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11. Flight visibility and vision at high altitudes

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Flight visibility and vision at high altitudes

Flight visibility is frequently different from the surface visibility. Such differences are caused by the unequal distribution of the obscuring particles, such as smoke, haze, fog-particles with height and because the pilot sees the conditions from a different angle than does the observer on the ground.

Some conditions which cause a variation of visibility with height are:

    – If an inversion exists at the surface with fog, smoke or mist accumulating near the ground, the visibility improves with height.
    – If a cloud layer exists beneath an inversion at some distance above the ground, the visibility will usually deteriorate with height below the clouds and a very sudden deterioration is often observed in the vicinity of the cloud base.
    – Mist or haze layers are often associated with inversion at different heights, such layers often cannot be observed from the ground, but they can cause an appreciable deterioration in the flight visibility.
    – If precipitation occurs, the visibility deteriorates rapidly upwards toward the freezing level.

At great altitudes, as in the stratosphere, clouds are rare and dust and haze almost non-existent, but this
does not mean that conditions for seeing an object are very good.

The fact that the apparent brightness of the sun increases with height while that of the sky decreases leads to greater dazzle than that which occurs at lower altitudes.

The importance of wearing optically correct anti-glare spectacles can, therefore, not be over-stressed.

Moreover, a second problem associated with the type of sky encountered frequently at high altitude is that of accommodation or focusing of the eyes, which has been referred to in the past as ’space myopia’ or ’empty field myopia’. This is not a true ’myopia’ or permanent shortsightedness.

If there is no detail for the eye to judge whether it is in or out of focus, there is no possibility of obtaining an error signal and, therefore, the eye will not be able to change its accommodation to the level required, in order to be in focus so as to detect some distant object which is as yet too small to arouse attention. This is an experience which most pilots have had, although some observe it less frequently than others, but it is a perfectly normal response under the circumstances.

Source: FCRG

See also:
1. Horizontal visibility aspects
2. Visual range
3. Observing techniques of RVR
4. Observing of RVR
5. Availability of RVR observations at ATS units
6. Reporting procedures of RVR reports
7. Accuracy of RVR reports
8. Slant visual range
9. The visual segment
10. Variation in visual segment
11. Flight visibility and vision at high altitudes

The presented material is for training purpose only!