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Clouds of blowing sand or dust.
Common in arid or semi-arid areas, particularly the Sahara and the Arabian peninsula, a sand storm is created by:
- An advancing Gust Front
- ahead of a Cumulonimbus (Cb) cloud (typical in the Sahara)
- along a cold front where cool air passing over hot ground creates instability in the air above (typical in the US mid-west).
- Strong winds: when the wind speed increases above the threshold speed needed to lift and transport surface particles.
An advancing sand storm associated with a gust front is a spectacular sight and looks like an advancing wall of swirling sand. The height of this wall can be 1 nm or more; in situations where there is significant atmospheric instability, dust can reach as high as 20,000 feet.
Sand storms move material by three mechanisms:
- Suspension - dust and very light sand particles carried, often to great heights, by the wind.
- Saltation - sand particles vibrate as the wind increases and then bounce along dislodging other particles. In so doing they also become negatively charged, which has the effect of increasing the amount of dust and sand particles becoming dislodged.
- Creep - blowing large particles along the ground.
The frequency of sand storms, particularly across the lands bordering the Sahara desert, has increased dramatically over the past few decades, caused by and perhaps a major causal factor behind desertification. It is therefore becoming more commonplace for aircraft to encounter a sand or dust storm when operating in these regions.
The diurnal variation in wind speed is usually significant in the regions where sand storms are common and therefore sand storms do not normally occur at night.
Sand Storm activity results in reduced visibility and the ingestion of sand and dust particles into engines, pitot static systems and conditioning packs causing blockage and corrosion.
- Awareness. A knowledge of local climatology and the weather forecast for the route to be flown is essential. Schedules should be designed to avoid the times when sand storms are most prevalent.
- Plans to deal with the consequences of a sand storm, such as blowing sand across aircraft operating surfaces, can be developed by airport authorities.
If already on the ground and a sandstorm is approaching:
- Consider turning the aircraft into wind and tiedown if the winds are forecast to be strong.
- Fit covers to intakes, vents and tubes to prevent ingress of sand and dust. Within the aircraft, cover any areas where the ingress of dust could conceivably interfere with flying controls e.g. throttle quadrant and condition levers.
- Before subsequent flight check and clear intakes and vents of any sand and dust. If feasible, vacuum the flight deck to remove as much dust as possible.