Single cell Thunderstorm moves in line with medium level winds (generally 10,000 feet). Active period is less than one hour.
– Updrafts 3-4000 fpm
– Turbulence mixes rising air with environmental cold air and limits its development.
– Moderate to sever turbulence
– No precipitation
– Last for 15-20 mins
– Updrafts and downdrafts.
– Rain begins to fall.
– Downdraft 3-4000 fpm may reach 6,000 fpm in severe storms.
– First gust
– Severe wind shear under thunderstorm.
– Activity ends by two factors:
a) Mixing of rising air with dry cold environmental air – reducing instability.
b) Downdrafts suppressing the updrafts.
– Mature stage lasts about 30-40 minutes
– Cessation of continous rain.
– Beginning of sporadic showers
– Air subsides, vertical currents weaken
– Higher clouds have anvil appearance
The cold air descending from a thunderstorm can also run ahead of the storm producing a mini cold front with windshear, turbulence and squally conditions. This is called a gust front. Gust fronts can extend as far as 15 to 20nm ahead of the thunderstorm and up to 6,000ft. When there is a line or there are group of thunderstorms gust fronts can extend to twice this distance. Windshear around a gust front has been measured with speed changes up to 80KT and direction changes of 90 deg.
The Super-Cell Thunderstorm
– Good supply of warm moist air at low level. Held down by a thin stable layer above so that the energy supply is not dissipated by turbulence or small-scale convection.
– A change of direction and strengthening of the winds aloft which will tilt the towering CB.
– The updraft is no longer on the same axis as the downdraft and the two exist side by side enabling the convection to continue.
– The increased size and intensity of the storm isolates the central core of the convection from the cold dry environmental air allowing it to reach high vertical speeds.
– This carries hail aloft and takes the top of the storm through the tropopause.
– Occur most frequently at the boundary between sub-tropical and polar air.
– The main area for super-cells is in the central area of the American Mid-West.
– Can last for several hours in their fully active state.
– Tend to move at an angle to the medium level wind and at a different speed, either 20 deg to the right and slower or 20 deg to the left and faster. The 20 deg right and slower option being more common in the Northern Hemisphere.
– Flying restriction under the anvil relates to hail rather than to CAT.
The recommended turbulence avoidance limits are:
– In visual flight – Avoid by 10 NM
– With weather radar – Avoid the echoes by:
Between 0 to FL250 by 10 NM
Between FL 250 to 300 by 15 NM
Over FL 300 by 20 NM
Note: Wx radar echoes show the core of the cloud not the edge so the avoidance limits are greater.
Exam Question Tips
At temperate latitudes, hail may be expected in connection with a CB from ground up to a maximum of FL 450.
The building stage of a thunderstorm last for approximately 20 mins.
The mature stage of a thunderstorm lasts for approximately 20 to 30 mins.
Airmass thunderstorms are the most difficult to forecast and detect.
If you cannot avoid penetrating a thunderstorm, the best area to penetrate is the “Sides”.
Icing is possible in temperatures lower than -23 deg C in a CB with thunderstorm in its mature stage.
The highest probability for severe thunderstorms is when there is advection of maritime cold air over a warm sea surface.
Thunderstorms are often preceded by Altocumulus Castellanus.
A cold front approaching a mountain range in the evening favors the formation of heavy thunderstorms?
Aircraft struck by lightning may sometimes get considerable damage and at least temporarily the manoeuvring of the aircraft will be made more difficult. Aircraft made by composite material may get severely damaged, the crew may be blinded and temporarily lose the hearing.
The aircraft is temporarily part of the lightning trajectory.
In North America tornadoes are most likely to occur in Spring and Summer.
The diameter of a typical tornado is 100-150 meters.