Thunderstorm hazards

Hazardous weather conditions can be packed into very concentrated zones in and around thunderstorms. Hazards include microbursts, severe wind shear and turbulence, large hail, icing, lightning, tornadoes, heavy rain and poor visibility. Lesser

known hazards can include engine water ingestion and marked variations in altimeter readings.

The capacity for thunderstorms to congregate into huge complexes over short periods of time is a danger in itself. Late morning clear skies may be filled with active thunderstorms by early afternoon.

Turbulence

All thunderstorms are characterised by internal turbulent flows generated by up and downdraught winds, with most of them possessing the potential to produce extreme turbulence that potentially poses a significant threat to aircraft.

The most likely areas in the vicinity of a thunderstorm where turbulence may be encountered are:

  • updraft/ downdraft boundaries within the cloud;
  • the leading edge and the upper surface of the gust front (where strong vertical and horizontal wind shears exist);
  • funnel clouds extending from cloud base (sometimes reaching the ground as tornadoes);
  • the upper parts of the updraft within the cloud (note the airflow in the updraft near cloud base is usually quite smooth).

Pilots are advised to monitor the movement and development of thunderstorms in order to avoid them by at least 20 nm (37 km).

Updraft winds

Updrafts tend to be strongest in the middle and upper parts of the storm, but the updraft beneath a growing cumulus may be strong enough to suck a glider or light powered aircraft into the cloud.

Updraft/downdraft Wind Shear

Wind shear between adjacent up and downdrafts within thunderstorms can generate extreme turbulence. The danger in extreme turbulence is twofold:

  • severe loadings may be imposed on the aircraft structure
  • violent changes in aircraft attitude may induce a stall or other conditions in which an attempted recovery may exceed the design limitations of the aircraft.

Straight-line downdraft winds

As a thunderstorm goes through its life cycle, the wind field under the cloud undergoes significant changes. During the early part of storm growth, the surface winds converge beneath the cloud. However, the onset of the downdraft transforms

the sub cloud environment. When the mass of cool descending air strikes the ground, it rushes outward in all directions. The outward flowing cold air undercuts the warmer environmental air and continues to move outward, forced by the downdraft.

The forward edge of the boundary, the gust front (Figure below), is like a miniature cold front because it separates regions of air having distinctly different properties. As it spreads horizontally, it forces the warmer air to rise and can lead to the formation of new convective clouds and thunderstorms.

The strongest lift coincident with moderate to severe turbulence is at the leading edge of the gust front. At the gust front, a roll cloud is sometimes observed, with strong upward motion on the leading edge, and general rotation along its horizontal axis.

Measurements of the wind gusts at the leading edge of the cold outflow occasionally exceed 100 knots.

Although the wind gusts accompanying the arrival of the cold outflow are quite pronounced at the surface, the wind speeds are usually strongest 450 feet to 1200 feet above the ground, due to frictional retardation nearer to the surface associated with surface roughness.

The initial depth of the cold outflow varies between 1800 to 4500 feet. The outflow gradually weakens and becomes shallower as it flows away from the thunderstorm.

Low-level horizontal, wedge-shaped cloud called a shelf cloud (Figure below) is often observed attached to the convective cell’s cloud base. At times the cloud looks much like a curtain, with a hanging terraced appearance, with several layers of cloud. It can be associated with turbulence and sharp wind shifts. Up motion can usually be detected on the forward flank of the shelf cloud while the underside appears turbulent.

SURFACE WEATHER CHANGES ASSOCIATED WITH THUNDERSTORMS

1). WIND

2). SURFACE PRESSURES

3). VISIBILITY

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