Ch 14 – Instrument Meteorological Conditions (IMC)

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions

(IMC)(IMC)

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Section A – BackgroundSection A – Background• Section B – Causes of IMCSection B – Causes of IMC

– Fog and Low Stratus CloudsFog and Low Stratus Clouds– PrecipitationPrecipitation

•Weather SystemsWeather Systems

– Smoke and HazeSmoke and Haze– DustDust

• Section C - ClimatologySection C - Climatology

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• IntroductionIntroduction

– Instrument Meteorological Conditions (IMC) Instrument Meteorological Conditions (IMC) refers to any state of the atmosphere where refers to any state of the atmosphere where ceiling and visibility are below specific minimum ceiling and visibility are below specific minimum valuesvalues

– Low ceilings and visibilities are common Low ceilings and visibilities are common occurrences in the meteorological environment occurrences in the meteorological environment in which you flyin which you fly

– Unless the causes and properties of these Unless the causes and properties of these weather conditions are understood and weather conditions are understood and respected, serious flight problems can resultrespected, serious flight problems can result


– The purpose of this chapter is to describe the The purpose of this chapter is to describe the characteristics and primary causes of characteristics and primary causes of meteorological phenomena which limit ceiling meteorological phenomena which limit ceiling and visibilityand visibility

– When you complete this chapter, you will know When you complete this chapter, you will know the technical terminology used to specify the technical terminology used to specify current and forecast ceilings and visibilitiescurrent and forecast ceilings and visibilities

– You will understand how they develop and the You will understand how they develop and the large-scale conditions under which they formlarge-scale conditions under which they form

– Finally, you will learn some useful rules of Finally, you will learn some useful rules of thumb that will help you deal with instrument thumb that will help you deal with instrument meteorological conditionsmeteorological conditions


– As you read this chapter, keep in mind that a As you read this chapter, keep in mind that a current instrument rating is required to operate current instrument rating is required to operate in instrument conditions and that even in instrument conditions and that even experienced instrument-rated pilots will choose experienced instrument-rated pilots will choose not to fly in some types of IMCnot to fly in some types of IMC

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Section A: BackgroundSection A: Background

– Visual meteorological conditions (VMC) – Visual meteorological conditions (VMC) – The counterpart to IMCThe counterpart to IMC•These two terms are a rather broad These two terms are a rather broad

classification used to describe the state of classification used to describe the state of the ceiling and/or visibility with regard to the ceiling and/or visibility with regard to aviation operations. aviation operations.

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)

•Key terminology used in the Key terminology used in the evaluation of IMC conditions includes:evaluation of IMC conditions includes:

– CeilingCeiling– Cloud amountCloud amount– Cloud heightCloud height– Cloud layerCloud layer– ObscurationObscuration– Prevailing visibilityPrevailing visibility– Radar summary chartRadar summary chart


•Key terminology used in the Key terminology used in the evaluation of IMC conditions includes:evaluation of IMC conditions includes:

– Relative humidityRelative humidity– Runway visibility (RVV)Runway visibility (RVV)– Runway visual range (RVR)Runway visual range (RVR)– Sector visibilitySector visibility– Temperature-dew point spreadTemperature-dew point spread– Tower visibilityTower visibility– Vertical visibility Vertical visibility – Weather depiction chartWeather depiction chart


– Slant range visibility – Slant range visibility – another important another important consideration is slant visibility on final consideration is slant visibility on final approach. approach.

•This is the oblique distance at which you can This is the oblique distance at which you can see landing aids, such as runway lights and see landing aids, such as runway lights and markings.markings.

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Section B: Causes of IMC – Section B: Causes of IMC – visibility is visibility is

decreased by particles that absorb, scatter, and decreased by particles that absorb, scatter, and reflect light. reflect light. – We can separate atmospheric particles into We can separate atmospheric particles into

two groups: two groups:

•1. Those composed of water, such as water 1. Those composed of water, such as water droplets and ice crystalsdroplets and ice crystals

•2. Those composed of dry particles, such as 2. Those composed of dry particles, such as those from combustion, wind-borne soil, and those from combustion, wind-borne soil, and volcanoes.volcanoes.

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Fog and Low Stratus CloudsFog and Low Stratus Clouds

– Radiation fog/Advection fog – Radiation fog/Advection fog – fog forms in fog forms in stable airstable air•……that is, it is cooled to saturation by contact that is, it is cooled to saturation by contact

with the cold groundwith the cold ground– Upslope fog – Upslope fog – fog caused by adiabatic cooling fog caused by adiabatic cooling

of stable airof stable air– Steam fog – Steam fog – fog that forms in unstable air (at fog that forms in unstable air (at

least in the lowest layers)least in the lowest layers)•water evaporates and saturates a thin layer water evaporates and saturates a thin layer

of colder air, which causes the fog.of colder air, which causes the fog.


– Ice fog – Ice fog – forms in cold climatesforms in cold climates

•a radiation-type fog which is composed of ice a radiation-type fog which is composed of ice crystalscrystals

– Forms at low temperatures (-20 degrees F or Forms at low temperatures (-20 degrees F or less) and may be quite persistent, especially less) and may be quite persistent, especially in cities or industrial areas where many in cities or industrial areas where many combustion particles are present to act as combustion particles are present to act as cloud nucleicloud nuclei

– At colder temperatures (-30 F or colder), the At colder temperatures (-30 F or colder), the sudden addition of moisture and particulates sudden addition of moisture and particulates can cause ice fog to rapidly formcan cause ice fog to rapidly form

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• PrecipitationPrecipitation

– Fractocumulus or fractostratus clouds – Fractocumulus or fractostratus clouds – sometimes called scudsometimes called scud

•Form below the original cloud base, causing Form below the original cloud base, causing the ceiling to lower over time.the ceiling to lower over time.

– Precipitation fog – Precipitation fog – may develop when rain may develop when rain saturates the layer near the groundsaturates the layer near the ground

– Blowing snow (BLSN) – Blowing snow (BLSN) – reported when the reported when the wind raises snow particles more than 6 feet wind raises snow particles more than 6 feet above the surface and reduces visibility to 6 s.m. above the surface and reduces visibility to 6 s.m. or less. or less.


– Blizzard – Blizzard – exists when low temperatures combine exists when low temperatures combine with winds that exceed 30 knots and great amounts of with winds that exceed 30 knots and great amounts of snow, either falling or blowing.snow, either falling or blowing.

– Weather Systems – Weather Systems – fog and low stratus clouds fog and low stratus clouds develop under identifiable larger scale weather develop under identifiable larger scale weather conditionsconditions• IMC conditions may also occur when warm, moist IMC conditions may also occur when warm, moist

air over runs cold air trapped in valleysair over runs cold air trapped in valleys•Radiation fog favors clear skies, cold ground and Radiation fog favors clear skies, cold ground and

light windslight winds•Radiation fog typically dissipates after the sun risesRadiation fog typically dissipates after the sun rises•Advection fog is common whenever warm, moist air Advection fog is common whenever warm, moist air

is carried over a cold surfaceis carried over a cold surface

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Weather maps have cross sections showing Weather maps have cross sections showing

precipitation falling from over running warm air precipitation falling from over running warm air saturating shallow cold air near the groundsaturating shallow cold air near the ground– Low clouds and fog occur in a broad band on Low clouds and fog occur in a broad band on

the cold-air side of the frontthe cold-air side of the front

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Smoke and HazeSmoke and Haze

– Smoke – Smoke – is the suspension of combustion is the suspension of combustion particles in the airparticles in the air

– Haze (HZ) – Haze (HZ) – is a suspension of extremely is a suspension of extremely small, dry particlessmall, dry particles

– Air pollution – Air pollution – as with smoke, some of the as with smoke, some of the worst haze problems occur in large industrial worst haze problems occur in large industrial areas and cities where many air pollution areas and cities where many air pollution sources add gases and more particulates to sources add gases and more particulates to any naturally occurring haze particles.any naturally occurring haze particles.

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• DustDust

– Dust (DU) – Dust (DU) – refers to fine particles of soil refers to fine particles of soil suspended in the airsuspended in the air

– Blowing dust (BLDU) – Blowing dust (BLDU) – dust raised by the dust raised by the wind to 6 feet (2 m) or more, restricting wind to 6 feet (2 m) or more, restricting visibility to 6 statute miles (10 km) or lessvisibility to 6 statute miles (10 km) or less

– Dust storm – Dust storm – visibility less than 5/8 sm (1km)visibility less than 5/8 sm (1km)– Severe dust storm – Severe dust storm – visibility less than 5/16 visibility less than 5/16

sm (500 m)sm (500 m)

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Weather depiction chart – Weather depiction chart – one of the most one of the most

useful charts for evaluating current ceiling and useful charts for evaluating current ceiling and visibility conditions at a glancevisibility conditions at a glance

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Section C: Climatology- Section C: Climatology- knowledge of the knowledge of the

favored areas of IMC is useful background for favored areas of IMC is useful background for flight planning, especially in unfamiliar flight planning, especially in unfamiliar geographical regions.geographical regions.

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Weather maps can illustrate annual average Weather maps can illustrate annual average

number of hours when visibility is reduced to 6 number of hours when visibility is reduced to 6 statute miles or less by duststatute miles or less by dust

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• SummarySummary

– IMC occurs when ceilings and visibilities are IMC occurs when ceilings and visibilities are reduced by clouds, fog, precipitation, haze, and reduced by clouds, fog, precipitation, haze, and any other particles produced by natural or any other particles produced by natural or anthropogenic sourcesanthropogenic sources

– Many of these conditions are associated with Many of these conditions are associated with identifiable large-scale weather systemsidentifiable large-scale weather systems

•Occur during certain seasons of the yearOccur during certain seasons of the year

•Are more common in particular geographical Are more common in particular geographical areasareas


– Surface visibility and ceiling observations are Surface visibility and ceiling observations are critical to your assessment of IMCcritical to your assessment of IMC

– Although the observations are made on the Although the observations are made on the basis of specific definitions and procedures, basis of specific definitions and procedures, they are, at best, approximations of a complex they are, at best, approximations of a complex situation situation

– Surface observations alone give little Surface observations alone give little information with respect to inflight conditions information with respect to inflight conditions

– Although imperfect, the system of weather Although imperfect, the system of weather observations is the only one that existsobservations is the only one that exists


– The wise pilot knows IMC rules and regulations The wise pilot knows IMC rules and regulations and the technical language and shortcomings and the technical language and shortcomings of observations, reports, and forecasts of IMC of observations, reports, and forecasts of IMC conditionsconditions

– When uncertain about conditions, the wise When uncertain about conditions, the wise pilot takes a conservative approach to preflight pilot takes a conservative approach to preflight and inflight decision making with respect to and inflight decision making with respect to IMCIMC

Ch 14 – Instrument Ch 14 – Instrument Meteorological Conditions Meteorological Conditions (IMC)(IMC)• Ceiling and visibility categories define the Ceiling and visibility categories define the

following:following:– VFR (Visual Flight Rules)VFR (Visual Flight Rules)– MVFR (Marginal Visual Flight Rules)MVFR (Marginal Visual Flight Rules)– IFR (Instrument Flight Rules)IFR (Instrument Flight Rules)– LIFR (Low Instrument Flight Rules)LIFR (Low Instrument Flight Rules)

Ch 14 – Instrument Meteorological Conditions (IMC)

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