, “Dimensionless Vertical Coordinate”).

The Udunits syntax that allows scale factors and offsets to be applied to a unit is not supported by this standard. The application of any scale factors or offsets to data should be indicated by the scale_factor and add_offset attributes. Use of these attributes for data packing, which is their most important application, is discussed in detail in Section 8.1, “Packed Data”.

Udunits recognizes the following prefixes and their abbreviations.

Table 3.1. Supported Units

FactorPrefixAbbreviation FactorPrefixAbbreviation
1e1deca,dekada 1e-1decid
1e2hectoh 1e-2decic
1e3kilok 1e-3millim
1e6megaM 1e-6microu
1e9gigaG 1e-9nanon
1e12teraT 1e-12picop
1e15petaP 1e-15femtof
1e18exaE 1e-18attoa
1e21zettaZ 1e-21zeptoz
1e24yottaY 1e-24yoctoy