G.4.5. DOWNBURST
The downburst is a powerful downdraft that can induce significant damages on the ground (e.g. felled trees). Horizontal winds from downburst may be as high as 100 kt.
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The macroburst is a downburst on a horizontal extent of more than 4 km.
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The microburst is a powerful downburst on a horizontal extent of less than 4 km. It can be either dry or wet:
- A dry microburst occurs with little or no precipitation when reaching the ground. The dry microburst is the result of an evaporation of rain in a dry air. The rain that evaporates cools the air. The cool air descends and accelerates as it approaches the ground. The visible signs of a dry microburst are:
o A cumulus or cumulonimbus with virga (precipitation that evaporates before reaching the ground),
o A ring of blowing dust on the ground, beneath the virga.
- A wet microburst occurs with moderate or heavy precipitation on the ground. The wet microburst forms with the drag of precipitations. The visible sign of wet microburst is a “rain foot” (prominence of precipitation) forming near the ground.
G.4.6. GUST FRONT
A gust front is the result of a thunderstorm downdraft hitting the ground and spreading out on the ground surface. Gust fronts may produce severe turbulence, and generally spread out downwind.
G.4.7. WIND SHEAR
The wind shear is a variation of wind in speed and/or direction on a short distance. It is a well-known cause of fatal accidents during take-off or landing. However, there are several types of wind shears with different levels of danger.
There are several causes for wind shears. The main ones are the following:
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Downburst: It causes the most dangerous wind shear for aircraft as the wind shear presents a significant wind speed difference, and occurs at low levels during take-off and landing.
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Wind around obstacles: A steady wind that blows on obstacles (buildings, mountain ranges, extensive forests, etc) becomes turbulent and induces wind shears.
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Wind associated with frontal surfaces: When a cold air mass slips beneath a warmer air mass (clod air is denser than warm air), the contact of air masses defines the frontal surface. At frontal surfaces, there are wind velocity discontinuities (i.e. wind shears) due to the dynamics of the frontal system (different air densities, temperatures, displacement of air masses).
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Front of sea breezes: Sea breeze appears due to the different temperatures over the land and sea. The front of the sea breeze induces wind shears when it encounters average surface winds.
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Wake vortices: Wake vortices are a kind of wind shear. They may induce severe turbulence when the encounter occurs at a certain distance behind the generating aircraft.
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Radiation inversion and low-level jet streams: At night with fair weather conditions, the land may cool down faster than the air above. There is a heat transfer from the warm air to the cool ground. A radiation inversion occurs: the temperature increases with height. The height of a radiation inversion is approximately 100 m and goes up to 1 km. Surface winds tend to be light or calm. When a low level jet stream passes over a radiation inversion area, wind shears appear.
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