Thunderstorms

Development

● Thunderstorms require three major ingredients to form:

➢ A high moisture content.

➢ A steep lapse rate (unstable air).

➢ A lifting agent.

● Thunderstorm frequency is the greatest in tropical regions

Development / Structure

Step 1: Cumulus or Developing Stage – Updraft dominated
Step 2: Mature Stage – Updrafts and Downdrafts
Step 3: Dissipating Stage – Downdraft dominated

Cumulus Stage

● Warm, moist, unstable air is forced to rise.

● The moisture rapidly cools into liquid drops of water due to the cooler temperatures at high altitude, which appears as cumulus clouds

● As the water vapour condenses into liquid, latent heat is released which warms the air, causing it to become less dense than the surrounding dry air.

● The upward growth rate of these clouds is approximately 5 to 20 m/s (10 to 45 MPH)

● During the cumulus stage the updraft holds all the water droplets and ice crystals so rain is unable to fall.

● The cloud typically extends above the freezing level.

● Usually no precipitation in this stage

Mature Stage

● The warmed air continues to rise until it reaches existing air which is warmer and then the air can rise no further.

● Often this 'cap' is the tropopause.

● The air is instead forced to spread out, giving the storm a characteristic anvil shape. 

● Water droplets coalesce into larger and heavier droplets and freeze to become ice particles.

● As these fall they melt to become rain

● The cloud may have already reached a height of 60 000 feet and the updraft may be travelling at speeds of 6 000 FPM

● While updrafts are still present the falling rain creates downdrafts as well.

● Strong downdrafts tend to form in the areas of the heaviest precipitation.

● The heavy rain cools and drags down the air with it so downdrafts may reach speeds of up to 2 000 FPM

● Precipitation, turbulence, thunder and lightning are the most intense during this phase.

● Turbulence is greatest in this stage due to the opposite rushing air currents at the middle level

● Updrafts continue to dominate the inner portions of the cloud.

● Most of the downdrafts form on the outside edges.

● Typically, the mature stage lasts about 15 minutes – but it is possible for it to last for an hour or more

Dissipating Stage 

● As the heavy precipitation falls through the cloud, the cloud cools, and then downdrafts dominate the base of the cloud.

● If atmospheric conditions do not support "SuperCell" or "Squall" development, this stage occurs rather quickly.

● The downdraft will push down out of the thunderstorm, hit the ground and spread out causing a microburst

● This cooling causes the cloud to lose energy and the rainfall gradually ceases.

● Cool air carried to the ground by the downdraft cuts off the inflow of the thunderstorm, the updraft disappears and the thunderstorm will dissipate

Thunderstorm Types

● Embedded Thunderstorms

➢ Obscured by cloud decks.

● Orographic

➢ Form in mountainous areas.

➢ Created as air is forced to move up a steep slope.

● Convection (Air mass thunderstorms)

➢ Rising hot air creates the energy source that thunderstorms require.

➢ Often seen on a summer afternoon.

➢ Even a forest fire can trigger them.

● Frontal (Steady-state thunderstorms)

➢ Created by frontal lift.

➢ Often form thunderstorm lines extending for hundreds of miles.

➢ Very hazardous because they are often embedded in cloud decks

Lower temperature spread (higher dewpoint) = lower cloud base = higher cloud/troposhere ceiling and quicker release of latent heat

Squall Line

● A fast moving line of thunderstorms.

● A SIGMET will be issued.

➢ Often you will get the SIGMET in flight, it is a good idea to take extra fuel.

● Frequently found well ahead of a fast moving cold front.

● Exercise extreme caution when near them!

● The squall line's leading edge is where the updrafts and downdrafts will be most severe.

➢ The greatest danger.

● If you intend to deviate around them be sure to take extra fuel.

➢ Squall lines are known to intensify or change shape without notice.

● If the flight will take you through a line of TCUs or CBs, penetrate the less dark areas

Squall Line Type Thunderstorm

cold front at left side. cold air undercuts warm, making each move faster

Lightning

● In order for lightning to occur, a voltage potential or difference of charge between two objects must exist.

● An atom is made up of a positive nucleus and negatively charged electrons swarming around the nucleus.

● An object can gain or lose electrons in a number of ways

● Air has an electrical resistance.

● When the electric potential or difference is large enough to break down this resistance, the electrons flow to the positive charge forming lightning

● The probability of lightning strikes occurring to aircraft is greatest when operating at altitudes where temperatures are between -5°C and 5°C.

● Lightning can strike aircraft flying in clear air in the vicinity of a thunderstorm.

● Lightning can puncture the skin of an aircraft, damage electronic equipment, cause engine failure, ignite fuel vapours, or induce permanent error in magnetic compasses.

● The crew may become blinded or temporarily lose their hearing

Hail 

● Strong updrafts push the water droplets up above the freezing level and the water droplets freeze.

● The updraft dissipates and the frozen droplets fall.

● As they fall they collect more water.

● Another updraft may push the droplets up and refreeze them at a larger size

● This cycle may repeat many times until the hail is too heavy for the updraft to carry it aloft again, when this happens, the hail will fall to the ground.

➢ This frequently happens under the anvil.

● Pilots should be alert for hail when radar echoes are well defined

Flight Near Thunderstorms 

There is no useful correlation between the external visual appearance of a thunderstorm, and the severity or amount of turbulence or hail within it.

Large hail is most commonly found in thunderstorms which have strong updrafts and high liquid water content.

The visible thunderstorm cloud is only a portion of a turbulent system of updrafts and downdrafts that often extend far beyond.

Severe turbulence may extend up to 20 NM from severe thunderstorms

● Airborne or ground-based weather radar will normally reflect areas of precipitation.

● The frequency and severity of turbulence associated with the areas of high water content generally increases the radar return.

● No flight path through an area of strong or very strong radar echoes that is separated by 40 NM or less can be considered free of severe turbulence

● Turbulence beneath a thunderstorm should not be underestimated.

➢ This is especially true even when the relative humidity is low.

● Virga (rain that evaporates before reaching the ground) is commonly associated with thunderstorms and typically related to microbursts.

● The pressure ahead of a thunderstorm falls rapidly and rises abruptly after the rain starts

Thunderstorm Considerations 

Whenever possible:

● Do not land or takeoff when a thunderstorm is approaching.

➢ The sudden wind shift of the gust front or low level turbulence could result in loss of control.

➢ The gust front speed and direction may change up to 50 KT and 180° in a matter of seconds.

➢ Violent wind shear may exist in all directions from a thunderstorm.

● Do not attempt to fly under a thunderstorm even when you can see through to the other side.

➢ Turbulence under the storm could be disastrous.

● Avoid any area where thunderstorms are covering 5/8 or more of the sky.

➢ Do not fly into a cloud mass containing embedded thunderstorms without airborne radar

● A microburst is a rapid downburst that can occur within 10 NM of a thunderstorm.

● Microbursts can cause wind shear at a considerable distance from a thunderstorm and have vertical speeds as high as 6 000 FPM.

If You Enter a Thunderstorm

● Plan a course that will take you through the storm area in a minimum time and hold it.

➢ Usually, altering heading to the right due to the cyclonic flow to have a tailwind on that side.

● Avoid the most critical icing areas by penetrating at an altitude below the freezing level or above the level of -15°C

● Disengage the autopilot altitude hold mode and the speed hold mode.

➢ The automatic altitude and speed controls will increase manoeuvres of the aircraft which increases structural stresses.

● Do not attempt to keep a constant rigid altitude.

➢ Let the aircraft “ride the waves.”

● Maintaining constant altitude increases stress on the aircraft

Hurricanes 

● These are destructive cyclonic (extremely low pressure systems) storms that originate in tropical waters.

➢ Basically they are huge systems of organized thunderstorms.

● They are quite large, being typically 100 miles or greater across