What are "Alternate Minimums"?
FAR 91.169 states that IFR flight plans must include an alternate airport unless the weather is at least 2000 ft ceiling and 3 miles visibility, from one hour before to one hour afterwards (1-2-3 rule). 

The same regulation also states that the alternate airport must meet the following critera:


  (c) IFR alternate airport weather minima. Unless otherwise authorized
  by the Administrator, no person may include an alternate airport in an
  IFR flight plan unless appropriate weather reports or weather
  forecasts, or a combination of them, indicate that, at the estimated
  time of arrival at the alternate airport, the ceiling and visibility
  at that airport will be at or above the following weather minima:
  
  
    (1) If an instrument approach procedure has been published in part 97
    of this chapter, or a special instrument approach procedure has been
    issued by the Administrator to the operator, for that airport, the
    following minima:  
    
    
      (i) For aircraft other than helicopters: The
      alternate airport minima specified in that procedure, or if none are
      specified the following standard approach minima:  
      
      (A) For a precision approach procedure. Ceiling 600 feet and visibility 2 statute miles. 
      
      (B) For a non-precision approach procedure. Ceiling 800 feet and visibility 2 statute miles.