From what I tried this should be adjusted so that scale is "scale" in every lane and to separate each argument for preprocess with ,

SELECT pgml.train(
project_name => 'preprocessed_model',
task => 'classification',
relation_name => 'weather_data',
target => 'rain',
preprocess => '{
"month": {"encode": {"ordinal": ["jan", "feb", "mar", "apr", "may", "jun", "jul", "aug", "sep", "oct", "nov", "dec"]}}
"clouds": {"encode": "target", scale: "standard"}
"humidity": {"impute": "mean", scale: "standard"}
"temp": {"scale": "standard"}
}'
);