full text of the classic FAA guide
In your preflight preparation, be aware of or alert for phenomena that may produce IFR or marginal VFR flight conditions. Current charts and special analyses along with forecast and prognostic charts are your best sources of information. You may get your preflight weather from a briefer; or, you may rely on recorded briefings; and you always have your own inflight observations. No weather observation is more current or more accurate than the one you make through your cockpit window. In any event, your understanding of IFR producers will help you make better preflight and inflight decisions.
Do not fly VFR in weather suitable only for IFR. If you do, you endanger not only your own life but the lives of others both in the air and on the ground. Remember, the single cause of the greatest number of general aviation fatal accidents is “continued VFR into adverse weather.” The most common cause is vertigo, but you also run the risk of flying into unseen obstructions. Furthermore, pilots who attempt to fly VFR under conditions below VFR minimums are violating Federal Aviation Regulations.
The threat of flying VFR into adverse weather is far greater than many pilots might realize. A pilot may press onward into lowering ceiling and visibility complacent in thinking that better weather still lies behind him. Eventually, conditions are too low to proceed; he no longer can see a horizon ahead. But when he attempts to turn around, he finds so little difference in conditions that he cannot re-establish a visual horizon. He continued too far into adverse weather; he is a prime candidate for vertigo.
Don't let an overwhelming desire to reach your destination entice you into taking the chance of flying too far into adverse weather. The IFR pilot may think it easier to “sneak” through rather than go through the rigors of getting an IFR clearance. The VFR pilot may think, “if I can only make it a little farther.” If you can go IFR, get a clearance before you lose your horizon. If you must stay VFR, do a 180 while you still have a horizon. The 180 is not the maneuver of cowards. Any pilot knows how to make a 180; a good pilot knows when.
Be especially alert for development of:
Fog the following morning when at dusk temperature—dew point spread is 15° F or less, skies are clear, and winds are light.
Fog when moist air is flowing from a relatively warm surface to a colder surface.
Fog when temperature-dew point spread is 5° F or less and decreasing.
Fog or low stratus when a moderate or stronger moist wind is blowing over an extended upslope. (Temperature and dew point converge at about 4° F for every 1,000 feet the air is lifted.)
Steam fog when air is blowing from a cold surface (either land or water) over warmer water.
Fog when rain or drizzle falls through cool air. This is especially prevalent during winter ahead of a warm front and behind a stationary front or stagnating cold front.
Low stratus clouds whenever there is an influx of low level moisture overriding a shallow cold air mass.
Low visibilities from haze and smoke when a high pressure area stagnates over an industrial area.
Low visibilities due to blowing dust or sand over semiarid or arid regions when winds are strong and the atmosphere is unstable. This is especially prevalent in spring. If the dust extends upward to moderate or greater heights, it can be carried many miles beyond its source.
Low visibility due to snow or drizzle.
An undercast when you must make a VFR descent.
Expect little if any improvement in visibility when:
Fog exists below heavily overcast skies.
Fog occurs with rain or drizzle and precipitation is forecast to continue.
Dust extends to high levels and no frontal passage or precipitation is forecast.
Smoke or haze exists under heavily overcast skies.
A stationary high persists over industrial areas.