ATPL Meteorology - Class 10: Visibility.
Summary
TLDRThis video script delves into the critical aspect of visibility for aviation, explaining how fog and mist can impede safe landings at airports. It outlines various types of fog, including radiation, advection, evaporation, frontal, and orographic fog, all resulting from the cooling of humid air below the dew point. The script also distinguishes between fog and mist based on visibility metrics and addresses man-made visibility issues like smog and haze. It further clarifies the measurement of visibility, the concept of runway visual range (RVR), and the impact of visual segment on pilots' perception of fog density during landing approaches.
Takeaways
- đ«ïž Visibility is crucial for safe aircraft landings at airports, and understanding what causes low visibility can help determine the safety of landing.
- đ Reduced visibility can be caused by natural environmental factors like sunlight glare or cloud cover, which reflects light and impairs viewing through a windscreen.
- âïž At higher altitudes, pilots rely on instruments and instrument flight rules, but near the ground, they depend on visual cues for takeoff and landing.
- đ Fog or mist, which are types of clouds at ground level, are significant factors affecting visibility, with fog defined as having visibility less than a thousand meters and mist as less than 5000 meters.
- đĄïž Radiation fog forms when cool ground temperatures radiate and cool the air above it to below the dew point, causing moisture to condense into fog, typically occurring in the early morning.
- đ€ïž Advection fog occurs when warm, humid air moves over a cold surface, cooling the air to below the dew point and forming fog, often seen in coastal areas.
- đ§ Evaporation fog happens when water vapor evaporates and then cools the air above it to below the dew point, forming fog, common in the Arctic regions.
- đŠïž Frontal fog is created when a warm air front moves over a cold air mass, causing the warm air to rise and cool, leading to condensation and fog formation at the surface.
- đïž Orographic fog results from air being forced to rise due to terrain, like mountains, cooling as it ascends and forming fog if it cools below the dew point.
- đ Man-made visibility issues can arise from pollution, where dust and smoke particles accumulate at low levels and create haze or smog, especially in industrial or densely populated areas.
- đ Visibility is measured in increments and can be assessed using physical objects at known distances or more accurately with a transmissometer, which measures light transmission through the air.
Q & A
Why is visibility important for aircraft landing at airports?
-Visibility is crucial for pilots to safely land aircraft at airports, especially during takeoff and landing maneuvers, which are not conducted using instruments but by visually assessing the environment outside the aircraft.
What is the primary difference between fog and mist?
-Fog is characterized by visibility less than a thousand meters, while mist has visibility less than 5000 meters. This distinction is based on the level of visibility they create.
How does radiation fog form?
-Radiation fog forms when the cold surface of the earth at night cools the air just above it through radiation, causing the air to cool down to below the dew point, leading to the condensation of moisture and the formation of clouds.
What type of fog is commonly seen in coastal areas?
-Advection fog is commonly seen in coastal areas, which occurs when warm humid air blows over a cold surface, cooling the air to below the dew point and causing water vapor to condense into fog.
How does evaporation fog differ from other types of fog?
-Evaporation fog forms when warm water vapor evaporates into colder air above it, causing the air to cool down to below the dew point and water vapor to condense into fog. This type of fog is often seen in the Arctic.
What causes frontal fog?
-Frontal fog is created when a warm air front moves in and rises over a colder air mass. Precipitation from the warm front falls into the cooler air below, causing the warm air to cool down to below the dew point and form fog at the surface.
What is orographic fog and how does it form?
-Orographic fog forms when humid air is forced to rise due to terrain, such as a mountain. As the air rises and cools adiabatically, if it cools to below the dew point, moisture condenses out to form fog close to the land.
Why do visibility levels differ between ground level and when in flight?
-Visibility levels differ because when you are above a layer of fog, you are looking at it from an angle, allowing you to see further. However, as you descend and approach the ground, you look through more of the fog, reducing visibility.
How is visibility measured at airports?
-Visibility at airports is measured in increments of 50 meters up to 800 meters, then 100 meters up to 5000 meters, and in thousand-meter steps up to 10 kilometers. Beyond 10 kilometers, visibility might be reported as '10k plus' or with 'four nines'.
What is Runway Visual Range (RVR) and why is it important?
-Runway Visual Range (RVR) is a runway-specific visibility measurement that provides an accurate assessment of the visibility conditions on a particular runway. It is important for pilots to determine if it is safe to conduct an approach, especially when using automatic landing systems.
How does the presence of haze or smog affect visibility for aircraft operations?
-Haze or smog, which are man-made visibility issues, occur when stable atmospheric conditions trap dust and smoke particles at low levels, causing reduced visibility during takeoff and landing. This is particularly prominent in areas with high industry and population.
Outlines
đ«ïž Understanding Visibility Issues at Airports
This paragraph discusses the challenges of landing at airports during foggy conditions, which can force pilots to divert or wait for visibility to improve. The importance of visibility for safe aircraft landings is highlighted, and the natural environmental factors causing low visibility levels are introduced. The paragraph explains how sunlight and clouds can reduce visibility, with a focus on ground-level clouds known as fog or mist. Different types of fog, such as radiation fog, advection fog, evaporation fog, frontal fog, and orographic fog, are briefly described, emphasizing how they form and affect visibility.
đŹïž Types of Fog and Their Impact on Visibility
The paragraph delves into the various types of fog and their formation processes. It explains how radiation fog forms on cold nights when the earth's surface cools the air above it, leading to condensation. Advection fog occurs when warm, humid air blows over a cold surface, causing the air to cool and condense. Evaporation fog is described as forming when warm water vapor from the sea evaporates into cold air above, common in the Arctic. Frontal fog is associated with warm air fronts moving over colder air masses, leading to condensation. Orographic fog forms when air is forced to rise over terrain like mountains, cooling and condensing. The paragraph also touches on man-made visibility issues caused by pollution, known as haze or smog, and how they differ from natural fog.
đ Measuring Visibility and Runway Visual Range (RVR)
This paragraph focuses on how visibility is measured at airports, which is crucial for determining whether it is safe for aircraft to land. It explains the difference between general visibility and runway visual range (RVR), with the latter being a more precise measurement for specific landing runways. The use of transmissometers to measure the opacity of the air is mentioned, along with the reporting increments for visibility. The paragraph also discusses the concept of visual segment, illustrating how visibility changes with the pilot's altitude relative to the fog layer. It concludes by summarizing the different types of fog and the distinction between fog and mist based on visibility levels, as well as the significance of accurate visibility measurements for safe aircraft operations.
Mindmap
Keywords
đĄVisibility
đĄFog
đĄMist
đĄDew Point
đĄRadiation Fog
đĄAdvection Fog
đĄEvaporation Fog
đĄFrontal Fog
đĄOrographic Fog
đĄRunway Visual Range (RVR)
đĄTransmissometer
Highlights
Visibility is crucial for safe aircraft landings at airports.
Low visibility can be caused by natural environmental factors such as clouds and fog.
Fog impairs visibility by reflecting light off suspended water and ice particles.
High altitude flights use instruments, not visibility, for navigation.
Fog and mist are differentiated by visibility levels: fog is less than 1000 meters, mist is less than 5000 meters.
Radiation fog forms when a cold surface cools the air above it to below the dew point.
Advection fog occurs when warm humid air blows over a cold surface, cooling it below the dew point.
Evaporation fog forms when warm water vapor evaporates into colder air above it.
Frontal fog is created by warm air rising over colder air, causing condensation at the surface.
Orographic fog is caused by air being forced to rise and cool when it encounters a mountain.
Man-made visibility issues can arise from pollution, causing haze or smog.
Visibility is measured in increments and reported based on airport conditions.
Runway Visual Range (RVR) provides a runway-specific visibility measure for accurate landing assessments.
Transmissometers are used to accurately measure visibility by detecting light transmission through the air.
Visibility differs depending on the angle and height from which it is measured.
Pilots must consider their visual segment when landing in low visibility conditions.
Transcripts
sometimes when we're trying to land at
an airport it can be a bit too foggy for
us to make an approach and landing
safely and we need to either divert
somewhere else or wait until the fog
clears and the visibility improves but
what is it that causes these visibility
issues in the first place let's find out
[Music]
hi i'm grant and welcome to the 10th
class in the meteorology series where
we're going to be taking a look at
visibility
visibility is vitally important to land
and aircraft at an airport safely so
we've got to understand why we get low
levels of visibility so we can use that
information to help judge whether an
airport is safe or not to land that
levels of visibility lower due to a
number of natural environmental factors
the sun in your eyes makes you squint
and reduces your personal visibility for
example
but more importantly and more
prominently
is problems due to clouds
when you're inside a cloud and light
reflects off of the suspended water and
ice particles it can greatly impair
your ability to see out of the
windscreen
and which reduces your visibility
obviously when high up in the clouds
this isn't really an issue as if you're
flying in the clouds you were flying
using instruments under something called
instrument flight rules which is a way
of flying where you're not looking at
the window you're just looking at the
instruments
when you're close to the ground though
and trying to take off and land
you don't do these maneuvers off of
instruments you use
these
you do these maneuvers
and by actually looking out the window
do you need to be able to see where
you're going
this means that we are most interested
in the clouds that are at ground level
which is what we call fog or mist
fog is when you have visibility which is
less than a thousand meters and missed
is when it's less than 5000 meters
that's the only difference between the
two
there are a few different types of fog
and in the class on clouds i explained
what radiation fog is but i'll just give
a quick reminder now so when we have
relatively high humidity levels and on a
cold night the surface temperature of
the earth cools down
this cold surface cools the air just
above it through radiation
and if it cools the air down to below
the dew point then the moisture in the
air will condense and form into clouds
depending on the levels of wind the cold
air can then travel upwards and cool
down air higher up causing it more
condensation and a thicker layer of fog
or mist to form radiation fog is most
common first thing in the morning
as
more heat comes in throughout the day
the sun
starts to heat the surface back up that
surface starts to heat the air back up
and the air can move back above the dew
point temperature and the fog will
evaporate back into water vapor in the
air and it will clear you also get
advection fog which is a term used to
describe fog by warm humid air
blowing in over a cold surface
the cold surface then cools down this
warm humid air to below the dew point
and the water vapor condenses out into
fog
you most often see this in coastal areas
so a mass of moisture filled sea air
blows into shore and as it hits the cold
land
and or slightly colder land
this cools down this humid air and forms
fog spock
evaporation fog forms
when water vapor
evaporates
so warm water vapor evaporates into
colder air above it and the colder air
above it immediately cools the air back
down to below the dew point and the
water vapor will condense out to form
fog
these considerations for formation of
the fog are all relative and what i mean
by that is
when i say warm air
or cold air
and i don't mean warm air that's 25
degrees celsius i mean it's warmer when
compared to the cold condition
so with evaporation fog for example this
is very common in the arctic
the warm sea you know it's not really
warm it's arctic sea temperature
evaporates into the cold air above it
in reality both temperatures are pretty
cold but the water is just a fraction
warmer
and it is often called arctic sea smoke
as a result of this
frontal fog moves in with a front of
warm air
so as the warm air front moves in it
rises over the colder air mass because
it is less dense and any precipitation
that falls from the clouds in the warm
front
fall into cooler air below it
this precipitation also pools air down
with it as goes
and this
warm air descends into colder air and
causes the warmer air to cool down to
below the dew point and for clouds to
form at the surface in the form of fog
the precipitation that falls
also increases the levels of humidity at
the surface and will lead to a higher
relative humidity
which has the effect of increasing the
dew point temperature
which means that they don't need to cool
down as much for the air to condense out
orographic fog is caused by the air
being forced to rise
if a mountain gets in the way of humid
air blowing in it forces the air to rise
and as it does this it cools
adiabatically and if it cools to below
the dew point temperature the moisture
condenses out to form fog it's kind of
just like clouds but because the land
rises with the rising air it's
considered surface level so it's
fog so all these types of fog have
something in common and it's basically
just the cooling down
of warm humid air to below the dew point
it's as simple as that there's just a
few methods of doing that so these are
the main causes of natural fog or mist
and problems to do with visibility but
you can also get man-made or unnatural
visibility problems caused by pollution
essentially
so if we have very stable conditions
with a high pressure and low levels of
rising air then all the dust and smoke
particles don't rise into the air and
spread out they stay at low levels
and cause visibility issues when we're
trying to take off and land
so this happens most prominently in
areas with levels of
industry
high levels of industry high population
levels
and it's just
referred to as haze or smog rather than
fog or mist
as pilots we are interested in
visibility at the surface which will be
measured at the air drone but before we
go into that there's an important point
to make
so the visibility on the ground and the
visibility in the air will be slightly
different
so say we had an aircraft up here it
doesn't really matter what the height is
but we're above
a 200 meter layer
of fog and the visibility within that
fog is 500 meters
so while we're on the ground in our
aircraft and we look straight ahead we
would be able to see 500 meters ahead of
us but when we are directly above this
layer and we look directly down because
the layer of fog is less than 500 meters
deep it's only 200 meters deep we would
be able to see the surface
and as we look further and further away
from the nose of the aircraft we start
to look through more and more of the fog
and the visibility starts to reduce
because we're then getting close to this
500 meter maximum distance we're only
looking through 200 meters here but by
the time we get here we're looking
through a lot more and eventually we'll
get to the point where we're looking
through 500 meters
so you can think of it as either looking
further and further away
or if we get lower and lower and lower
we're going to therefore naturally have
to look through more and more of the fog
until we reach that point when we're on
the ground looking through the whole 500
meters of the fog in essence this means
that when we are higher up
we have a larger visual segment as we're
looking at the fog from an angle
and when we're lower down we're looking
through more and more of the fog and
less of the
air that is free of the fog so when you
have low visibility reported and you
look straight down and you can see all
the runway clearly
this is the reason why they have a low
visibility because when you're on the
ground
you can actually you're looking through
more of the fog and the visibility will
be a lot worse
so hopefully that makes sense
and it's all just to do with the angle
at which you're looking through it
and you're looking through more or
you're looking through less depending on
where you're actually aiming for so say
the runway start point was here the
lower and lower you get you're going to
be looking through more and more of the
fog and your visibility is going to be
worse
anyway
we measure visibility in
50 meter increments up to 800 meters
then 100 meter steps up to 5000 meters
and a thousand meter steps up to 10
kilometers
and if the visibility is more than 10
kilometers you'll just see a little plus
um on a weather report or you might see
four nines about to say nine nine nine
nine which means visibility is more than
ten visibility is a measure of
the opacity of the air how opaque it is
and we can either use physical objects
that are known distances away to gauge
it so
say you're standing at one side of the
airport you know the tower is 500 meters
away and you can't see it you know that
the visibility will therefore be less
than 500 meters
but more accurately you can use a bit of
kit called a transmissometer
which basically uses a light and a
sensor
to detect how much light is passing
through the air and therefore gives a
value of the opaqueness and opacity of
the air and the visibility levels so
when the visibility reaches certain low
levels depending on the airport a more
accurate measure of visibility can be
provided which is called a runway visual
range an rvr
and it applies only to the specific
landing runway
the reason for this is that some
aircraft will have a
bit of equipment called an automatic
landing system
which allows the aircraft to land in
very very low visibility conditions
this requires a high level of accurate
measurement
and accuracy in the visibility readings
which will allow the pilots to assess
the situation and see whether it's safe
or not to
carry out one of these types of approach
on that specific runway
so the airport might have a general
visibility of 700 meters for example
but the specific landway
lam way the specific landing runway
might have a reported runway visual
range of 300 meters so
and that would require special
procedures and the rvr is basically
runway specific visibility
whereas visibility in general is just
the airport's overall visibility so
quick class just to summarize then
you've got various types of cooling down
of humid air to form fog that's the
fundamental process behind all these
types of fog you cool down humid air and
it forms into fog you cool below the dew
point
so radiation is when the cold surface
cools the air above it to below the dew
point forming fog
advection is when warm humid air blows
in over a colder surface and the colder
surface cools down that warm humid air
to below the dew point forming fog
evaporation fog is when
there's cold air above a warmer sea
in the arctic for example and the
evaporating air immediately cools that
down to below the dew point and
condenses out to form fog
frontal fog is formed by falling
precipitation and air
coming into contact with colder
conditions
and the precipitation raises the
humidity levels so you've got humid air
and the colder air cools that down
and to form fog to below the dew point
an orographic fog
is when
terrain a mountain causes the air to
rise it cools as it rises and forms a
cloud but because that cloud is very
close to the land because it's close to
the mountain it's considered fog
and the difference between fog and
visibility a fog and mist is just the
visibility below a thousand meters is
fog and below five thousand meters is
missed
and if you've got visibility problems
due to man-made factors you do they call
it smog or haze visibility at airports
is reported in 50 meter increments up to
800 meters
100 meter increments up to 5000 meters
and a thousand meter increments up to 10
kilometers
and if it's more than 10 kilometers
you'll see a 10k plus or you might just
see
four nines on a report all the nines as
people call it so the difference between
visibility
and runway visual range is
visibility is just airport general
whereas rvr is runway specific which you
might need a very accurate measure of to
know if you can do
a more accurate type of approach for
example
and using the
automatic landing systems and then
you've got a important point to know is
your visual segment
basically the lower down you are the
more of the fog
or mist you're going to be looking
through so your visibility is going to
be worse say you're aiming for a point
this is the runway here you're aiming
for the runway as you get lower and
lower you're going to be looking through
more and more of the fog as you come
into land
so when some the airport reports a
visibility of 800 meters but you're you
can see the runway from the cruise
this is the reason why as you get lower
and lower you're going to be looking
through more
of the fog as you come into land
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