ATPL Meteorology - Class 3: Pressure.
Summary
TLDRIn 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
🌦️ 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.
✈️ 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.
🌤️ 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.
Mindmap
Keywords
💡Pressure
💡Hectopascals
💡Altitude
💡Isobar
💡QFE, QNH, QFF
💡Lapse Rate
💡Altimeters
💡Iso Deviation
💡Meteorology
💡Weather Prediction
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
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