Surface Wind Structure and Operational Consequences
5.1.1. Gusts
01. Some Remarks Relating to the Structure of the Surface Wind
The airflow in the lower levels of the atmosphere is nearly always turbulent. The normal laminar flow is broken up into a very complicated pattern of eddies.
The degree of turbulence depends on the roughness of the terrain and on the wind speed. The stability of the atmosphere also plays an important role.
The eddy motions in the atmosphere are detected as fluctuations of both wind speed and wind direction. These fluctuations, particularly in speed, are usually referred to as “gusts”.
Gust definition
A gust can be defined as a positive or negative departure of the wind speed from its mean value, which departure lasts for a relatively short time.
A “negative gust” may alternatively be termed as “lull”.
Aerodrome wind reporting
For the satisfactory understanding of the effects of a gusty wind on an aircraft during approach, landing and take-off, it is necessary to be familiar with the detailed structure of the wind. Criteria in use in aerodrome weather reporting are not precise enough to reveal this detailed structure and are not standardized worldwide.
In reporting the surface wind in aerodrome weather reports the meteorological observer is generally guided by directives along the following lines:
- mean value of wind speed and wind direction is taken over a period of ten minutes;
- variations from the mean wind speed (gusts) are reported only when during the past 10 minutes the variation from the mean wind speed has exceeded 10 kt.
Summarizing, it can be said that the complicated wind structure is reproduced in aerodrome weather reports by the following parameters only: mean wind speed and wind direction and positive gusts when applicable.
Gust parameters
The parameters used to describe gusts include gust peak speed, gust minimum speed, gust duration, gust frequency and gustiness factor.
P = gust peak speed: the instantaneous maximum wind speed obtained in a positive gust;
M = gust minimum speed: the instantaneous minimum wind speed obtained in a negative gust;
Tg = gust duration: the interval from the beginning of a gust to the end of the gust;
gust frequency: the number of positive gusts which occur within the specified time interval;
gustiness factor: maximum wind speed during a specified interval of time divided by the mean speed of the wind during the same interval of time.
In reports for take-off and landing, variations in the wind direction shall be given when the total variation is 60° or more with mean wind speeds above 5 kt. Such directional variations shall be expressed as the two extreme directions between which the wind has varied during the past 10 minutes. Variations from the mean wind speed (gusts) shall be reported only when the variation from the mean speed has exceeded 10 kt. Such speed variations (gusts) should be expressed as the maximum and minimum speeds attained.
In reports for take-off, surface winds of 5 kt or less shall include a range of wind directions, whenever possible.
Present day aerodrome anemometers are not generally designed to measure all gust parameters of aeronautical significance. Before reproducing some records obtained with special equipment, the most important gust parameters are illustrated in the preceding hypothetical figure.
The significance of the above parameters has to be based on the mean speed and direction of the wind during a specified interval of time, i.e. normally over a period of ten minutes.
During speed changes the direction of the wind may change as well. On the whole the change is likely to be in the sense that the wind veers during a positive gust and backs during a negative gust, but this is by no means an invariable rule. Direction changes generally are of the order of 10-25 degrees on either side of the mean direction.
Figures 2, 3 and 4 show the wind speed fluctuations during three rather arbitrarily chosen ten minute intervals. Figure 5 gives the gustiness factor for altitudes up to 180 metres for a five hour period.
From these and other records, although limited in amount, it appears that:
- the gustiness factor of the surface wind varies between 1.2 and 1.8; the factor most frequently found is 1.5 – 1.6;
- gust frequency varies from 20 – 40 per ten minutes;
- gust duration is of the order of ten seconds;
- gustiness factor decreases markedly with altitude.
02. Operational Consequences
For the pilot executing a landing or a take-off it is of great importance to be familiar with the properties of the wind.
Cases are known where hard landings could be proven to have been a result of a sudden wind change.
Sudden wind changes may cause aircraft to stall, which may lead to landings before the threshold or falling back on the runway just after the aircraft has become airborne. Changes in wind speed and wind direction may also lead to landings with a sideward load on the landing gear.
In a gusty wind pilot and aircraft reaction time are the tools with which the changing wind must be encountered.
As figure 5 shows, the gustiness factor increases markedly with decreasing altitude. The pilot being aware of this will choose a higher approach speed to avoid stalling near the ground, or may select less flap extension. A heavy aircraft will be less affected than a light aircraft, but on the other hand a heavy aircraft reacts slower to increased power.
It will be clear that the stronger the mean wind, the far more important the gustiness factor. Therefore, it makes a great difference whether e.g. a cross-wind component of 20 knots is caused by a wind that is actually blowing at right angles to the runway or by a component of a much stronger wind blowing at an angle less than 90 degrees, particularly because in a gust the wind direction also changes.