ATPL Meteorology - Class 3: Pressure.

Summary

In this meteorology class, we explore atmospheric pressure, its role in weather, and how it varies with altitude. The formula for pressure, p = f/a, is introduced, explaining how air pressure decreases with altitude due to fewer air particles and reduced weight. We learn about different pressure settings like QFE, QNH, and QFF, crucial for aviation, and their impact on altimeter readings. The video also delves into isobar charts, illustrating lines of equal pressure, and how they help predict weather patterns, such as wind strength indicated by the proximity of isobars.

Takeaways

• 🌍 Pressure is the force exerted per unit area and is a key factor in weather patterns.
• 📉 As altitude increases, air pressure decreases due to fewer air particles and reduced weight.
• 🔍 The standard rate of pressure decrease is approximately 1 hectopascal per 27 feet or 30 feet for quick calculations.
• 📏 Pressure is measured in hectopascals (hPa), which is equivalent to millibars (mb).
• 🌡 The international standard atmosphere at sea level has a pressure of 1013.25 hPa or 29.92 inches of mercury.
• ✈️ Altimeters use the difference in pressure to calculate altitude, applying a standard lapse rate.
• 🌡️ Temperature variations affect the lapse rate, which is why temperature corrections are crucial for accurate altitude readings.
• ✈️ QFE, QNH, and QFF are different pressure settings used in aviation, each with specific applications.
• 📊 Isobars on weather maps represent lines of equal pressure, and their spacing can indicate wind strength.
• 🌬️ Close isobars signify stronger winds, while widely spaced isobars indicate calmer conditions.

Q & A

What causes the sensation of ears popping during an aircraft's climb?

The sensation of ears popping is due to the sudden drop in pressure experienced as the aircraft ascends. This change in pressure is a result of the decrease in air density at higher altitudes.

How does pressure affect weather patterns?

Pressure changes on the Earth's surface are a significant driving factor in weather patterns. High and low-pressure systems influence the movement of air masses, which in turn affects temperature, humidity, and precipitation.

What is the formula for calculating pressure?

The formula for calculating pressure is p = F/A, where 'p' stands for pressure, 'F' is the force applied, and 'A' is the area over which the force is distributed.

Why does pressure decrease with an increase in altitude?

Pressure decreases with altitude because there are fewer air particles above exerting force per unit area. As altitude increases, the weight of the air column above decreases, leading to a reduction in pressure.

What is the standard rate of pressure decrease with altitude?

The standard rate of pressure decrease is approximately one hectopascal for every 27 or 30 feet of altitude increase.

How is pressure measured, and what units are commonly used?

Pressure is commonly measured in hectopascals (hPa), which is the same as millibars (mb). In some regions, inches of mercury (inHg) is also used. One hectopascal is equivalent to one millibar.

What is the sea level pressure in the international standard atmosphere?

In the international standard atmosphere, the sea level pressure is 1013.25 hectopascals (hPa) or 1013.25 millibars (mb), which is equivalent to 29.92 inches of mercury (inHg).

How do altimeters work, and what are the different pressure settings they use?

Altimeters work by sensing the difference in pressure between a datum point and the current pressure. They use different pressure settings such as QFE (field elevation), QNH (mean sea level pressure), and QFF (flight level pressure) to calculate altitude.

What is the difference between QNH and QFF in aviation?

QNH uses a standard lapse rate of 27 feet per hectopascal to calculate sea level pressure, while QFF factors in temperature corrections to provide a more accurate sea level pressure for the day's conditions.

How can temperature affect the pressure altitude reading in an aircraft?

In cold air, pressure levels become compressed, causing the true altitude to be lower than the indicated altitude. This is accounted for by applying temperature corrections to the pressure altitude reading.

What is an isobar, and how are they used in weather prediction?

An isobar is a line on a weather map connecting points of equal pressure. Isobars help predict weather patterns as they indicate areas of high and low pressure, which influence wind strength and direction.

Outlines

00:00

🌦️ Understanding Atmospheric Pressure

This paragraph introduces the concept of atmospheric pressure and its role in weather patterns. It explains that pressure changes, such as those experienced during an aircraft climb, are crucial for weather prediction. The pressure is defined by the formula p=f/a (force over area), and it decreases with altitude due to fewer air particles and less weight. The standard rate of pressure decrease is one hectopascal per 27 or 30 feet of altitude gain. Pressure is measured in hectopascals (hPa), which are equivalent to millibars, and the standard sea-level pressure is 1013.25 hPa or 29.92 inches of mercury. Altimeters use the pressure difference from a datum point to calculate altitude, with adjustments for temperature variations affecting the pressure lapse rate.

05:00

✈️ Altitude and Temperature Corrections in Aviation

The second paragraph delves into the practical application of pressure and temperature in aviation. It discusses the calculation of obstacle clearance for an aircraft flying at a specific altitude and temperature. The example provided walks through the process of determining the aircraft's true altitude by applying temperature corrections to the indicated altitude. The concept of isobars, lines of equal pressure, is introduced, and it's explained how meteorologists use these to predict weather. The paragraph also touches on the difference between QNH, which uses a standard lapse rate, and QFF, which includes temperature corrections for a more accurate sea-level pressure reading.

10:01

🌤️ Predicting Weather with Isobars

The final paragraph focuses on isobar charts, which display lines of equal pressure (isobars) and are used by meteorologists to forecast weather. It explains that the spacing of isobars can indicate wind strength, with closer isobars suggesting stronger winds. The paragraph summarizes the importance of understanding pressure, its measurement in hPa or millibars, and the significance of the standard atmospheric pressure at sea level. It also reiterates the use of QNH, QFE, and QFF in altimetry and weather prediction, emphasizing the role of temperature corrections in providing accurate altitude and pressure readings.

Keywords

💡Pressure

Pressure, as discussed in the video, refers to the force exerted per unit area. It is a fundamental concept in meteorology and aviation. The video explains that pressure decreases with altitude due to fewer air particles above exerting less force. This concept is crucial for understanding weather patterns and is used to predict changes in weather conditions. For instance, the video mentions that pressure changes are a 'huge driving factor in the weather we experience'.

💡Hectopascals

Hectopascals (hPa) are a unit of pressure used in meteorology. The video states that pressure is measured in hectopascals, with one hectopascal being equivalent to one millibar. This unit is used to quantify changes in atmospheric pressure, which is essential for weather forecasting and aviation. The standard atmospheric pressure at sea level is given as 1013.25 hPa.

💡Altitude

Altitude is the height of an object or point above a fixed reference point, usually sea level. In the context of the video, altitude is directly related to pressure changes. As altitude increases, the pressure decreases, which is a key principle in aviation. The video uses the example of a standard rate of pressure decrease: 'one hectopascal drop for every 27 feet increase in altitude'.

💡Isobar

An isobar is a line on a weather map connecting points of equal atmospheric pressure. The video explains that isobars are used to predict weather patterns, with closer isobars indicating stronger winds and wider spacing suggesting calmer conditions. Isobars are part of isobar charts, which are essential tools for meteorologists to analyze and forecast weather.

💡QFE, QNH, QFF

These terms refer to different pressure settings used in aviation. QFE is the pressure setting measured at the airfield corrected down to sea level, QNH is the pressure setting based on the standard atmospheric pressure at sea level, and QFF is the pressure setting measured at the airfield corrected for temperature variations. The video explains that these settings are crucial for accurate altitude readings and safe flight operations.

💡Lapse Rate

The lapse rate in the video refers to the rate at which temperature decreases with an increase in altitude. It is an important concept in meteorology and aviation because it affects the density of the air and thus the pressure. The video mentions that temperature variations influence the lapse rate, which in turn affects the pressure settings used by pilots.

💡Altimeters

Altimeters are instruments used to measure altitude. The video explains that altimeters work by sensing the difference in pressure between a datum point and the current pressure. This is crucial for pilots to determine their height above the ground or a specific pressure level, which is essential for safe navigation.

💡Iso Deviation

Iso Deviation, or isothermal deviation, refers to the difference between the actual temperature at a given altitude and the standard temperature for that altitude. The video uses iso deviation in the context of calculating true altitude, where colder-than-standard temperatures compress the pressure levels, leading to a lower true altitude than indicated.

💡Meteorology

Meteorology is the scientific study of the atmosphere, especially as it relates to weather forecasting. The video is part of a meteorology series, focusing on the understanding of atmospheric pressure and its role in predicting weather. The video explains various meteorological concepts like pressure, altitude, and isobar charts, which are essential for predicting weather patterns.

💡Weather Prediction

Weather prediction is the process of forecasting the future state of the atmosphere, including temperature, humidity, wind, and precipitation. The video emphasizes the importance of understanding pressure patterns, such as isobars, for predicting weather. It provides examples of how meteorologists use these patterns to anticipate changes in weather conditions.

Highlights

Pressure changes in aircraft can cause ears to 'pop' due to rapid adjustments to altitude and pressure.

Surface pressure patterns are crucial for predicting weather conditions.

Pressure is defined by the formula p = f/a, where p is pressure, f is force, and a is area.

Air pressure decreases with altitude due to reduced weight and fewer air particles at higher levels.

The standard pressure decrease rate is approximately 1 hectopascal per 27 or 30 feet of altitude gain.

Pressure is measured in hectopascals (hPa), which is equivalent to millibars (mb).

The international standard atmosphere at sea level has a pressure of 1013.25 hPa or 29.92 inches of mercury.

Altimeters use the difference in pressure and a standard lapse rate to calculate altitude.

QFE, QNH, and QFF are different pressure settings used in aviation for various altitude readings.

QFF accounts for temperature variations, providing a more accurate sea level pressure for weather prediction.

Temperature corrections in altitude calculations are essential, especially in cold air where pressure levels compress.

A practical example is provided to calculate obstacle clearance with temperature corrections.

Isobars on weather maps represent lines of equal pressure and are used to predict weather patterns.

Close isobars indicate stronger winds, while widely spaced isobars suggest calmer conditions.

Meteorologists use isobar charts with QFF to predict weather more accurately.

The video concludes with a summary of key points about pressure, its measurement, and its role in weather prediction.

Transcripts

when climbing in an aircraft your ears

will suddenly pop due to that sudden

drop of pressure and the pressure

changes on the earth's surface are a

huge driving factor in the weather that

we experience

so what is pressure anyway how do we use

surface pressure patterns to help

predict what the weather is going to be

let's find out

[Music]

hi i'm grant and welcome to the third

class in the meteorology series in this

class we're going to be continuing our

breakdown of the atmosphere and take a

deeper dive into the pressure element

pressure is given by the formula that p

equals f over a force over area

if we consider a column of air maybe

i don't know one kilometer high

something like that it's not really

important with a fixed um

surface area as we travel up

then as we get higher up the column the

weight of the air which is the mass

times the acceleration due to gravity

reduces and reduces as we travel up the

column

there's also fewer particles higher up

and more particles lower down

because they're basically being pulled

down by gravity

so if we look at this section of air

down near the surface it has a larger

amount of air particles pushing down on

it when you compare that to

this section of air up near the top

this means that the force element the

weight element

of our

pressure equation reduces

as

we get

higher up

and that in turn

makes the pressure reduce and then down

near the bottom there's more weight so

that's more force

and that means there's more pressure

so we can say that as altitude increases

the pressure decreases

the standard rate at the which this

happens

is

we reduce in pressure by one heck to

pascal for every 27 or 30 feet for quick

calculations

we measure pressure in hectopascals

which is exactly the same thing as a

millibar so one hectopascal is one

millibar

and in the night the united states they

measure things and in some other

countries i'm sure they measure things

in inches of mercury as well and in the

international standard atmosphere the

sea level pressure

is equal to

1013.25 hits pascals or 101 3.25

millibars which is the equivalent of

29.92

inches of mercury as well

altimeters work by sensing the

difference in pressure between a datum

point and the current pressure

and then you multiply the difference by

that 27 foot per hectare pascal lapse

rate

we can set a few things as our datum we

can have the qfe the qnh or the standard

setting

i've made a video previously on

altimetry in the general navigation

series where

i go into a bit more detail i'll link

that below

and if you're happy with pressure

settings and stuff like that what q and

h are what qfe are in standard then

continue watching this video but if

you're unhappy i'd recommend you pause

this go and watch that other one first

to explain all of these terms

okay

so

we have the date and pressure settings

of q and h qfe and standard and

meteorologists we use one more which is

called the qff

this is the pressure setting

measured at the airfield or weather

station

corrected down to the sea level pressure

setting for the actual day's conditions

this is different to qnh

because qnh uses the standard 1 hecta

pascal every 27 feet

whereas the qff

is giving the

adjusted lapse rate you can think of it

as that because temperature variations

have an influence on this lapse rate

temperature corrections are very

important when flying

usually in cold air because the air

levels the pressure levels become

compressed together and your true

altitude is lower than your indicated

altitude

so the qff factors in this temperature

correction and the correction that we

normally apply is four feet

for every thousand feet

for every degree of iso deviation

or

as a good rule of thumb you can use a

one percent

altitude error

for every 2.5 degrees of iso deviation

so that's where the main difference is

between qff and qna is q h is using 27

feet per hectare pascal and the qff is

factoring in this temperature correction

to in order to give us a different lapse

rate so you'll get the more accurate sea

level pressure for that day when using

qff which is why meteorologists like to

use it because it's better for

predicting weather whereas us aviators

we use q h because

it's not really that important um from

the majority of things it's only when it

gets really cold that you see

significant changes in your altitude

so i'm just going to do a quick example

of the temperature error calculation

for more information as i said before go

back and watch that video on altimetry

that was in the gnab series

so this example will assume that you're

comfortable with a few of the

definitions vertical distances et cetera

and

yeah so anyway

an aircraft is at flight level 200 where

the temperature is minus 40 degrees c

the q h at nearby airfield is 998

hectopascals there's an obstacle on

route at 800 feet what obstacle

clearance does the aircraft have

so draw the effing picture

flight level 200 equals 20 000 feet in

pressure altitude and that's based off

of standard which is one zero one three

so i'll draw a line here one zero one

three

and twenty thousand feet above that is

our aircraft

we then have the q and h which is going

to be a lower pressure of 998 and that's

going to be higher up so it's going to

be somewhere up here

and then we can find out the distance in

here to find out our indicated altitude

which is above the q and h

so this distance is very easy to

calculate 1013 minus 998 times 27

which is equal to 405 feet so this

distance in here

405 feet which means our indicated

altitude our height above the q and h is

going to be 20 000 minus 405

so our indicated out

i don't know why i've done that i out um

equals 20

000

minus

405

which is equal to

one nine

five nine five

feet

so that's our indicated altitude we

apply temperature corrections to this to

get our true altitude

so we have to figure out the iso

deviation

so normally

at 20 000 feet

the temperature would be 15 degrees and

then 2 degrees per thousand feet so 15

minus 40

is going to be

minus 25 so iso at 20 000 feet so iso

temperature at 20 000 feet is minus 25

degrees celsius and it today is minus 40

so the iso

deviation

is 15 degrees colder

equals minus 15 degrees

and then we apply the temperature

correction so it's four feet

for every thousand feet that we're above

so let's call that 20. or more

accurately 19.6 i suppose it should be

that's our indicated altitude 0.6

and then we

per also multiply that and multiply this

by the 15 degrees

4 times the 19.6

for in thousand of feet above

the q h

and times that by our iso deviation 15.

and that's one one seven six one one

seven six feet of altitude correction to

make and we're gonna be lower because it

is colder than i said everything's

getting squished together so it's gonna

be this answer take away this number

so our true out is equal to

uh let's just do that on the calculator

19595

minus one one seven six

that's going to be eighteen thousand

four hundred and nineteen feet

and we're asking for the obstacle

clearance

and so how far above this eight thousand

through oscar just take away the eight

thousand

and our obstacle clearance

is equal to 10

419

feet

or

and you don't have to do the temperature

correction this way

you can do that um

one percent per 2.5 degrees of iso

deviation i was talking about

so we'll do a quick calculation of that

so the iso deviation

is minus 15. so 15 divided by 2.5 is

gonna be six right so we're doing a six

percent change

um

in altitude so 19595

times 0.06 that's six percent

and we're looking at one one

seven

point

and the actual difference is one one

seven six so it's really really close

it's really quite a good estimation and

then we would take that from the 19595

and that is our uh answer in here

and then you do this obstacle clearance

the same way so the one percent per 2.5

degrees

is just as good if not maybe a bit

quicker than the temperature correction

of four feet per thousand feet per

degree of iso deviation so this is

probably a diagram you've seen somewhere

before or something a bit more colorful

and a bit better than this but it's

showing what we call isobars

these lines are all lines of equal

pressure and it's the calculated qff

from an airfield or the actual measured

pressure at sea level

and every point along this line has

exactly the same pressure

and the difference between the lines is

normally

either two or four hectopascal so this

would be one

thousand this would be one

thousand and two

as with this one

or this could be the other way around so

this could drop down to or they're both

going to highs but this one here would

be nine nine eight this one here would

be nine nine eight and so on

and then you drop down these eyes of our

charts can be named and labeled in

various ways we give the highest

pressure

um on the chart in h and the lowest in l

but there can be obviously secondary

high points with the same high pressure

as this and secondary low points

and ice charts are very useful

as pressure systems

and areas have fairly predictable

weather

and which is something we'll learn about

more in future classes but just a quick

example um for you it would be if the

isobars are close together it means that

the wind is going to be stronger than if

they're quite far apart like this so

this is going to be a very windy area

this will be very calm over here

there are many more patterns and

predictions that can be made using

isobar as well as i said we'll look at

some of them in future classes to

summarize then pressure is the force

over the area and because we have fewer

particles above us that means our weight

is lower and our force is lower as we

climb up through the atmosphere so that

means our pressure reduces so as

altitude increases the pressure

decreases the rate at which this happens

is

one hecta pascal drop for every 27 feet

increase in altitude or 30 feet for easy

calculations

we measure pressure in hectopascals one

hit pascal is equivalent to one millibar

and in the international standard

atmosphere at sea level we have a 101

3.25 hectopascal pressure or 101 3.25

millibar pressure

or if you're measuring it in somewhere

that uses inches of mercury it's 29.92

in higgs inches of mercury hgs being the

chemical symbol for mercury so in terms

of altimetry as i said there's a good

class well i think it's quite good um i

did in the gnab series explaining about

this a bit more but basically if you're

setting q h you're reading indicated

altitude if you're setting standard

which is one zero one three then you're

reading a pressure altitude or a flight

level if you

round it up and take off the last two

zeros

and if you're setting qfe you're reading

height above the ground

um and the height of the ground from the

sea level to the

highest point of the ground is known as

elevation

so qnh is calculated

by sensing the pressure at the airfield

or the weather station then using 27

feet per every hecta pascal with your

known elevation to calculate an

equivalent sea level pressure

the qff

does this but doesn't use a standard 27

feet per every hit pascal it factors in

temperature corrections

so it uses the daily lapse rate in a

sense

um so it's the 27 feet adjusted for

temperature and that temperature

correction is four feet for every

thousand feet

um for every degree of iso deviation or

a good estimation well very accurate

decimal estimation is one percent of

altitude

for every 2.5 degrees of iso deviation

and then we have isobar charts

which use the qff which is the

equivalent sea level pressure or the

actual measured sea level pressure and

the isobars all have equal pressure iso

meaning

uh the same i believe so

iso

bar same bar lines i don't know

um

but yeah they're all the same pressure

and there's usually about two or a four

hectopascal difference between them all

so that'll be one thousand this would be

one thousand and two

1004 and this would be 1006 which is

actually not a very high pressure but

yeah and we use the ice bar charts to

help predict weather which we'll look at

in future

but just a quick sneak peek if the

isobars are really close together it

means it's gonna be a lot more windy

than if they're quite spread apart