FOG Types of Fog • • • • • • • Radiation Fog Advection Fog Smoke Fog (Smog) Hill Fog Frontal Fog Thaw Fog Arctic Sea Smoke (Steam Fog) Fog/Mist/Haze • Fog: – is composed of small water droplets in suspension (or ice crystals in ice fog). – Visibility is reduced to less than 1000 m. – Relative humidity is generally 100%. • Mist: – is also caused by small droplets in suspension. – Visibility is 1000m or more. – METAR and TAF codes impose an upper limit of 5000 m. – RH is at least 95% but less than 100%. • Haze: – caused by solid particles in suspension. Radiation Fog • Caused by long wave radiation cooling from ground at night. • Favourable conditions are – High relative humidity – little or no cloud – light winds, 2-8 knots.(or 1-5 kt.) • Fog dispersal caused – by incoming insolation heating environment to above saturation temperature, or by – turbulent mixing in the boundary layer with increasing wind and lifting fog into low stratus or by – mixing with dry air from above. Synoptic Conditions for Rad. Fog • These conditions are often met in anticyclones, ridges and cols, in late autumn, winter and early spring. • Radiation fog is usually between 300 and 1000 ft in depth. • Cloud cover arriving early in the night will arrest the heat loss, so that the radiation fog will not develop further, and may clear. Rad. Fog Cont. • Suitable diversion airfields in radiation fog would be: – A high level airfield if early in the night. – A coastal airfield with onshore wind. – An airfield in the lee of high ground, there is at least 7 - 8 kt of wind. providing Advection Fog • Formed by warm moist air moving over a colder surface below dew point of the air. • Can form over sea as well as land. • Wind strength is not material to the formation of advection fog. • Sea Fog which has formed over the sea may drift inland, as does the HAAR on east coasts of the UK in summer. • Depth of advection fog is typically 1,000-1,500 ft. • A change of airmass is usually necessary for the clearance of advection fog. Smoke Fog (Smog) • A form of radiation fog. • Formed when – A marked low level inversion exists. – A source of pollution. • The inversion traps industrial and domestic pollution in the lower atmosphere. • Fog visibilities may be found at slightly less than 100% relative humidity. Hill Fog • Also referred to as “cloud covering hills.” • Formed by cloud in contact with high ground – forming orographic stratus due to forced acsent over high ground or, – simply high ground extending into an existing low cloud layer. • If forecast the visibility is automatically assumed to be less than 200m. • Usually encountered in warm sectors with Tropical Maritime air. Frontal Fog • Formed by the lowering of frontal cloud to the surface (eg. at warm front passage). • Forms ahead of a warm front by the saturation of air due to the continuous rain. Thaw Fog • Thaw fog is a particular case of advection fog. • Warm air arriving over a snow-covered surface will often produce widespread fog while the snow melts • The melting snow surface (at 0°C) both cools the air and increases the moisture content as it melts and evaporates. • It is a particular feature of central and eastern European plains in the spring. Arctic Sea Smoke • Also known as steam fog or frost smoke. • Occurs when very cold air moves over a relatively warm water surface. • The very cold air mixes with the warm air above the sea surface to a depth of about 500 feet, cooling to below dew point temperature. • There must also be a marked inversion. • Occurs frequently in winter – over the north-west Atlantic near Greenland or northern Canada when cold air moves over the relatively warm ocean. – very cold Siberian air moves over the sea by the Kamchatka Peninsula off eastern Russia, and – when cold katabatic air moves down the mountains into the fjords of Scandinavia. Visibility Meteorological Visibility Meteorological Visibility • Is a measure of the clarity of the atmosphere. • It is more properly known as the Meteorological Optical Range (MOR). • Visibility (Met Vis) – is the greatest horizontal distance at which suitable objects can be recognised for what they are in daylight or – at which lights of specified intensity can be seen at night by a person with normal sight. • Where the visiblity is variable the lowest value is reported. Visiblity Reporting • Lowest value is normally reported for Met Vis. • In METARS the lowest value is always given and the highest values may be reported under certain conditions. • Runway Visual Range (RVR) reporting begins when the Met Vis or the RVR fall to below 1500 m. • RVR is the maximum distance in direction of takeoff or landing at which designated runway markers or lights can be seen from a centreline height corresponding to the eye-level of the pilot on touchdown. Visibility Reporting (cont.) • RVR reporting begins when the horizontal visibility or the RVR is less than 1500 m. • At a/d’s with Instrumented RVR (IRVR) systems, RVR may also be reported when – the observed value is at or below the maximum reportable value (usually 1500m) or – when shallow fog is forecast or reported. • RVR is passed to a/c before take-off and during approach to land. • Changes in RVR are passed to a/c during approach or if pilot reports or ATC observation indicate the RVR is worse than that indicted by equipment Visibility Reporting (cont) • RVR is reported by human observer (OBS) or by IRVR systems. • Multi site IRVR systems report touchdown (TDz), midpoint (MID) and stop-end (END) values. • Mid-point and/or stop-end values are suppressed when: – they are 800 m or more or – they are equal to or higher than the touch-down zone unless they are less than 400 m. Visibility Reporting Problems • Lack of reference objects e.g. at sea or level snow areas. • Depends on size, shape and colour of the object. • Object illumination and background. • Transparency of wind shield. • Visibility varies in layers and in direction. • Slant visual range in flight. • Met vis and RVR may vary widely. In-Flight Visibility in Poor Conditions Summary • Inside an obscuring layer – Increasing height decreases circle of visibility. – Decreasing height increases circle of visibility. In-flight visibility above a layer of poor visibility Summary • Above an obscuring layer – decreasing height decreases circle of visibility – increasing height increases circle of visibility In-flight visibility on the approach Summary • On the approach – Airfield clearly visible from directly above. – On glide path forward visibility lost due to slant range through obscuring layer. – Contact with airfield will be lost. – Instrument approach will be necessary. Effect of Precipitation on Visibility • Drizzle commonly restricts visibility to between 500 metres and 3,000 metres. • Light rain has virtually no effect. • Moderate rain can reduce visibility to 3-10 km. • Heavy rain showers of temperate latitudes rarely cause a reduction below 1,000 metres. • Snow and blown snow are both very effective in restricting visibility to less than 1,000 metres. • Heavy snow may lead to visibilities of the order of 50-250 metres. Flight in Rain • Rain on the windscreen can have two opposite effects. • It can scatter light and so reduce its intensity. – the runway lighting may appear further away than it is or, • It may cause the runway lights to “bloom” thus – they appear larger and closer than they really are. • Care must be taken to avoid an inadvertent descent well below a safe approach path, when subject to such illusion. • In heavy rain a film of water on the windscreen can cause refraction.