WINDS (Meteorology Notes)
Wind is defined as air in horizontal motion. Although the atmosphere has both horizontal and vertical motions of air, the horizontal motions have in general, larger speeds and their measurement both near the ground and at upper levels can be made with a greater degree of accuracy than in the case of vertical motions. Wind blows from region of high pressure to region of low pressure. The wind is closely related to the horizontal variation of pressure and consists of a succession of strong winds (gusts) and weak winds (lulls). It has two components direction and speed.
The direction of the wind is the direction from where it is blowing or the direction that the wind is coming from. Example, a southerly wind blows from South to North and Northerly winds blow from North to South. Wind direction is indicated on 16 points of compass (N,NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW, WSW, W, WNW, NW, NNW) or in degrees at intervals of 100, for example 3500, 3300, 0200, 0900, 1600 etc. The direction is measured from True North.
True North Datum
The wind direction of all meteorological materials such as charts, forecasts and METAR are related to True North. The exception to this rule occurs only at extremely high latitudes. In Arctic areas the compass variation may be significant. In these areas the ATC and Met Report give the Magnetic wind direction, so that both runway and reported wind direction will use the same datum.
Wind speed is measured in knots.
1 kt = 0.514 m/s
= 1.85 Km/Hr
= 1.15 Miles/ hr (Statute)
Wind Speed is measured by Anemometer.
At every airfield the runways are constructed after studying the wind records of the locality over many years. Their orientation is chosen in such a way that for a large part of the year the prevailing wind is along one of the runways. In spite of this, occasions arise during adverse weather or transit seasons when the surface wind blows at an appreciable angle to the runway. Such winds could be dangerous for flying as aircraft may tend to swing or drift on takeoff / landing.
The component of the wind at right angles to the runways in use is known as Cross-wind component. For each type of aircraft, critical cross wind components have been specified. When the critical value is exceeded flying has either to be suspended or great caution exercised.
Crosswinds of appreciable strength occur in unusual weather situations and in months in which a transition takes place in the prevailing winds.
These are measured by RAWIN and Pilot Balloon equipment. These Hydrogen filled balloons are tracked by Radars and optical Theodolite.
Recording and Annotations of Wind Directions and Speeds
To record surface winds the anemometer and wind vane are installed at a height of 10 meters at an area free of obstructions. The wind is averaged for 10 min for all weather observations. For Take off and Landing purpose, however, wind is averaged for 2 min.
A circle is used to indicate the station location. Wind direction is annotated by a straight line from the periphery of the circle. The line indicates the direction from which the wind is blowing. The speed is shown by the feathers at the end of the line. A long barb forming the feathers indicate 10 kts, a small barb indicates 5 kts and a shaded triangle indicates 50 kts.
Factors Influencing Speed and Direction of Winds
The wind closest to the surface of earth is referred to as Surface Winds. This is influenced by local pressure distribution, friction and terrain. The winds at upper level is referred to as Wind Aloft. This is influenced by regional pressure distribution and forces including Pressure Gradient Force, Geostrophic Force and Cyclostrophic Force.
Forces Influencing Wind Aloft
Pressure Gradient Force
Like any other fluid, air has got a natural tendency to move from a region of air surplus (high pressure) to one of air deficit (low pressure). PGF is a force caused by the pressure Gradient, which tries to move the winds from an area of high pressure to an area of low pressure. Isobars are lines joining places with equal surface pressure. The rate of change of pressure with distance which is normal to the Isobars determines the magnitude of the pressure gradient force. PGF is inversely proportional to the distance between Isobars. It means the closer the Isobars are the higher is the PGF. The speed of movement is proportional to the gradient of pressure, i.e. the rate of change of pressure from high to low. Thus, if the gradient were the only force, we should expect the wind to blow directly from high pressure to low pressure i.e. at right angles to the isobars.
A look at any chart wherein isobars are drawn and surface winds are plotted shows that this is not the case. In fact the surface winds blow more or less along the isobars. The reason is that apart from the pressure gradient force, there is another apparent force, which deflects air in motion. This apparent force is known as the Coriolis force or the geostrophic force and arises out of the rotation of the earth on its axis. The Coriolis force is greatest in magnitude at the poles and zero at the equator. At a given latitude it is proportional to the wind speed. As regards the direction in which it operates, it deflects a wind towards the right in the Northern Hemisphere if we look along the air motion.
Geostrophic Force or Coriolis Force
It is a force caused by the rotation of earth and will cause a moving mass of air to turn to the right in Northern Hemisphere and to the left in Southern Hemisphere.GF is max at Poles and Min at Equator. It is expressed as:
Geostrophic force = Coriolis Force = 2 WV Sinθ
Where W = Angular Vel of Earth
V = Wind Speed
θ = Latitude
The wind which results from balance between the pressure gradient and the Coriolis force is known as geostrophic wind. This balance can only occur when it is not affected by any other force. Therefore the Geostrophic Wind can only blow above the friction layer at 2- 3000 ft and above. At higher latitudes where the magnitude of the Coriolis force is large, the observed wind closely corresponds to the calculated geostrophic wind.
A wind, which starts blowing from high to low pressure, may be assumed to start initially at right angles to the isobars. However, once the motion commences, the coriolis force starts acting on it, deflecting it to the right, with the result that ultimately it blows along the isobars with low pressure to its left. illustrates this. The adjustment of wind directions brought in by the coriolis force is quick at higher latitudes but may take as long as 24 hours in the tropics.
Limitations of Geostrophic Winds
Geostrophic Winds are applicable when:
- Isobars are straight and parallel
- The pressure distribution is fixed and not changing rapidly with time
- The air motion is steady along the horizontal. Appreciable departure occurs when the motion is unsteady or vertical motions occur
- Balance is reached between PGF and Coriolis force without influence of any other force.
Cyclostrophic Force and Cyclostrophic Wind
When the path of motion is not straight there is yet another force, which acts on air in motion, this arises out of the curved motion. It means that it is the force acting towards the centre of pressure system when the Isobars are curved This force is a centrifugal type and in meteorology it is known as cyclostrophic force. Its magnitude and sign depends upon the amount of curvature and the fact whether the curvature is cyclonic or anticyclonic. Its effect is to reduce the speed in cyclonic motion and increase it in anticyclonic motion.
The wind which results due to the balance of PGF and Cyclostrophic force is called Cyclostrophic Winds. The frictional Force is disregarded. In a tropical Cyclone, both PGF and Cyclostrophic Force is of large magnitude in comparison to Coriolis Force. Hence the latter can be neglected and the winds in a tropical cyclone closely resembles Cyclostrophic Winds.
.A wind which results from the balance between the pressure gradient force, Coriolis force and the cyclostrophic force is known as gradient wind. The Gradient Wind also occurs when the Isobars are curved.
If Isobars are straight and parallel, Gradient Wind is equal to Geostrophic Winds. When in lower latitudes the corilois force is negligible then Gradient wind is eaqual to Cyclostrophic Wind.
Effect of Friction
There is yet another force which acts on wind in the layers close to the ground. This is friction and its magnitude depends upon the roughness of the ground and the presence of obstacles. Its primary effect is to reduce the wind speed and deflect the direction in such a way that the wind blows a little towards low pressure. A second but more important effect is to disturb the smooth horizontal motion and cut it up to a greater or lesser extent into minor broken circulations embedded in the flow. These are known as “eddies”.
The eddies move in the general flow causing irregular variation in the speed as well as the direction of the wind. This is known as gustiness. Pronounced gustiness is associated with bumpy motions of an aircraft as it encounters the eddies. The entire phenomenon is known as mechanical turbulence.
The effort of friction persists upto about 600 m to 1 km, above ground level beyond which it is negligible.
Veering and Backing
A wind is said to veer when its direction changes in clockwise sense and to back when it changes in an anti-clockwise sense.
At the ground frictional force is maximum and therefore the wind speed is minimum. In general over land the wind speed decreases to 1/3rd of the normal free flow and over sea where friction is much less, the wind speed reduces to 2/3rd of the free flow. Also the wind blows across the isobars, towards low pressure, angle of intersection being max at the surface. As we go up through the frictional boundary layer, the frictional force decreases and the wind speed increases turning to the right i.e. Veers and cross isobaric angle decreases. At the free flow (Frictionless level), approximately 0.9 Km, the wind speed attains geostrophic gradient value and starts blowing parallel to the Isobars as per Buy’s Ballots Law. All these can be seen in Ekman Cycle.
Diurnal Variation of Surface Winds
Surface Winds strengthens during daytime and becomes light and calm during night. This is primarily due to lack of surface heating with no solar radiation during night.
Buys Ballot’s Law
The Buys Ballots Law states that in the northern hemisphere, if an observer stands with his back to the wind, the low pressure is to his left. In the Southern Hemisphere the reverse is true. From Buys Ballot’s Law it follows that the wind circulation around a low in the Northern Hemisphere is anti clockwise, and around a high is clockwise.
In Northern Hemisphere if an ac experiences tail winds then the Low Pressure would be to the left and if the ac experiences Head Winds then the Low Pressure would be to the right.
Strong drift to the right in Northern Hemisphere, implies that the ac is heading towards the Low pressure and strong drift to the left implies that ac is heading away from Low Pressure.
Defining and Recording Chages in Wind Speeds and Direction
A gust is sudden increase in wind speeds, often with a change in direction. It lasts only for a few seconds and is very local. A lull is a sudden decrease in wind speed. A gust Factor is sometimes used to indicate the amount of gustiness.
Gust Factor = Maximum Wind – Minimum Wind/ Mean Wind
Squall is defined as the sudden increase in wind speed by 16 kts, should last for 1 min or more and speed should increase to 22 kts or more. Squall is associated with Large CB cells or violent convective activity. Squall extends some Km’s horizontally and several thousand feet vertically. Both speed and direction may differ widely from the prevailing winds in a squall. Violent Squalls are experienced in Norwesters, Thunderstorms and Dust storms from March to June. The main difference between Squall and a gust is the duration.
Gale is defined as the persistent mean wind of speed 34 kts or more. It is associated with depressions and cyclonic storms.
Convergence and Divergence
Air blowing in towards an area of falling low pressure is called Convergence and blowing outwards from an area of increasing high pressure is called as Divergence.
The large-scale distribution of pressure determines the general direction and speed of wind at any location. This scheme is, however, sometimes completely upset by local wind circulation set up by smaller scale topographical features. When such circulations are set in the general wind pattern due to local topographical features, the pressure distribution is either partially or completely masked in the locality. Since these local wind circulations occur with great regularity, they have to be taken note of while assessing the surface winds at any location. A few important types of local winds are discussed below.
Sea and Land Breezes. Coastal areas are subjected to the characteristic alternation of sea breeze during daytime and land breeze at night. These are set up because land gets heated in the day more quickly than the adjacent sea, while at night it cools more quickly than the sea. Some of the important characteristics of these breezes are given below:
- Sea breeze sets in abruptly around noon or somewhat later. It dies down by sunset.
- Average speed of sea breeze is 12 – 15 kts, but in some localities it could be higher due to peculiarities in the coastline.
- Initially the direction of sea breeze is at right angles to the coastline. Later it tends to blow keeping land towards the left.
- It penetrates to about 50 Kms inland.
- Land breeze is less pronounced than see breeze, the speed being rarely more than 7 kts during night.
Katabatic and Anabatic Winds. These are characteristic of hilly areas. At night the ground on a hill slope cools rapidly with the result that the temperature at any point close to the hill slope is lower and the air close to it being heavier slides down the slope to give rise to a wind known as Katabatic wind. During daytime the reverse process takes place and an upslope wind known as Anabatic wind occurs.
Fohn Wind. This is local name in the Alps region for a warm dry wind on the leeward side of the mountains, but has now become a general terms of winds of this nature.
Valley Winds. When a mountain is broken by a valley, the wind tends to blow along the valley at a speed appreciably greater than in neighbouring area on either side. Such winds are known as valley winds.
Some Important Local Winds
Many small-scale wind circulations have acquired local names as they make their impact on the life of the people in region. A few important local winds are as follows:
- A wind of Katabatic origin which blows in violent gusts on the shores of the Arabic sea from the mountains to the northeast.
- A wind of the Fohn variety blowing down the eastern slopes of the Rockies in the U.S.A. and Canada.
- A hot dust-laden wind that blows during summer afternoons over the plains of northern India.
- A strong offshore northerly wind that blows along the north coast of the Mediterranean.
- A gusty northwesterly wind which blows over Iraq and the Persian Gulf in the summer.
Surface winds are of great importance in landing and take-off. A pilot must, therefore, be thoroughly familiar with the prevailing winds in different seasons on the airfield on which he normally operates. He should also have a good knowledge of the changes that take place in the surface wind in different weather situations.
Development and Growth of Lows
For the surface low to be maintained the convergence at the surface must be compensated or supported by a divergence aloft. Surface Convergence can be maintained only if the divergence aloft is at a rate equal to or more than the surface convergence. Therefore, the airflow aloft is an important aspect in maintaining cyclonic or anticyclonic circulation on the surface.
|Types of Disturbances||Associated Wind Speed in the Circulation|
|Low Pressure area||< 17 kts (31 Kmph)|
|Depression||17-27 Kts (31-49Kmph)|
|Deep Depression||28-33 kts (50-61 Kmph)|
|Cyclonic Storm||34-47 kts (62- 88 Kmph)|
|Severe Cyclonic Storm||48-63 Kts (89- 118 Kmph)|
|Very severe Cyclonic Storm||64-119 Kts (119-221 Kmph)|
|Super Cyclonic Storm||120 Kts and above (222 Kmph and above)|
- Choose correct answer / answers.
- Wind is:
(i) Horizontal motion of air. (ii) Vertical Motion of air.
- Both (a) & (b).
- Wind is directly proportional to:
(i) Pressure. (ii) Pressure gradient.
- Pressure change in last one hr.
- Wind we experience is:
(i) Geostrophic. (ii) Cyclostrophic.
- Gradient wind.
- Veering means wind direction changing:
(e) Backing means wind direction changing:
(i) Clockwise. (ii) Anti-clockwise.
- Jet stream means winds stronger than:
(i) 50 kt. (ii) 60 kt.
- 70 kt.
- Sea breeze is observed in:
(i) Morning. (ii) Afternoon.
- Cross wind means component of wind at:
(i) 90° to runway. (ii) 60° to runway.
- Not along or opposite runway.
- Anabatic winds are winds which blow:
- Down slope & at night. (ii) Up slope in afternoon.
- Up slope and at night.
- As per Buy Ballots’ law, winds blow in Northern Hemisphere keeping:
(i) Low to right. (ii) Low to left. (iii) High to left.