Charcoal could be damp, or your wood may not be dry enough.....
Keep in mind that in colder weather there will be more visible smoke due to cold air being heated and releasing the humidity (like when your glasses fog up when coming in from the cold). However once the cooker heats up this condition will no longer exist and there should be no large amounts of white smoke.
Make sure your charcoal hasn't been stored in a damp location, or try another brand.
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