2.5 Differences between Attributes and Variables
In contrast to variables, which are intended for bulk data, attributes
are intended for ancillary data, or information about the data. The
total amount of ancillary data associated with a netCDF object, and
stored in its attributes, is typically small enough to be
memory-resident. However variables are often too large to entirely fit
in memory and must be split into sections for processing.
Another difference between attributes and variables is that variables
may be multidimensional. Attributes are all either scalars
(single-valued) or vectors (a single, fixed dimension).
Variables are created with a name, type, and shape before they are
assigned data values, so a variable may exist with no values. The
value of an attribute is specified when it is created, unless it is a
zero-length attribute.
A variable may have attributes, but an attribute cannot have
attributes. Attributes assigned to variables may have the same units
as the variable (for example, valid_range) or have no units (for
example, scale_factor). If you want to store data that requires units
different from those of the associated variable, it is better to use a
variable than an attribute. More generally, if data require ancillary
data to describe them, are multidimensional, require any of the
defined netCDF dimensions to index their values, or require a
significant amount of storage, that data should be represented using
variables rather than attributes.
