2. Ch 14 – Instrument Meteorological Conditions (IMC)

3. Ch 14 – Instrument Meteorological Conditions (IMC)
Section A – Background
Section B – Causes of IMC
Fog and Low Stratus Clouds
Precipitation
Weather Systems
Smoke and Haze
Dust
Section C - Climatology

4. Ch 14 – Instrument Meteorological Conditions (IMC)
Introduction
Instrument Meteorological Conditions (IMC) refers to any state of the atmosphere where ceiling and visibility are below specific minimum values
Low ceilings and visibilities are common occurrences in the meteorological environment in which you fly
Unless the causes and properties of these weather conditions are understood and respected, serious flight problems can result

5. Ch 14 – Instrument Meteorological Conditions (IMC)
Introduction
The purpose of this chapter is to describe the characteristics and primary causes of meteorological phenomena which limit ceiling and visibility
When you complete this chapter, you will know the technical terminology used to specify current and forecast ceilings and visibilities
You will understand how they develop and the large-scale conditions under which they form
Finally, you will learn some useful rules of thumb that will help you deal with instrument meteorological conditions

6. Ch 14 – Instrument Meteorological Conditions (IMC)
Introduction
As you read this chapter, keep in mind that a current instrument rating is required to operate in instrument conditions and that even experienced instrument-rated pilots will choose not to fly in some types of IMC

7. Ch 14 – Instrument Meteorological Conditions (IMC)
Section A: Background
Visual meteorological conditions (VMC) – The counterpart to IMC
These two terms are a rather broad classification used to describe the state of the ceiling and/or visibility with regard to aviation operations.

8. Ch 14 – Instrument Meteorological Conditions (IMC)
Key terminology used in the evaluation of IMC conditions includes:
Ceiling
Cloud amount
Cloud height
Cloud layer
Obscuration
Prevailing visibility
Radar summary chart

9. Ch 14 – Instrument Meteorological Conditions (IMC)
Key terminology used in the evaluation of IMC conditions includes:
Relative humidity
Runway visibility (RVV)
Runway visual range (RVR)
Sector visibility
Temperature-dew point spread
Tower visibility
Vertical visibility
Weather depiction chart

10. Ch 14 – Instrument Meteorological Conditions (IMC)
Slant range visibility – another important consideration is slant visibility on final approach.
This is the oblique distance at which you can see landing aids, such as runway lights and markings.

12. Ch 14 – Instrument Meteorological Conditions (IMC)
Section B: Causes of IMC – visibility is decreased by particles that absorb, scatter, and reflect light.
We can separate atmospheric particles into two groups:
1. Those composed of water, such as water droplets and ice crystals
2. Those composed of dry particles, such as those from combustion, wind-borne soil, and volcanoes.

13. Ch 14 – Instrument Meteorological Conditions (IMC)
Fog and Low Stratus Clouds
Radiation fog/Advection fog – fog forms in stable air
…that is, it is cooled to saturation by contact with the cold ground
Upslope fog – fog caused by adiabatic cooling of stable air
Steam fog – fog that forms in unstable air (at least in the lowest layers)
water evaporates and saturates a thin layer of colder air, which causes the fog.

14. Ch 14 – Instrument Meteorological Conditions (IMC)
Ice fog – forms in cold climates
a radiation-type fog which is composed of ice crystals
Forms at low temperatures (-20 degrees F or less) and may be quite persistent, especially in cities or industrial areas where many combustion particles are present to act as cloud nuclei
At colder temperatures (-30 F or colder), the sudden addition of moisture and particulates can cause ice fog to rapidly form

15. Ch 14 – Instrument Meteorological Conditions (IMC)
Precipitation
Fractocumulus or fractostratus clouds – sometimes called scud
Form below the original cloud base, causing the ceiling to lower over time.
Precipitation fog – may develop when rain saturates the layer near the ground
Blowing snow (BLSN) – reported when the wind raises snow particles more than 6 feet above the surface and reduces visibility to 6 s.m. or less.

16. Ch 14 – Instrument Meteorological Conditions (IMC)
Blizzard – exists when low temperatures combine with winds that exceed 30 knots and great amounts of snow, either falling or blowing.
Weather Systems – fog and low stratus clouds develop under identifiable larger scale weather conditions
IMC conditions may also occur when warm, moist air over runs cold air trapped in valleys
Radiation fog favors clear skies, cold ground and light winds
Radiation fog typically dissipates after the sun rises
Advection fog is common whenever warm, moist air is carried over a cold surface

17. Ch 14 – Instrument Meteorological Conditions (IMC)
Weather maps have cross sections showing precipitation falling from over running warm air saturating shallow cold air near the ground
Low clouds and fog occur in a broad band on the cold-air side of the front

19. Ch 14 – Instrument Meteorological Conditions (IMC)
Smoke and Haze
Smoke – is the suspension of combustion particles in the air
Haze (HZ) – is a suspension of extremely small, dry particles
Air pollution – as with smoke, some of the worst haze problems occur in large industrial areas and cities where many air pollution sources add gases and more particulates to any naturally occurring haze particles.

20. Ch 14 – Instrument Meteorological Conditions (IMC)
Dust
Dust (DU) – refers to fine particles of soil suspended in the air
Blowing dust (BLDU) – dust raised by the wind to 6 feet (2 m) or more, restricting visibility to 6 statute miles (10 km) or less
Dust storm – visibility less than 5/8 sm (1km)
Severe dust storm – visibility less than 5/16 sm (500 m)

21. Ch 14 – Instrument Meteorological Conditions (IMC)
Weather depiction chart – one of the most useful charts for evaluating current ceiling and visibility conditions at a glance

22. Ch 14 – Instrument Meteorological Conditions (IMC)
Section C: Climatology- knowledge of the favored areas of IMC is useful background for flight planning, especially in unfamiliar geographical regions.

24. Ch 14 – Instrument Meteorological Conditions (IMC)
Weather maps can illustrate annual average number of hours when visibility is reduced to 6 statute miles or less by dust

26. Ch 14 – Instrument Meteorological Conditions (IMC)
Summary
IMC occurs when ceilings and visibilities are reduced by clouds, fog, precipitation, haze, and any other particles produced by natural or anthropogenic sources
Many of these conditions are associated with identifiable large-scale weather systems
Occur during certain seasons of the year
Are more common in particular geographical areas

27. Ch 14 – Instrument Meteorological Conditions (IMC)
Surface visibility and ceiling observations are critical to your assessment of IMC
Although the observations are made on the basis of specific definitions and procedures, they are, at best, approximations of a complex situation
Surface observations alone give little information with respect to inflight conditions
Although imperfect, the system of weather observations is the only one that exists

28. Ch 14 – Instrument Meteorological Conditions (IMC)
The wise pilot knows IMC rules and regulations and the technical language and shortcomings of observations, reports, and forecasts of IMC conditions
When uncertain about conditions, the wise pilot takes a conservative approach to preflight and inflight decision making with respect to IMC

29. Ch 14 – Instrument Meteorological Conditions (IMC)
Ceiling and visibility categories define the following:
VFR (Visual Flight Rules)
MVFR (Marginal Visual Flight Rules)
IFR (Instrument Flight Rules)
LIFR (Low Instrument Flight Rules)