No variable or dimension names are standardized by this convention. Instead we follow the lead of the NUG and standardize only the names of attributes and some of the values taken by those attributes. The overview provided in this section will be followed with more complete descriptions in following sections. Appendix A, Attributes contains a summary of all the attributes used in this convention.
We recommend that the NUG defined attribute
Conventions be given the string value "
CF-1.6" to identify datasets
that conform to these conventions.
The general description of a file's contents should be contained in
the following attributes:
references (Section 2.6.2, “Description of file contents”). For backwards compatibility
with COARDS none of these attributes is required, but their use is
recommended to provide human readable documentation of the file
Each variable in a netCDF file has an associated description which
is provided by the attributes
long_name attributes are
defined in the NUG and the
standard_name attribute is
defined in this document.
units attribute is required for all variables
that represent dimensional quantities (except for boundary variables
defined in Section 7.1, “Cell Boundaries”. The values of the
units attributes are character strings that are
recognized by UNIDATA's Udunits package [UDUNITS], (with exceptions allowed as
discussed in Section 3.1, “Units”).
standard_name attributes are used to describe the
content of each variable. For backwards compatibility with COARDS neither
is required, but use of at least one of them is strongly recommended. The
use of standard names will facilitate the exchange of climate and forecast
data by providing unambiguous identification of variables most commonly
Four types of coordinates receive special treatment by these conventions: latitude, longitude, vertical, and time. Every variable must have associated metadata that allows identification of each such coordinate that is relevant. Two independent parts of the convention allow this to be done. There are conventions that identify the variables that contain the coordinate data, and there are conventions that identify the type of coordinate represented by that data.
There are two methods used to identify variables that contain
coordinate data. The first is to use the NUG-defined "coordinate
variables." The use of coordinate variables is required for all
dimensions that correspond to one dimensional space or time
coordinates. In cases where coordinate variables are not
applicable, the variables containing coordinate data are identified by the
Once the variables containing coordinate data are identified,
further conventions are required to determine the type of coordinate
represented by each of these variables. Latitude, longitude, and time
coordinates are identified solely by the value of their
units attribute. Vertical coordinates with units of
pressure may also be identified by the
Other vertical coordinates must use the attribute
positive which determines whether the direction of
increasing coordinate value is up or down. Because identification of a
coordinate type by its units involves the use of an external software
package [UDUNITS], we provide the optional attribute
axis for a direct identification of coordinates that
correspond to latitude, longitude, vertical, or time axes.
Latitude, longitude, and time are defined by internationally
recognized standards, and hence, identifying the coordinates of these
types is sufficient to locate data values uniquely with respect to time
and a point on the earth's surface. On the other hand identifying the
vertical coordinate is not necessarily sufficient to locate a data value
vertically with respect to the earth's surface. In particular a model may
output data on the dimensionless vertical coordinate used in its
mathematical formulation. To achieve the goal of being able to spatially
locate all data values, this convention includes the definitions of common
dimensionless vertical coordinates in Appendix D, Dimensionless Vertical Coordinates. These definitions provide a mapping
between the dimensionless coordinate values and dimensional values that
can be uniquely located with respect to a point on the earth's surface.
The definitions are associated with a coordinate variable via the
attributes. For backwards compatibility with COARDS use of these
attributes is not required, but is strongly recommended.
It is often the case that data values are not representative of
single points in time and/or space, but rather of intervals or
multidimensional cells. This convention defines a
bounds attribute to specify the extent of intervals or
cells. When data that is representative of cells can be described by
simple statistical methods, those methods can be indicated using the
cell_methods attribute. An important application of
this attribute is to describe climatological and diurnal
Methods for reducing the total volume of data include both packing
and compression. Packing reduces the data volume by reducing the precision
of the stored numbers. It is implemented using the attributes
are defined in the NUG. Compression on the other hand loses no precision,
but reduces the volume by not storing missing data. The attribute
compress is defined for this purpose.