AC 00-6A - AVIATION WEATHER
For Pilots and Flight Operations Personnel
PART I. WHAT YOU SHOULD KNOW ABOUT WEATHER
Chapter 12 - COMMON IFR PRODUCERS
Most aircraft accidents related to low ceilings and visibilities involve pilots who are not instrument qualified. These pilots attempt flight by visual reference into weather that is suitable at best only for instrument flight. When you lose sight of the visual horizon, your senses deceive you; you lose sense of direction - you can't tell up from down. You may doubt that you will lose your sense of direction, but one good scare has changed the thinking of many a pilot. "Continued VFR into adverse weather" is the cause of about 25 percent of all fatal general aviation accidents.
Minimum values of ceiling and visibility determine Visual Flight Rules. Lower ceiling and/or visibility require instrument flight. Ceiling is the maximum height from which a pilot can maintain VFR in reference to the ground. Visibility is how far he can see. AVIATION WEATHER SERVICES (AC 00-45) contains details of ceiling and visibility reports.
Don't let yourself be caught in the statistics of "continued VFR into adverse weather." IFR producers are fog, low clouds, haze, smoke, blowing obstructions to vision, and precipitation. Fog and low stratus restrict navigation by visual reference more often than all other weather parameters.
Fog is a surface based cloud composed of either water droplets or ice crystals. Fog is the most frequent cause of surface visibility below 3 miles, and is one of the most common and persistent weather hazards encountered in aviation. The rapidity with which fog can form makes it especially hazardous. It is not unusual for visibility to drop from VFR to less than a mile in a few minutes. It is primarily a hazard during takeoff and landing, but it is also important to VFR pilots who must maintain visual reference to the ground.
Small temperature-dew point spread is essential for fog to form. Therefore, fog is prevalent in coastal areas where moisture is abundant. However, fog can occur anywhere. Abundant condensation nuclei enhances the formation of fog. Thus, fog is prevalent in industrial areas where byproducts of combustion provide a high concentration of these nuclei. Fog occurs most frequently in the colder months, but the season and frequency of occurrence vary from one area to another.
Fog may form (1) by cooling air to its dew point, or (2) by adding moisture to air near the ground. Fog is classified by the way it forms. Formation may involve more than one process.
Radiation fog is relatively shallow fog. It may be dense enough to hide the entire sky or may conceal only part of the sky. "Ground fog" is a form of radiation fog. As viewed by a pilot in flight, dense radiation fog may obliterate the entire surface below him; a less dense fog may permit his observation of a small portion of the surface directly below him. Tall objects such as building, hills, and towers may protrude upward through ground fog giving the pilot fixed references for VFR flight. Figure 117 illustrates ground fog as seen from the air.
Figure 117. Ground fog as seen from the air.
Conditions favorable for radiation fog are clear sky, little or no wind, and small temperature-dew point spread (high relative humidity). The fog forms almost exclusively at night or near daybreak. Terrestrial radiation cools the ground; in turn, the cool ground cools the air in contact with it. When the air is cooled to its dew point, fog forms. When rain soaks the ground, followed by clearing skies, radiation fog is not uncommon the following morning.
Radiation fog is restricted to land because water surfaces cool little from nighttime radiation. It is shallow when wind is calm. Winds up to about 5 knots mix the air slightly and tend to deepen the fog by spreading the cooling through a deeper layer. Stronger winds disperse the fog or mix the air through a still deeper layer with stratus clouds forming at the top of the mixing layer.
Ground fog usually "burns off" rather rapidly after sunrise. Other radiation fog generally clears before noon unless clouds move in over the fog.
Advection fog forms when moist air moves over colder ground or water. It is most common along coastal areas but often develops deep in continental areas. At sea it is called "sea fog." Advection fog deepens as wind speed increases up to about 15 knots. Wind much stronger than 15 knots lifts the fog into a layer of low stratus or stratocumulus.
The west coast of the United States is quite vulnerable to advection fog. This fog frequently forms offshore as a result of cold water as shown in figure 118 and then is carried inland by the wind. During the winter, advection fog over the central and eastern United States results when moist air from the Gulf of Mexico spreads northward over cold ground as shown in figure 119. The fog may extend as far north as the Great Lakes. Water areas in northern latitudes have frequent dense sea fog in summer as a result of warm, moist, tropical air flowing northward over colder Arctic waters.
Figure 118. Advection fog off the coast of California.
Figure 119. Advection fog over the southeastern United States and Gulf Coast. The fog often may spread to the Great Lakes and northern Appalachians.
A pilot will notice little difference between flying over advection fog and over radiation fog except that skies may be cloudy above the advection fog. Also, advection fog is usually more extensive and much more persistent than radiation fog. Advection fog can move in rapidly regardless of the time of day or night.
Upslope fog forms as a result of moist, stable air being cooled adiabatically as it moves up sloping terrain. Once the upslope wind ceases, the fog dissipates. Unlike radiation fog, it can form under cloudy skies. Upslope fog is common along the eastern slopes of the Rockies and somewhat less frequent east of the Appalachians. Upslope fog often is quite dense and extends to high altitudes.
When relatively warm rain or drizzle falls through cools air, evaporation from the precipitation saturates the cool air and forms fog. Precipitation induced fog can become quite dense and continue for an extended period of time. This fog may extend over large areas, completely suspending air operations. It is most commonly associated with warm fronts, but can occur with slow moving cold fronts and with stationary fronts.
Fog induced by precipitation is in itself hazardous as is any fog. It is especially critical, however, because it occurs in the proximity of precipitation and other possible hazards such as icing, turbulence, and thunderstorms.
Ice Fog occurs in cold weather when the temperature is much below freezing and water vapor sublimates directly as ice crystals. Conditions favorable for its formation are the same as for radiation fog except for cold temperature, usually -25° F or colder. It occurs mostly in the Arctic regions, but is not unknown in middle latitudes during the cold season. Ice fog can be quite blinding to someone flying into the sun.
Stratus clouds, like fog, are composed of extremely small water droplets or ice crystals suspended in air. An observer on a mountain in a stratus layer would call it fog. Stratus and fog frequently exist together. In many cases there is no real line of distinction between the fog and stratus; rather, one gradually merges into the other. Flight visibility may approach zero in stratus clouds. Stratus tends to be lowest during night and early morning, lifting or dissipating due to solar heating during the late morning or afternoon. Low stratus clouds often occur when moist air mixes with a colder air mass or in any situation where temperature-dew point spread is small.
Haze is a concentration of salt particles or other dry particles not readily classified as dust or other phenomenon. It occurs in stable air, is usually only a few thousand feet thick, but sometimes may extend as high as 15,000 feet. Haze layers often have definite tops above which horizontal visibility is good. However, downward visibility from above a haze layer is poor, especially on a slant. Visibility in haze varies greatly depending upon whether the pilot is facing the sun. Landing an aircraft into the sun is often hazardous if haze is present.
Smoke concentrations form primarily in industrial areas when air is stable. It is most prevalent at night or early morning under a temperature inversion but it can persist throughout the day. Figure 120 illustrates smoke trapped under a temperature inversion.
Figure 120. Smoke trapped in stagnant air under an inversion.
When skies are clear above haze or smoke, visibility generally improves during the day; however, the improvement is slower than the clearing of fog. Fog evaporates, but haze or smoke must be dispersed by movement of air. Haze or smoke may be blown away; or heating during the day may cause convective mixing spreading the smoke or haze to a higher altitude, decreasing the concentration near the surface. At night or early morning, radiation fog or stratus clouds often combine with haze or smoke. The fog and stratus may clear rather rapidly during the day but the haze and smoke will linger. A heavy cloud cover above haze or smoke may block sunlight preventing dissipation; visibility will improve little, if any, during the day.
BLOWING RESTRICTIONS TO VISIBILITY
Strong wind lifts blowing dust in both stable and unstable air. When air is unstable, dust is lifted to great heights (as much as 15,000 feet) any may be spread over wide areas by upper winds. Visibility is restricted both at the surface and aloft. When air is stable, dust does not extend to as great a height as in unstable air and usually is not as widespread. Dust, once airborne, may remain suspended and restrict visibility for several hours after the wind subsides. Figure 121 is a photograph of a dust storm moving in with an approaching cold front.
Figure 121. Aerial photograph of blowing dust approaching with a cold front. The dust cloud outlines the leading surface of the advancing cold air.
Blowing sand is more local than blowing dust; the sand is seldom lifted above 50 feet. However, visibilities within it may be near zero. Blowing sand may occur in any dry area where loose sand is exposed to strong wind.
Blowing snow can be troublesome. Visibility at ground level often will be near zero and the sky may become obscured when the particles are raised to great heights.
Rain, drizzle, and snow are the forms of precipitation which most commonly present ceiling and/or visibility problems. Drizzle or snow restricts visibility to a greater degree than rain. Drizzle falls in stable air and, therefore, often accompanies fog, haze, or smoke, frequently resulting in extremely poor visibility. Visibility may be reduced to zero in heavy snow. Rain seldom reduces surface visibility below 1 mile except in brief, heavy showers, but rain does limit cockpit visibility. When rain streams over the aircraft windshield, freezes on it, or fogs over the inside surface, the pilot's visibility to the outside is greatly reduced.
OBSCURED OR PARTIALLY OBSCURED SKY
To be classified as obscuring phenomena, smoke, haze, fog, precipitation, or other visibility restricting phenomena must extend upward from the surface. When the sky is totally hidden by the surface based phenomena, the ceiling is the vertical visibility from the ground upward into the obscuration. If clouds or part of the sky can be seen above the obscuring phenomena, the condition is defined as a partial obscuration; a partial obscuration does not define a ceiling. However, a cloud layer above a partial obscuration may constitute a ceiling.
An obscured ceiling differs from a cloud ceiling. With a cloud ceiling you normally can see the ground and runway once you descend below the cloud base. However, with an obscured ceiling, the obscuring phenomena restricts visibility between your altitude and the ground, and you have restricted slant visibility. Thus, you cannot always clearly see the runway or approach lights even after penetrating the level of the obscuration ceiling as shown in figure 122.
Figure 122. Difference between the ceiling caused by a surface-based obscuration (B) and the ceiling caused by a layer aloft (A). When visibility is not restricted, slant range vision is good upon breaking out of the base of a layer aloft.
Partial obscurations also present a visibility problem for the pilot approaching to land but usually to a lesser degree than the total obscuration. However, be especially aware of erratic visibility reduction in the partial obscuration. Visibility along the runway or on the approach can instantaneously become zero. This abrupt and unexpected reduction in visibility can be extremely hazardous especially on touchdown.
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 obstruction. 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 reestablish 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:
1. Fog the following morning when at dusk temperature-dew point spread is 15° F or less, skies are clear, and winds are light.
2. Fog when moist air is flowing from a relatively warm surface to a colder surface.
3. Fog when temperature-dew point spread is 5° F or less and decreasing.
4. 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.)
5. Steam fog when air is blowing from a cold surface (either land or water) over warmer water.
6. 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.
7. Low stratus clouds whenever there is an influx of low level moisture overriding a shallow cold air mass.
8. Low visibilities from haze and smoke when a high pressure area stagnates over an industrial area.
9. 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.
10. Low visibility due to snow or drizzle.
11. An undercast when you must make a VFR descent.
Expect little if any improvement in visibility when:
1. Fog exists below heavily overcast skies.
2. Fog occurs with rain or drizzle and precipitation is forecast to continue.
3. Dust extends to high levels and no frontal passage or precipitation is forecast.
4. Smoke or haze exists under heavily overcast skies.
5. A stationary high persists over industrial areas.