ATPL Meteorology - Class 10: Visibility.

Summary

This 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

• 🌫️ Visibility is crucial for safe aircraft landings at airports, and understanding what causes low visibility can help determine the safety of landing.
• 🌞 Reduced visibility can be caused by natural environmental factors like sunlight glare or cloud cover, which reflects light and impairs viewing through a windscreen.
• ✈️ At higher altitudes, pilots rely on instruments and instrument flight rules, but near the ground, they depend on visual cues for takeoff and landing.
• 🌁 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.
• 🌡️ 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.
• 🌤️ 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.
• 💧 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.
• 🌦️ 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.
• 🏞️ 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.
• 🏭 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.
• 📏 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

00:00

🌫️ 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.

05:00

🌬️ 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.

10:00

📏 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.

Keywords

💡Visibility

Visibility refers to the ability to see and be seen, especially in terms of the clarity of the atmosphere. In the context of the video, visibility is crucial for safe aircraft landings at airports. The video discusses how visibility can be reduced due to natural factors like fog and mist, and also due to man-made factors like pollution. It explains that visibility is measured in increments and is vital for pilots to judge the safety of landing conditions.

💡Fog

Fog is a natural phenomenon where tiny water droplets or ice crystals are suspended in the air, reducing visibility to less than a thousand meters. The video explains different types of fog, such as radiation fog which forms when the ground cools the air above it, and advection fog which occurs when warm humid air blows over a cold surface. Fog is a primary factor affecting visibility near airports, impacting the decision to land or divert an aircraft.

💡Mist

Mist is similar to fog but has a visibility range between 1,000 and 5,000 meters. It is also formed by the condensation of water vapor in the air when it cools. The distinction between fog and mist in the video is based on the level of visibility they create, with mist being less dense and allowing for greater visibility compared to fog.

💡Dew Point

The dew point is the temperature at which air becomes saturated and water vapor begins to condense into liquid water. In the video, it is mentioned as a critical threshold in the formation of fog and mist. When the air cools to below the dew point, the moisture condenses, leading to the formation of clouds close to the ground, which impair visibility.

💡Radiation Fog

Radiation fog is a type of fog that forms when the ground cools the air above it through radiation, particularly during the night. As the ground loses heat, it cools the air, causing the moisture in the air to condense into fog. This type of fog is most common in the early morning and typically clears as the sun warms the ground and air throughout the day.

💡Advection Fog

Advection fog occurs when warm, humid air moves over a colder surface, causing the air to cool to below its dew point and form fog. This type of fog is often seen in coastal areas where moist sea air moves over the colder land. The video uses the example of coastal areas to illustrate how advection fog forms and its impact on visibility.

💡Evaporation Fog

Evaporation fog, also known as steam fog or Arctic sea smoke, forms when warm water evaporates and the resulting water vapor comes into contact with colder air, causing it to cool and condense. This type of fog is common in regions with cold climates, such as the Arctic, where the relatively warmer sea water evaporates into the cold air above.

💡Frontal Fog

Frontal fog is associated with weather fronts, particularly warm fronts. When a warm front moves in, it rises over the colder air mass below. Precipitation falling from the warm front encounters the colder air, causing the air to cool and the moisture to condense into fog at the surface. The video explains how this process increases humidity levels and dew point temperature, contributing to the formation of fog.

💡Orographic Fog

Orographic fog is formed when air is forced to rise, often due to the presence of a mountain or other terrain features. As the air rises, it cools adiabatically, and if it cools to below the dew point, the moisture condenses to form fog. This type of fog is closely associated with the landform that causes the air to rise, as opposed to clouds that form at higher altitudes.

💡Runway Visual Range (RVR)

Runway Visual Range (RVR) is a measure of visibility specific to a particular runway, which is especially important for pilots when landing. The video explains that RVR can differ from the general visibility at an airport, as it accounts for the specific conditions on the runway. This measurement is crucial for determining whether it is safe to land using visual references or if special procedures are required.

💡Transmissometer

A transmissometer is a device used to measure visibility by emitting a light beam and measuring how much light is transmitted through the air. It provides a quantitative measure of the opacity of the air, which is essential for assessing visibility conditions at airports. The video mentions the use of a transmissometer as a more accurate method of measuring visibility compared to relying on physical objects at known distances.

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