Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and short term forecasting (in contrast with climatology). Aviation meteorology (MET) deals with the impact of weather on Air Traffic Management (ATM). It is important for air crews to understand the implications of weather on their flight plan as well as their aircraft.

Weather conditions concern all aspects of ATM operations, for example, by variations in head and tail-wind components, through changes in pressure and temperature values at airports, and in imposing low visibility operating conditions. Adverse meteorological conditions have the greatest impact on the ATM system creating disruption and the consequent problems of disturbed flow rates, lost capacity and induced additional costs

One of the greatest threats to aircraft operations are thunderstorms. Relatively recent meteorological studies have confirmed the existence of microburst phenomenon around these dangerous weather phenomena. Microbursts are small scale intense downdrafts which, on reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be extremely hazardous to all types and categories of aircraft, especially at low altitudes. Due to their small size, short life span, and the fact that they can occur over areas without surface precipitation, microbursts are not easily detectable using conventional weather radar or wind shear alert systems.

The life cycle of a microburst as it descends in a convective rain shaft .An important consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground. Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types. Note, however, that microbursts commonly occur within the heavy rain portion of thunderstorms, and in much weaker, benign appearing convective cells that have little or no precipitation reaching the ground.

Characteristics of microbursts include:

1. Size. The microburst downdraft is typically less than 1 mile in diameter as it descends from the cloud base to about 1,000-3,000 feet above the ground. In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter.

2. Intensity. The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground.

3. Visual Signs. Microbursts can be found almost anywhere that there is convective activity. They may be embedded in heavy rain associated with a thunderstorm or in light rain in benign appearing virga. When there is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only visual clue of its existence.

4. Duration. An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds lasting approximately 2-4 minutes. Sometimes microbursts are concentrated into a line structure, and under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.

Exercise: Using the following vocabulary list, match the lettered clouds in the diagram with the appropriate terminology.

A___________     B___________     C___________     D___________     E___________     

F___________      G___________     H___________    I ___________      J ___________

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Answers: (A) cirrus-stratus (B) cirrus (C) cirro-cumulus (D) alto-stratus (E) cumulus (F) alto-cumulus (G)stratus (H) nimbo-stratus (I) cumulo-nimbus (J) strato-cumulus