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Any expected precipitation; or

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Operating at night.

Know the minimum safe altitude for each segment of your flight.  All VFR sectional charts include a maximum elevation figure (MEF) in each quadrangle. The MEF is determined by locating the highest obstacle (natural or man-made) in each quadrangle, and rounding up by 100 to 300 feet.  
Charts for IFR navigation include a Minimum En route Altitude (MEA) and a Minimum Obstruction Clearance Altitude (MOCA). Jeppesen charts depict a Minimum Off Route Altitude (MORA), while FAA/NACO charts show an Off Route Obstruction Clearance Altitude (OROCA) that guarantees a 1,000-foot obstacle clearance in non-mountainous terrain and a 2,000 foot obstacle clearance in mountainous terrain.
In addition to these sources, many GPS navigators
(both panel-mount and handheld) include a feature showing the Minimum Safe Altitude (MSA), En route Safe Altitude (ESA), or Minimum En route Altitude (MEA) relative to the aircraft’s position.  If you have access to such equipment, be sure you understand how to access and interpret the information about safe altitudes.
The Air Safety Foundation’s Terrain Avoidance Plan is another helpful resource.
Passenger Plan: A number of GA weather accidents have been associated with external or social pressures, such as the pilot’s reluctance to appear “cowardly” or to disappoint passengers eager to make or continue a trip.  There is almost always pressure to launch, and pressure to continue.  Even the small investment in making the trip to the airport can create pressure to avoid “wasted” time. 

For this reason, your weather planning should include preflighting your passengers (and anyone waiting at your destination) as well as your aircraft.  If you jointly plan for weather contingencies and brief your passengers before you board the aircraft, you as the pilot will be less vulnerable later on to the pressure to continue in deteriorating weather conditions. Suggestions:
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DO use the worksheet in Appendix 4 to develop personal minimums that will help you make the toughest go / no-go and continue / divert decisions well in advance of any specific flight.

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DO be aware that the presence of others can influence your decision-making and your willingness to take risks, and let your passengers know up front that

safety is your top priority. Share your personal minimums with your passengers and anyone who might be waiting for you at the destination.

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DO establish “weather check” checkpoints every 25-30 nm along the route, at which you will reevaluate conditions. If possible, have your passengers assist by tracking progress and conditions at each weather checkpoint.

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DO use your pre-established personal minimums to determine exactly what conditions will trigger a diversion at any given weather checkpoint. Let your passengers know what these conditions are.  
　
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本文鏈接地址：General Aviation Pilot’s Guide to Preflight Weather Planning, Weather Self-Briefings, and Weather Decision Making(10)