slight modification to the previous post, sorry, see
http://www.postgresql.org/docs/8.0/interactive/plpgsql-structure.html
for the real docs.
CREATE OR REPLACE FUNCTION "public"."cproc_hli" (bgtemp NUMERIC,
humidity NUMERIC, wspeed NUMERIC) RETURNS NUMERIC AS $$
BEGIN
IF ($1 < 25) THEN
SELECT (10.66 * (0.28 * $2)+(1.3 * $1) - $3) ;
ELSE
SELECT (8.62 * (0.38 * $2) + (1.55 * $1) - (0.5 * $3) + (exp(- $3 +
2.4))) ;
END IF ;
END ;
$$ LANGUAGE 'plpgsql' VOLATILE RETURNS NULL ON NULL INPUT SECURITY
INVOKER;