Download Section B: Causes of IMC

Ch 14 – Instrument
Meteorological Conditions (IMC)
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
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
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 largescale conditions under which they form
– Finally, you will learn some useful rules of thumb that
will help you deal with instrument meteorological
conditions
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 instrumentrated pilots will choose not to fly in some types of
IMC
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.
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
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
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.
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.
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.
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
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.
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
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
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.
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)
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
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.
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
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
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
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
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)