Wind & Pressure Systems — Summary Notes

1. Wind — Basic Concept

Wind is the horizontal movement of air over the Earth’s surface.

Cause of wind

• Wind is initiated by a horizontal difference in atmospheric pressure.
• The initiating force is the Pressure Gradient Force (PGF).
• PGF acts:
  • From High pressure → Low pressure
  • Perpendicular to isobars
• Coriolis force and friction modify the wind after it begins moving; they do not initiate it.

Pressure gradient

• Closely spaced isobars → strong pressure gradient → strong winds
• Widely spaced isobars → weak pressure gradient → light winds

2. Pressure Gradient Force (PGF)

Memory

Close isobars = steep pressure gradient = strong wind.

3. Coriolis Force

The Coriolis force is an apparent force caused by the rotation of the Earth.

Direction of deflection

• Acts at 90° to the direction of motion.
• Its magnitude is proportional to sin(latitude).
• It influences wind direction, not wind speed.
• It becomes negligible near the Equator.

Operational significance

The Coriolis force allows moving air above the friction layer to turn until it reaches an approximate balance with the PGF.

4. Geostrophic Wind

Geostrophic Wind (GW) is the theoretical wind produced when:

PGF = Coriolis Force

It applies above the friction layer with:

• Straight, parallel isobars
• Sufficient Coriolis force
• Typically above about 2,000–3,000 ft AGL
• Generally at latitudes greater than about 15° N/S

Characteristics

• Flows parallel to isobars.
• PGF acts toward low pressure.
• Coriolis force balances PGF.
• There is no cross-isobar flow in ideal geostrophic conditions.

5. Geostrophic Wind — Northern Hemisphere

In the NH:

• Coriolis deflects moving air to the right.
• The final geostrophic wind flows parallel to the isobars.
• Low pressure is on the left when facing downwind.
• High pressure is on the right.

Aircraft flying High → Low

If an aircraft flies directly from high pressure toward low pressure:

• PGF direction = aircraft track.
• Geostrophic wind is approximately 90° to the left of the aircraft’s track.
• The wind therefore produces port drift.

6. Geostrophic Wind — Southern Hemisphere

In the SH:

• Coriolis deflects moving air to the left.
• With your back to the wind, low pressure is on your right.
• When facing the wind, low pressure is on your left.

7. Friction and Surface Wind

Friction becomes important near the Earth’s surface.

Effect of friction

Friction → wind speed decreases → Coriolis force decreases

PGF remains essentially unchanged.

Therefore:

PGF > reduced Coriolis effect

and the surface wind is deflected across the isobars toward low pressure.

Surface wind compared with wind aloft

8. Surface Wind vs Wind at 2000–3000 ft

Above friction layer

• Wind is faster.
• Wind is approximately geostrophic/gradient.
• Flows approximately parallel to isobars.

At surface

• Friction slows the wind.
• Reduced speed means reduced Coriolis force.
• Wind crosses the isobars toward low pressure.

Northern Hemisphere

Surface wind is generally:

Backed relative to the wind above the friction layer.

9. Backing and Veering

Backing

Wind direction changes anticlockwise.

Example:

270° → 250° = backing

Veering

Wind direction changes clockwise.

Example:

310° → 020° = veering

The terminology applies in both hemispheres.

Easy memory

Veer = clockwise
Back = anticlockwise

10. Diurnal Variation of Surface Wind

The day/night variation is particularly important over land.

Daytime

Solar heating produces turbulence and vertical mixing.

• Surface air mixes with faster-moving air above.
• Surface wind becomes stronger.
• Wind direction becomes closer to the geostrophic direction → veers.
• Maximum surface wind is around 1500 LMT.

Night

Surface cooling stabilizes the atmosphere.

• Turbulence decreases.
• Friction has a greater effect.
• Wind becomes weaker.
• Surface wind becomes more deflected toward low pressure → backs.
• The source gives the minimum around 30 minutes after sunrise.

Nighttime deflection over land can reach approximately 45°.

11. Buys-Ballot’s Law

Buys-Ballot’s Law relates wind direction to pressure distribution.

Northern Hemisphere

With your back to the wind:

Low pressure is on your left.

Southern Hemisphere

With your back to the wind:

Low pressure is on your right.

Important

At the surface, winds cross the isobars toward low pressure because of friction.

12. Low-Pressure Circulation

Northern Hemisphere

Low-pressure/cyclonic circulation:

• Anticlockwise
• Inward
• Convergent at the surface
• Rising air
• Associated with clouds and precipitation

Southern Hemisphere

Low-pressure/cyclonic circulation:

• Clockwise
• Inward
• Convergent at the surface

13. High-Pressure / Anticyclonic Circulation

Northern Hemisphere

• Clockwise
• Outward/divergent at surface
• Descending air

Southern Hemisphere

• Counterclockwise
• Outward/divergent at surface
• Descending air

Do not confuse

Anticyclonic does NOT mean anticlockwise.

The direction depends on the hemisphere.

14. Strongest Winds

The strongest surface winds in temperate latitudes are normally associated with:

Low-pressure systems / depressions / cyclones

Reason:

Low → close isobars → steep PGF → strong winds

High-pressure systems generally have wider isobar spacing and lighter winds.

The source identifies temperate latitudes approximately as:

40°–65°

15. Gradient Wind

Gradient Wind (GRW) is wind flowing parallel to curved isobars above the friction layer.

It involves three forces:

1. Pressure Gradient Force
2. Coriolis Force
3. Centrifugal Force

Comparison

16. Gradient Wind Around a Low

Around a low-pressure system:

• PGF is directed inward.
• Centrifugal force acts outward.
• Centrifugal force therefore opposes PGF.
• Less Coriolis force is required.
• Since Coriolis force is related to wind speed, the resulting gradient wind is slower.

Therefore:

Low pressure

Gradient Wind < Geostrophic Wind

This is called:

Sub-geostrophic flow

17. Gradient Wind Around a High

Around a high-pressure system:

• PGF acts outward.
• Centrifugal force also acts outward.
• They act in the same general direction.
• Greater Coriolis force is required.

Therefore:

High pressure

Gradient Wind > Geostrophic Wind

This is called:

Super-geostrophic flow

Memory

Low → GRW slower than GW
High → GRW faster than GW

18. Cyclostrophic Wind

Cyclostrophic flow occurs when:

PGF ≈ Centrifugal Force

with the Coriolis force negligible.

Conditions

Most applicable to:

• Very low latitudes
• Near the Equator
• Very strong pressure gradients
• Very high wind speeds
• Highly curved flow

Examples

• Intense tropical revolving storms
• Tornadoes

The source gives tornado vortex speeds up to approximately 200 kt.

19. Coriolis Force and Latitude

Coriolis force depends on:

sin(latitude)

Therefore:

Because geostrophic wind requires Coriolis force, geostrophic calculations become unreliable close to the Equator.

The source gives approximately:

>15° N/S → generally suitable for geostrophic calculations

20. Geostrophic Wind Speed

The source gives the relationship conceptually as:

V ∝ PGF / (ρ × sin θ)

where:

• V = geostrophic wind speed
• PGF = pressure gradient force
• ρ = air density
• θ = latitude
• Ω = Earth’s angular rotation rate

Therefore

• PGF ↑ → wind speed ↑
• Air density ↓ → wind speed ↑
• Latitude ↑ → wind speed ↓ for the same PGF
• Latitude ↓ → wind speed ↑, provided geostrophic assumptions remain valid

21. Thermal Wind

Thermal Wind Component (TWC) represents the change in wind with height associated with the horizontal temperature gradient.

Formula

ULW = GW + TWC

Therefore:

TWC = ULW − GW

The subtraction is performed by taking the reciprocal/opposite vector of the lower-level wind and adding it to the upper wind.

22. Thermal Wind Direction

In the Northern Hemisphere:

• Thermal wind flows parallel to isotherms/isopleths.
• Cold air is on the left when facing downwind.

Key rule

Back to the thermal wind → cold air is on your left in NH.

23. Thermal Wind — Worked Examples

Example 1

Lower wind:

030°/08 kt

Upper wind:

030°/28 kt

Same direction, so:

TWC = 030°/20 kt

Example 2

Lower wind:

090°/10 kt

Upper wind:

090°/05 kt

The upper wind is 5 kt weaker.

Therefore TWC must oppose the lower wind:

TWC = 270°/05 kt

Example 3

Lower wind:

180°/10 kt

Upper wind:

360°/15 kt

The reciprocal of 180° is 360°.

Therefore:

15 + 10 = 25 kt

TWC = 360°/25 kt

Example 4

Upper wind:

200°/15 kt

Lower wind:

200°/30 kt

Reciprocal of lower wind:

020°/30 kt

The vectors oppose each other.

Difference:

30 − 15 = 15 kt

Direction follows the stronger vector:

TWC = 020°/15 kt

Example 5

Upper wind:

315°/15 kt

Lower wind:

135°/20 kt

Reciprocal of 135°:

315°/20 kt

Both vectors now point 315°:

15 + 20 = 35 kt

Therefore:

TWC = 315°/35 kt

24. Temperature Gradient and Upper Winds

In the Northern Hemisphere:

• Warmer air is associated with comparatively higher pressure aloft.
• Colder air is associated with lower pressure aloft.
• A horizontal temperature gradient therefore produces a thermal wind component.
• If warm air is south and cold air north, the thermal wind component can be westerly.
• A westerly thermal wind component can cause existing westerly winds to increase with height.

25. True Altitude, Pressure and Temperature

When an aircraft maintains a constant indicated altitude/Flight Level, its true altitude can change.

Flying toward warmer air / higher pressure

True altitude increases

Flying toward colder air / lower pressure

True altitude decreases

Important memory

High to Low — beware below!

If indicated altitude remains constant while true altitude decreases:

Altimeter over-reads the aircraft’s actual altitude.

26. Sea Breeze

A sea breeze is a daytime thermally driven circulation.

Formation

During the day:

Land heats faster than sea

→ Air over land becomes warmer and less dense
→ Relative low pressure develops over land
→ Cooler, relatively higher-pressure air over the sea flows toward land.

Characteristics

• Develops during the day.
• Usually begins during the early forenoon.
• Dies off around dusk/after sunset.
• Typically stronger than the land breeze.
• Typical temperate-latitude speed: about 10 kt.
• Can reach 15 kt or more in tropical areas.
• May extend approximately 8–14 NM inland.

27. Sea Breeze Front

When the sea breeze reaches an area:

• Cooler maritime air replaces warmer land air.
• Temperature shows a sharp fall.
• RH generally increases.
• Visibility may deteriorate.
• Haze, low cloud, fog or low stratus can occur.

This is particularly important at coastal aerodromes.

28. Land Breeze

A land breeze develops mainly at night.

Formation

After sunset:

Land cools faster than sea

→ Air over land becomes colder and denser
→ Pressure becomes relatively higher over land
→ Air flows from land toward the warmer sea.

Characteristics

• Nighttime phenomenon.
• Usually weaker than sea breeze.
• Typical speed: about 5 kt.
• Extends approximately 5 NM out to sea.
• Requires a relatively weak large-scale pressure gradient.

29. Sea Breeze vs Land Breeze

Memory

Day → Sea to Land
Night → Land to Sea

30. Anabatic Wind / Valley Breeze

Anabatic wind is a daytime upslope wind.

Cause

• Sun heats the mountain slope.
• Air adjacent to the slope warms.
• Warm air becomes less dense.
• It flows upslope.

Typical speed

Approximately:

5 kt

31. Katabatic Wind / Mountain Breeze

Katabatic wind is a nighttime downslope wind.

Cause

• Mountain slopes cool by terrestrial radiation.
• Air adjacent to the slope becomes cold and dense.
• Gravity causes it to flow downslope.

Typical speed

Approximately:

10 kt

It is generally stronger than the anabatic wind.

Effects

Cold air collects in valleys, increasing the likelihood of:

• Fog
• Frost

32. Anabatic vs Katabatic

33. Föhn / Foehn Wind

A Föhn is a warm, dry descending wind on the leeward side of a mountain barrier.

Process

1. Air rises on the windward side.
2. It cools.
3. Moisture condenses and precipitation removes water.
4. Air crosses the mountain.
5. It descends on the lee side.
6. Descending air is compressed.
7. It warms at the Dry Adiabatic Lapse Rate (DALR).
8. It becomes warm and dry.

Conditions on lee side

• Warm
• Dry
• Often clear
• Turbulent
• Possible mountain-wave activity
• Possible CAT

34. Föhn-Type Winds

Chinook

• North American equivalent of Föhn.
• Warm and dry.
• Can produce a temperature rise of 20°C (36°F) or more in one hour according to the source.

Other named winds

• Bora → cold, strong katabatic wind
• Harmattan → hot, dry, dusty wind from Sahara
• Ghibli → hot, dusty southerly wind associated with depressions over Libya

35. Rotor Clouds

Rotor clouds form:

• On the leeward side of mountains.
• Beneath strong standing mountain-wave crests.
• In the rotor zone.

Characteristics

• Turbulent cumuliform clouds.
• Strong vertical motions.
• Rotation about a horizontal axis.
• Severe to extreme turbulence.
• Can be as violent as severe thunderstorm turbulence.

Aviation significance

The rotor zone is a major aviation hazard and should be avoided.

36. Bora

The Bora is:

• A strong katabatic wind.
• Cold.
• Gale force.
• Northeasterly.
• Associated with the Balkan Plateau and Dalmatian coast.
• Blows toward the northern Adriatic.
• Particularly strong and frequent in winter.
• Speeds can reach approximately 70–100 kt.

37. Gust

A gust is a sudden, short-duration increase in wind speed.

Reporting criterion

A gust is reported when the peak wind speed exceeds the mean wind speed by:

≥10 kt

The source specifies:

• Mean wind: preceding 10 minutes
• Gust measurement: approximately 3 seconds

38. Gust vs Squall

Squall definition

A squall requires:

Increase ≥16 kt → wind reaches ≥22 kt → lasts ≥1 minute

Example:

10 kt → 30 kt for 2–3 minutes

= Squall

But:

10 kt → 30 kt → immediately falls back

= Gust, if the one-minute criterion is not met.

39. Gale

A gale is associated with strong sustained winds.

The source gives:

• Mean/sustained: ≥34 kt
• Gusting: ≥43 kt

Equivalent wording:

• Sustained >33 kt
• Gusts >42 kt

Gales are commonly associated with strong low-pressure systems and steep pressure gradients.

40. Wind Shear

Significant/severe wind shear is particularly associated with:

• Thunderstorms
• Squall lines
• Cumulonimbus clouds
• Gust fronts

Thunderstorm wind shear

Strong updrafts and downdrafts produce major changes in:

• Wind speed
• Wind direction

A microburst can produce very rapid wind changes, with the source giving values up to approximately:

80 kt in a few hundred feet

Squall lines

• Narrow bands of active thunderstorms.
• Can produce extreme turbulence and wind shear.
• Wind changes can reach approximately:
  • 80 kt in speed
  • 90° in direction
• Significant wind shear may be encountered up to approximately 20 NM laterally from a severe storm.

41. Local Winds and Buys-Ballot’s Law

Local winds generally do not follow Buys-Ballot’s Law in the same way as large-scale geostrophic/gradient flow.

Examples:

• Sea breeze
• Land breeze
• Anabatic wind
• Katabatic wind

Reason

These are relatively small-scale circulations where:

• Thermal differences dominate.
• Gravity may dominate.
• Coriolis force may be relatively small.

Sea breezes may therefore flow almost directly from high pressure toward low pressure.

42. Inertial Flow

The source notes:

• Inertial flow is anticyclonic in both hemispheres.
• Inertial effects become stronger toward higher latitudes and weaker toward lower latitudes.

For ordinary anticyclonic circulation:

43. Wind Chart Conventions

For upper-air wind charts:

• Wind direction is expressed as TRUE direction.
• Standard wind-speed unit is knots (kt).

Isotachs

Isotachs = lines joining points of equal wind speed.

Isobars

Isobars = lines joining points of equal pressure.

44. Surface Wind Instrument

The source states that ICAO surface wind sensors/anemometers should be positioned approximately:

10 m (33 ft) above aerodrome level

Purpose

• Provide representative wind measurements for takeoff and landing.
• Keep instruments clear of buildings and obstructions.
• Avoid airflow distortion.

The source states that the reported surface-wind layer is approximately:

6–10 m above the runway.

45. High-Yield Comparison Table

46. Essential Formulas

Pressure Gradient

Strong PGF → Strong wind

Close isobars → Strong PGF

Geostrophic Wind

PGF = Coriolis Force

Thermal Wind

ULW = GW + TWC

Therefore:

TWC = ULW − GW

Vector Subtraction

To calculate:

A − B

reverse B by 180°, then add it vectorially to A.

Wind Speed Relationship

Conceptually:

V ∝ PGF / (ρ × sin latitude)

Therefore:

• PGF ↑ → V ↑
• Density ↓ → V ↑
• Latitude ↑ → V ↓

47. Final Rapid-Revision Sheet

Forces

PGF → initiates wind

Coriolis → deflects wind

Friction → slows wind

Centrifugal → important in curved flow

Wind Flow

Above friction layer → parallel to isobars

At surface → across isobars toward Low

Hemisphere

NH → Coriolis right

SH → Coriolis left

NH Low → anticlockwise

SH Low → clockwise

NH High → clockwise

SH High → anticlockwise

Friction

More friction → slower wind → weaker Coriolis → greater cross-isobar flow

Land → more friction → ≈30° deflection

Sea → less friction → ≈15° deflection

Local Winds

Sea breeze → day → sea to land

Land breeze → night → land to sea

Anabatic → day → upslope

Katabatic → night → downslope

Mountain Winds

Föhn → warm + dry + descending + lee side

Chinook → Föhn of Rockies

Zonda → Andes

Bora → cold + strong + katabatic

Rotor cloud → severe/extreme turbulence

Wind Changes

Backing → anticlockwise

Veering → clockwise

Gust → short duration

Squall → ≥1 minute

Thermal Wind

TWC = ULW − GW

NH → TWC parallel to isotherms, cold air left

Altimetry

High → Low = true altitude decreases

Warm → Cold = true altitude decreases

High to Low — beware below!