Winds: IC Joshi Chapter 6, DGCA Notes and Practice Questions
Winds is a High-weight chapter and one of the biggest in IC Joshi. Almost every wind question comes back to four forces: the pressure gradient, Coriolis, centrifugal force and friction. Understand which ones are in balance, how they change with latitude, height and temperature, and the geostrophic wind, Buys Ballot's law, surface versus upper winds and the thermal wind all follow.
What DGCA asks from this chapter
- Terms. How wind direction is reported, veering and backing, gust, lull, squall and gale, and the Beaufort scale.
- The forces. Pressure gradient force, Coriolis, centrifugal force and friction, and how Coriolis varies with latitude and speed.
- Balanced winds. Geostrophic, gradient and cyclostrophic wind, where each applies, and how the geostrophic wind changes with latitude and air density.
- Surface versus upper wind. How friction changes speed and direction, and how both change through the day.
- Thermal wind. Why wind changes with height, and what warm and cold advection do to the direction.
- Local and global winds. Sea and land breezes, anabatic and katabatic winds, fohn, trade winds, westerlies and the monsoon.
Describing and measuring the wind
- Direction is always the direction the wind is blowing from. A westerly blows from the west. METARs, TAFs and upper wind forecasts give it in degrees true. The surface wind given by ATC for take-off and landing, and on ATIS, is in degrees magnetic, so it can be compared with the runway direction.
- Speed is given in knots (sometimes km/h or m/s). It's measured by a cup anemometer at a standard height of 10 m above the ground, with direction from a wind vane. The reported surface wind is a mean over 10 minutes ⚑.
- In a METAR, gusts are reported when they exceed the mean speed by 10 kt or more ⚑, for example 27015G28KT. VRB means variable direction, and a range such as 240V300 is given when the direction varies widely.
- Veering is a clockwise change in direction, for example from 270° to 300°. Backing is anticlockwise, for example from 270° to 240°. Both apply to a change in time at one place, or to a change with height.
| Term | Meaning |
|---|---|
| Gust | A brief, sudden increase in speed lasting a few seconds |
| Lull | A brief decrease in speed between gusts |
| Squall | A sudden increase of at least 16 kt, reaching 22 kt or more and lasting for at least a minute, often with a change in direction. Comes with thunderstorms, cold fronts and squall lines. |
| Gale | A sustained wind of Beaufort force 8, 34 to 40 kt |
Beaufort scale: the bands DGCA asks about
| Force | Description | Speed |
|---|---|---|
| 0 | Calm | under 1 kt |
| 4 | Moderate breeze | 11 to 16 kt |
| 7 | Near gale | 28 to 33 kt |
| 8 | Gale | 34 to 40 kt |
| 10 | Storm | 48 to 55 kt |
| 12 | Hurricane force | 64 kt and above |
The four forces
| Force | What it does |
|---|---|
| Pressure gradient force (PGF) | Pushes air from high pressure towards low, at right angles to the isobars. Proportional to the pressure gradient and inversely proportional to air density. It's what starts the wind. |
| Coriolis force | An apparent force from the earth's rotation, acting at right angles to the direction of motion. Deflects air to the right in the northern hemisphere and to the left in the southern. Its size is 2Ωv sin φ: it increases with wind speed (v) and with latitude (φ). Zero at the equator, greatest at the poles, and zero for still air. |
| Centrifugal force | Acts outward from the centre when air moves along curved isobars, around a low or a high. |
| Friction | Slows the wind near the ground. Because Coriolis depends on speed, it weakens too, the pressure gradient wins, and the wind turns towards low pressure. Effective up to about 2,000 to 3,000 ft, the friction layer ⚑. Greater over rough land than over sea. |
Balanced winds
| Wind | Forces in balance | Where it applies |
|---|---|---|
| Geostrophic | PGF and Coriolis | Straight, parallel isobars, above the friction layer. Not valid near the equator, where Coriolis is close to zero. |
| Gradient | PGF, Coriolis and centrifugal | Curved isobars. For the same isobar spacing it is weaker than geostrophic around a low (sub-geostrophic) and stronger around a high (super-geostrophic). |
| Cyclostrophic | PGF and centrifugal | Small, intense, fast-spinning systems where Coriolis doesn't matter: tornadoes, dust devils, waterspouts, and the inner core of tropical cyclones. |
What changes the geostrophic wind speed
In balance, Coriolis equals the pressure gradient force. Rearranged, the geostrophic wind speed is V = ΔP / (2Ω ρ sin φ × distance). Three exam conclusions follow:
- Closer isobars, stronger wind. Double the gradient and the wind doubles.
- Lower latitude, stronger wind for the same isobar spacing, because sin φ is smaller. The same chart spacing gives about twice the wind at 30° as at 90° ⚑.
- Lower density, stronger wind for the same pressure gradient. So with the same gradient, the wind is stronger at height, and stronger in warm air than cold.
Buys Ballot's law
In the northern hemisphere, stand with your back to the wind and low pressure is on your left. In the southern hemisphere it's on your right. It follows from the geostrophic balance, and it's why air circulates anticlockwise round a low and clockwise round a high in the northern hemisphere. For a pilot in the northern hemisphere, a wind from the left (drifting the aircraft to the right) means flying towards lower pressure, and the altimeter will over-read. See Chapter 2, Atmospheric Pressure.
Surface wind versus upper wind
| Over land | Over sea | |
|---|---|---|
| Surface wind backed from the 2,000 ft wind by about | 30° | 10° |
| Surface speed as a share of the 2,000 ft wind | about 50% | about 70% |
These are typical figures ⚑. The sea is smoother, so there's less friction. In the northern hemisphere, climbing out of the friction layer the wind veers and increases; descending into it, the wind backs and decreases. In the southern hemisphere the directions reverse: the surface wind is veered from the upper wind.
Diurnal variation
| Surface wind | Wind at about 1,500 to 2,000 ft | |
|---|---|---|
| By day | Stronger, veers towards the upper wind. Gusty. | Weaker, as momentum is mixed downwards |
| By night | Lighter, backs. Often near calm under a clear sky. | Stronger, and can form a low-level jet |
The cause is mixing. Daytime heating stirs the lower air and brings the faster upper wind down. At night a surface inversion forms, the layers decouple, and the wind just above the inversion speeds up. That's a classic source of low-level wind shear on night and early morning approaches. The effect is greatest over land, on clear days and nights.
Turbulence and eddies
Friction doesn't just slow the wind, it breaks it into eddies. Mechanical turbulence comes from wind over rough ground, buildings and hills, and increases with wind speed and surface roughness. Thermal turbulence comes from convection over heated ground. Both are confined mainly to the friction layer and are worst in the afternoon. Turbulence in clear air at height is covered in Chapter 16, Clear Air Turbulence.
The thermal wind
The thermal wind isn't a wind you can measure. It's the vector difference between the geostrophic wind at two levels, caused by a horizontal temperature gradient. Because cold columns are shorter than warm ones (see Chapter 2), pressure surfaces slope down towards cold air, and the slope increases with height. So the pressure gradient, and the wind, change with height.
- The thermal wind blows along the isotherms (strictly, the lines of equal thickness), with cold air on the left in the northern hemisphere. Its strength is proportional to the temperature gradient.
- Since the poles are colder than the tropics, the thermal wind in middle latitudes is westerly. That's why westerly winds increase with height up to the tropopause, ending in the jet streams. See Chapter 15, Jet Streams.
- Warm advection (wind blowing from warm air towards cold): the wind veers with height. Cold advection: the wind backs with height. This applies in the northern hemisphere.
Local winds
| Wind | When and why | Typical character |
|---|---|---|
| Sea breeze | By day. The land heats faster than the sea, pressure falls over land, and air flows in from the sea. | Starts late morning, peaks mid-afternoon. Around 10 to 15 kt, a few thousand feet deep, reaching up to about 15 to 20 nm inland. Can bring low cloud and fog in from the sea, and a sea breeze front with cumulus. |
| Land breeze | At night. The land cools faster, and air flows out to sea. | Weaker and shallower than the sea breeze. Strongest around dawn. |
| Anabatic | By day, upslope, as sun-heated slopes warm the air against them. | Gentle. Can build cumulus over ridges. |
| Katabatic | At night, downslope, as cold dense air drains off radiating slopes, especially under clear skies and light gradient winds. | Can be strong, and much stronger over snow and ice. Pools in valleys as frost and fog. |
| Fohn | Air forced over a mountain range loses moisture as rain on the windward side, then descends the lee side warming at the dry rate. | Warm, dry, often gusty. Clear skies on the lee side, the "fohn gap" in the cloud. |
| Ravine (gap) wind | Wind funnelled through a valley, gap or pass. | Accelerated, often turbulent at the exit. |
| Loo | Hot, dry westerly over the plains of north-west and north India in May and June. | Very hot and dusty in the afternoon, with poor visibility and high density altitude. |
Worked example: the fohn effect
Air at 20°C at sea level crosses a 3,000 m mountain. Use a dry rate of 1°C per 100 m and a saturated rate of 0.6°C per 100 m ⚑, with cloud base at 1,000 m on the windward side.
- Sea level to cloud base, 1,000 m dry: 20 − 10 = 10°C.
- Cloud base to the summit, 2,000 m saturated: 10 − 12 = −2°C. The moisture has fallen as rain or snow.
- Summit back down to sea level on the lee side, 3,000 m dry: −2 + 30 = 28°C.
The air arrives 8°C warmer and much drier than it started. The gain comes from latent heat released in the cloud on the windward side.
Large-scale winds
- Trade winds: steady winds in the tropics blowing out of the subtropical highs towards the equator. North-easterly in the northern hemisphere and south-easterly in the southern. They meet at the inter-tropical convergence zone. See Chapter 20, General Circulation.
- Westerlies: the prevailing winds of the middle latitudes, between the subtropical highs and the polar front. In the southern hemisphere, over open ocean with little land to slow them, they're strong enough to be called the roaring forties.
- The monsoon: a seasonal reversal of wind caused by the different heating of land and sea, and the seasonal shift of the ITCZ. Over India, the south-west monsoon blows from June to September and brings most of the year's rain. The north-east monsoon blows in winter, dry over most of India but bringing rain to Tamil Nadu. See Chapter 19, Climatology India.
Wind shear
Wind shear is a change in wind speed or direction over a short distance, vertically or horizontally. It changes the airspeed and lift of an aircraft passing through it. Low-level wind shear, below about 1,600 ft ⚑, is the most hazardous because there's little height to recover. Common causes:
- Thunderstorms and microbursts. See Chapter 13, Thunderstorm.
- Frontal passage, especially fast-moving cold fronts
- A low-level jet above a night-time inversion
- Strong surface winds over obstructions, and sea breeze fronts
- Mountain waves and rotors. See Chapter 17, Mountain Waves.
Common traps
Practice questions
Three questions from the 294 on this chapter. Each one cites the book it comes from.
1. What is the term used to describe the wind that results when the pressure gradient force and the Coriolis force are in exact balance?
Show answer
Answer: B. Geostrophic wind
The geostrophic wind is a theoretical wind that blows parallel to isobars when the pressure gradient force and the Coriolis force are in perfect balance, with no friction involved. This balance condition is generally achieved in the free atmosphere above approximately 2,000 to 3,000 feet, away from the influence of surface friction. The geostrophic wind speed is directly proportional to the pressure gradient — closer isobars indicate stronger geostrophic winds. In the Northern Hemisphere, the geostrophic wind blows with lower pressure to the left of the direction of motion, in accordance with Buys Ballot's Law. While purely theoretical, the geostrophic wind is an important reference concept in understanding upper-level wind patterns and aviation weather forecasting.
Source: Aviation Meteorology — IC Joshi · Chapter 6 — Winds · Page 67
2. In the Southern Hemisphere, the flow of air around a low-pressure system (cyclone) is:
Show answer
Answer: D. Clockwise, opposite to the Northern Hemisphere
The direction of circulation around pressure systems is governed by the Coriolis force, which is caused by the Earth's rotation. In the Northern Hemisphere, the Coriolis force deflects moving air to the right, producing anticlockwise (counterclockwise) flow around low-pressure systems and clockwise flow around high-pressure systems. In the Southern Hemisphere, the deflection is to the left, reversing the circulation: air flows clockwise around lows and anticlockwise around highs. This fundamental difference applies to both surface and upper-level winds and is a critical concept for pilots operating across hemispheres or interpreting synoptic charts from different regions.
Source: Aviation Meteorology — IC Joshi · Chapter 6 — Winds · Page 69
3. What is the primary reason why wind speed generally increases with altitude in the lower atmosphere?
Show answer
Answer: C. Surface friction, which retards wind flow, decreases with increasing altitude above the ground
Near the Earth's surface, friction between the moving air and the ground surface—including terrain features, vegetation, and buildings—acts as a retarding force that reduces wind speed. This frictional influence is greatest at the surface and diminishes progressively with altitude. Above the friction layer (planetary boundary layer, typically up to about 2,000 ft over sea and higher over rough terrain), wind tends to become more geostrophic in character, blowing more parallel to isobars and at greater speeds. Pilots must account for this increase in wind speed with altitude when planning flights, as headwinds and tailwinds at cruise altitude can differ substantially from surface observations.
Source: Aviation Meteorology — IC Joshi · Chapter 6 — Winds · Page 63
Frequently asked questions
Which chapter of IC Joshi covers winds?
Chapter 6, Winds, in the 2019 edition of Aviation Meteorology by IC Joshi. It covers how wind is described and measured, the forces that drive it, balanced winds, local winds and the large-scale wind systems including the monsoon.
What is Buys Ballot's law?
In the northern hemisphere, if you stand with your back to the wind, low pressure is on your left. In the southern hemisphere it is on your right.
What is the geostrophic wind?
The wind that blows when the pressure gradient force and the Coriolis force are in balance. It blows parallel to straight isobars, above the friction layer. It does not apply near the equator, where the Coriolis force is too weak.
Why does the wind veer and increase with height?
Near the ground, friction slows the wind and weakens the Coriolis force, so the wind is backed and slower. Above the friction layer the wind reaches its geostrophic speed and direction, so in the northern hemisphere it veers and increases with height. Higher up, the thermal wind usually adds a further westerly increase.
Related chapters
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