There are lots of information in official doc pages. In particular, there are two sections in Ferret Users Guide you should look carefully.
Sec. 2.5: ASCII DATA
Some ways to read ASCII data in Ferret. Pick one that apply for your case.
Sec. 2.2 NETCDF DATA
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Paulo Santiago
Doutorado em Meteorologia
Programa de Pós-Graduação em Meteorologia
Instituto Nacional de Pesquisas Espaciais
Tel.: (12) 3186 8637