2. Pressure

pressure defn

Pressure is the force per unit area exerted by molecules of air in random motion, acting

uniformly in all directions. This is called Static Pressure or Barometric Pressure.

dynamic pressure

When air is in motion it also carries an additional pressure, ½ρV², which it exerts on any surface facing into

the flow (this is what a pitot tube senses). This is called Dynamic Pressure or Wind Pressure. Static pressure

is still present in moving air.

unit of pressure

hectopascal

1 hPa = 1000 dyn/cm²

std sea lvl pressure

1013.25hpa = 760mm = 29.92 in = 101325 N/m3 (1bar)

vertical variation of px

pressure decreases with height at a decreasing rate

pressure at msl , 5k ft , 10k ft , 20k ft , 30k ft

5k ft or 1.46km = 850hPa 1hpa = 31t

10k ft = 700hPa 37

20k ft = 500hPa 48

30k ft = 300hPa 73

1hPa = 27ft = 8m

Height Difference with 1 hPa Change

Formula:

Height difference = 96 × T / p feet

(where T = temperature in Kelvin, p = pressure in hPa)

In ISA conditions, 1 hPa pressure change corresponds to approximately:

Level MSL 2,000 ft 20,000 ft 40,000 ft

Height equivalent 27 ft 30 ft 50 ft 100 ft

Variation of Pressure – Warm vs Cold Air

Cold air is denser than warm air. Pressure falls at a faster pace over a cold column of air than over a warm column.

isobars and variation of px

A pressure level such as 850 hPa lies at a greater height over warm air than over cold air.

Key Rule: Pressure at any given height will be higher over warm air than over cold air.

• Isobars Dip going from High to Low pressure

• Isobars Rise going from Low to High pressure

• Isobar = line joining places of equal pressure

Semi-Diurnal Variation of Pressure

Atmospheric pressure follows a bimodal curve during a day:

Maxima: 1000 hr and 2200 hr (local time)

• Minima: 0400 hr and 1600 hr (local time)

This semi-diurnal variation is due to solar influence.

Variations are very small at the Poles, about 1 hPa in temperate latitudes and about 3 hPa in the tropics

(some Indian texts quote up to 3–5 hPa at the Equator).

This is probably a natural oscillation of the atmosphere with a period of almost 12 hours .

PRESSURE IS LOWEST AT 4AM AND 4PM

Measurement Instruments

mercury barometer -atm px measured balanced by height of mercury column

aneroid barometer - uses metal capsule not as accurate as mercury

altimeter - aneroid barometer with scale graduated in altitude instead of pressure; can be

set to desired pressure value

barograph - Continuously records pressure; can be Daily Barograph or Weekly Barograph

Altimeter correction ALTICOR

The correction applied to indicated altitude to obtain true altitude is called Altimeter Correction (also

called Alticor).

This is frequently required for mountain flying and bombing operations.

D Value = True Altitude − Indicated Altitude

Correction to obtain true altitude = True Altitude − Indicated Altitude

D Value decreases when aircraft flies from High to Low pressure

rough calculation for alticor

(a) Pressure Correction:

Add 30 ft for every 1 hPa when MSL pressure is higher than 1013.2 hPa

Subtract 30 ft per 1 hPa when MSL pressure is lower than 1013.2 hPa

(b) Temperature Correction:

Add 1% of indicated altitude for every 3°C temperature is higher than ISA

Subtract 1% per 3°C when temperature is lower than ISA

(c) Add these algebraically for final correction

H-L-H

High to Low → Over Read (altimeter reads High = dangerous, you're lower than indicated)

Low to High → Under Read (altimeter reads Low; this alone does not establish terrain clearance)

Warm to Cold → Over Read | Cold to Warm → Under Read

If QNH is High → True Alt will be High

Contours (contour lines)

Lines joining places of equal height are called Contours. Contour lines may also be regarded as isobars.

In Contour Charts, contours are drawn at an interval of 40 gpm (geopotential metre) in 700 hPa and

500 hPa level charts; and at 80 gpm in 300 hPa and 200 hPa level charts.

1 gpm = 9.8 J/kg (gravitational potential energy per unit mass)

Contour lines numbered in geopotential decametres, e.g. 5280 gpm = 528 .

A geopotential height of 700 gpm is approximately 700 m. A chart label 700 in geopotential decametres

represents 7000 gpm; always check the chart units.

Centres of Low and High are marked as in surface pressure charts.

Thickness

The height interval between the lower and upper level is called Thickness. Low thickness = lower mean

temperature; High thickness = higher mean temperature. The isopleths of thickness coincide with the

isotherms of mean temperature of the layer

Pressure Gradient

Pressure gradient is the horizontal rate of change of pressure perpendicular to the isobars, directed from

high to low pressure.

• Isobar = line joining places of equal pressure.

• Steep pressure gradient → isobars are close together → strong winds.

• Flat/weak gradient → isobars far apart → light winds

pressure tendency

The change of pressure with time is called Pressure Tendency. In India, pressure tendency is worked out for

the past 24 hr and in higher latitudes for the past 3 hours of the current observation

Isallobars

Isallobars = lines joining places of equal pressure change. At a glance, isallobars indicate areas of rising or falling pressure tendency.

• Isallobaric Low = region of greatest fall enclosed by isallobars.

• Isallobaric High = region of highest rise.

Surface Lows tend to move towards Isallobaric Low.

Isallobaric High indicates weakening of a pressure system.

MSL defn

A reference level taking average of high and low tides

Altitude

Vertical distance from MSL.

Height

Vertical distance from a specific datum (e.g., from the ground).

Elevation

Vertical distance of a point or level on the surface of the earth from MSL.

Transition

Altitude (TA)

The altitude at or below which the vertical position of an aircraft is controlled by reference to altitude (local QNH)

Transition Level TL

The lowest flight level available for use above the transition altitude. At and above it the altimeter

is set to the Standard Pressure Setting, 1013.25 hPa , and vertical position is expressed as a

flight level

Transition Layer

The airspace between Transition Altitude (TA) and Transition Level (TL).

Flight Level

Surfaces of constant pressure related to the standard pressure datum of 1013.25 hPa, separated

by specified pressure intervals (usually 500 or 1,000 ft), expressed in hundreds of feet. E.g., FL50

= 5,000 ft , FL300 = 30,000 ft , FL200 = 20,000 ft

Pressure Altitude

When the altimeter sub-scale is set to 1013.2 hPa , the altimeter indicates Pressure Altitude.

Expressed as FL35, FL190, FL400 (avoid last two zeros)

Pressure setting

Corrections applied to barometer readings to reduce to a common level (MSL) are called Pressure Setting.

The corrections are: Index correction (instrumental error), Gravity correction (varies with latitude), and

Temperature correction (varies with temperature).

Pressure settings: QFE, QFF and QNH

QFE

Pressure at the specified aerodrome or runway pressure datum. With QFE

set, the altimeter reads approximately zero at that datum - height above the aerodrome

QFF

Barometric pressure of aerodrome reduced to MSL using the actual temperature of the place (column of air extending upto MSL ) - plotting synoptic charts and drawing isobars

QNH

Station pressure reduced to mean sea level using the standard atmosphere.

With QNH set, the altimeter reads the aerodrome elevation at its datum.

Absolute altitude means height above ground and is a different quantity.- Vertical separation of

aircraft and terrain

Regional

QNH

Forecast value of the lowest pressure expected in an Altimeter Setting

Region (ASR). Issued every hour, valid for one hour.

QNE

Altitude indicated when sub-scale set to 1013.25 hPa (29.92 in) . Used

for high altitude airfields.

Station elevation formula

Station Elevation ≈ (QNH − QFE) × 30 ft

QFF vs QNH and Temperature

Aerodrome ABOVE MSL: warmer than ISA → QFF < QNH; colder than ISA → QFF > QNH

Aerodrome BELOW MSL: reversed (warmer → QFF > QNH; colder → QFF < QNH)

At MSL: QFF = QNH whatever the temperature

Pressure set on altimeter < QNH → Indicated Altitude < True Altitude

Pressure set on altimeter > QNH → Indicated Altitude > True Altitude

standard isobaric settings

winds and altimeter

Critical: When flying with winds from port, the altimeter will over-read. With winds from starboard, it

will under-read. Error increases with strength of wind and length of route. In S hemisphere the opposite is

true.

buy ballots law = your back towards wind , then low is on your left

then if wind from left we are flying towards the low

Low definition

Area enclosed by isobars with lowest

pressure at centre. Two or more

closed isobars = Depression. if Severe = Cyclone

Cloud and precipitation can

occur with rising moist air;

visibility varies

N Hemisphere: winds

blow anticlockwise,

converging & upward

High (H) /

Anticyclone

Region enclosed by isobars with

highest pressure at centre.

Fair weather but visibility

may be poor

N Hemisphere: winds

blow clockwise, veer

and diverge outwards

Trough

"V" shaped extension of isobars from

a low. Wind direction abruptly

changes and backs.

Bad weather, associated

with monsoon trough

along Indo-Gangetic plains

Ridge

wedge like extension of isobars from high. Isobars are rounded not V shaped

Fair weather

Winds VEER at a ridge

COL

Region of almost level pressure

between two highs and two lows:

pressure is higher than in the lows

and lower than in the highs.

light variable winds

winds light variable

pressure is uniform

Cold Anticyclone (temporary)

forms in the cold air behind a depression , before the next one arrives

short lived upto 24hrs descending air may warm up by compression so it may warm up

Blocking

Anticyclone

Any High that is slow moving or near stationary that blocks or diverts migratory depressions , usually an extension of warm subtropical high .

settled weather , can persist for weeks

depressions are steered around it

Quick ref

Low = Cyclone = Convergence = Rising air = Bad Wx

High = Anticyclone = Divergence = Subsidence = Good Wx (but poor vis)

Trough = V-shape = Bad Wx | Ridge = Rounded = Good Wx | Col = Uniform = Light winds

Q11. Two aircraft flying at same indicated altitude with altimeters set to 1013.2 hPa. One is flying

over cold air mass, other over warm air mass. Which has greater altitude

Answer: (a) Aircraft flying over warm air mass — Warm air is less dense, so pressure falls more slowly

with height and a given pressure level lies higher. With 1013.2 hPa set, both aircraft are on the same

pressure surface, but over warm air that surface is higher, so the aircraft over the warm air mass has the

greater true altitude.

❌ (b) Over cold air the pressure levels are squashed downwards, so the same indication means a lower

true altitude.

💡 Warm air: true altitude above indicated. Cold air: true altitude below indicated.

iso

Iso = equal; Bar = pressure (like barometer). Standard meteorological lines: Isobar, Isotherm, Isohyet

(rainfall), Isoneph (cloudiness).

What type of inversion occurs when a stable layer lies in a high pressure area?

Subsidence — In a high-pressure area the air sinks, is compressed and warms, forming a

warm layer above cooler air below: a subsidence inversion. It traps haze and smoke and is a key cause of

poor visibility

Q39. Which of the following would cause true altitude to decrease when the altimeter indicates a

constant altitude? (a) Warm/Low (b) Cold/High (c) Cold/Low (d) Warm/High

✅ Answer: (c) Cold/Low — True altitude is less than indicated when the air is colder than standard (the

pressure levels are squashed downwards) and when you fly into lower pressure than the subscale setting.

Cold and Low together lower the true altitude.

❌ Warm and High both raise the true altitude above the indication; Warm/Low and Cold/High have one

effect each in opposite directions.

💡 Cold/Low: true below indicated , the dangerous direction over high ground