If air is cooled to the saturation point or moisture is added until saturation occurs, fog will form. Fog usually forms when the temperature/dewpoint spread is 5 degrees or less. Try to anticipate the formation of fog during flight planning by examining weather reports and forecasts. For example, if you are flying to an airport in the midwest during a period of cooling temperatures, such as after dusk, and calm winds are indicated along with a narrow temperature/dewpoint spread, you should be alert for the formation of fog. Fog is also common along coastal areas a few hours after sunset with a light sea breeze.

Fog has been known to form, even when the weather forecast says it will not. If you think unforecast fog formation might be possible, stay updated enroute by checking destination weather reports via ATIS, ASOS, AWOS, or a call to the flight service station. Also, take additional reserve fuel and prepare for diversion to an alternate.

Thin morning fog typically "burns off" under clear skies a few hours after sunrise, as a result of increased temperatures in the low lying air. In this case, the sun is able to penetrate through and heat the ground. Thicker fog or fog protected from sunlight from a higher overcast tends to lift into a low overcast layer.

Volcanic Ash

Clouds of volcanic ash contain an abrasive dust that poses a serious safety threat to flight operations. Should an ash cloud be entered, dust and smoke may be evident in the cabin, often along with the odor of an electrical fire. St. Elmo's fire may also be present on the windshield. The pitot-static system may fail as the result of port blockage. Ingestion of ash will likely result in severe engine damage.

Clouds of ash are not discernible from ordinary clouds when at some distance from the eruption. Therefore, flight planning is the key to avoid ash clouds. Check SIGMETs and PIREPs. Plan your flight upwind of the eruption. During an active eruption, the volcanic ash forecast transport and dispersion chart, or VAFTAF, will also be made available to pilots.

Wind Shear

Wind shear is a change in wind speed and/or direction in a short distance. It can exist in a horizontal or vertical direction, or both. It can occur at all levels in the atmosphere, but is of greatest concern during takeoff and landing. While typically associated with thunderstorms and low level temperature inversions, the jet stream and weather fronts are also sources of wind shear.

Wind shear is normally found before a warm front passes and after a cold front has passed. Warm front wind shear usually begins about 6 hours before frontal passage at altitudes below 5,000 feet. For a cold front, shear is usually present when the front is moving at 30 knots or more. It is normally contained to below 5,000 feet AGL and continues until 3 hours after the cold front passes.

A shear from tailwind to a headwind will cause airspeed to increase, the nose to pitch up, and the aircraft to balloon above the glide path. Lower than normal power is required, followed by further decrease as the shear is encountered. Then, power required increases as the glide slope is once again captured.

A shear from headwind to tailwind has the opposite effect, airspeed decreases, the nose pitches down, and aircraft sinks below the glide path.

It may be impossible to recover from a wind shear encounter at low altitude. Some airports have a low-level wind shear alert system, or LLWAS, consisting of a centerfield wind detector and several surrounding boundary wind detectors. This system alerts controllers of wind shear possibility and they then provide this information to pilots.

Warm front wind shear is normally more extreme than wind shear associated with cold fronts.