THUNDERSTORMS

TOPICS:

• What are Thunderstorms?

• Ingredients

• Stages

• Types

• Lightning

• Downburst and Microburst

What are Thunderstorms?

A storm produced by a cumulonimbus cloud that

contains lightning and thunder

Convective Ingredients

• Instability

• Moisture

• Lift

• Exhaust

– All of these must be present for thunderstorms

to occur

Instability

• A condition of the atmosphere where by a

parcel of air, given an initial start vertically,

then continues to move upward without any

outside force.

• Instability can be caused by:

– Surface heating

– Cooling aloft

– Evaporational cooling

• Dry air is advected into a moist area

Moisture

• Necessary for cloud formation

• Increasing moisture will enhance instability

Lift (trigger)

• Force needed to get a parcel of air to it’s

Level of Free Convection

• The three main triggers

– Boundaries

• Fronts, troughs, outflow boundaries and moist

discontinuities (dry-lines)

– Topography

• Mountains and coastlines

– Differential heating

• Strong temperature gradient over a short distance

Exhaust

• Upper level

divergence is

required

Evolution of Thunderstorm

Development

• Cumulus Stage

• Mature Stage

• Dissipating Stage

Cumulus Stage

• Begins with the formation of the first

convective cloud

• Dominated by updrafts

• Updrafts hold the prospective precipitation

aloft

Mature Stage

• Cloud particles grow too large for the rising air to keep them suspended

• The particles then fall against the updraft and create a downdraft

• The cloud reaches the stratosphere and the top begins to spread out

• Most violent and active stage

• Thunder and lightning begin in this stage

• The heavier the precipitation the stronger the downdraft

Mature cont.

• Updrafts and downdrafts reach their greatest strength in the middle of the cloud, creating severe turbulence

• Ice crystals suspended near the melting level alternate between freezing and melting, producing hail

• Hailstones continue to grow until it is too heavy to be supported by the updraft core

• Aircraft have encountered hail in clear air several kilometers away from the storm

Mature cont.

• The leading edge of the downdraft at the

surface is known as the “gust front” or

“outflow boundary”

• The gust front produces “Low-Level Wind

Shear”

Dissipating Stage

• Begins when the updraft collapses and the

downdraft dominates

• The downdraft cuts off the inflow of warm,

moist air into the storm

• Cloud droplets stop forming

• The cloud begins to stratify into layered

clouds

Types of Thunderstorms

• Single cell

• Multi-cell

• Supercell Thunderstorms

• Squall line

Single Cell Thunderstorms

• “Air mass thunderstorms”

• Short lived (30 to 60 minutes)

• Severe weather is rare

Multi-cell Thunderstorms

• Requires a greater degree of instability

• Strong mid level winds carry the

precipitation downstream so it doesn’t fall

into the updraft

• New cells continue to form on the right

flank and develop into mature storms

• Storms last longer due to their ability to

renew themselves

Supercell Thunderstorms

• Updrafts and downdrafts are so sufficiently

in balance that it can maintain itself for

hours on end

• Producers of tornadoes and destructive hail

Squall Line

• A line of thunderstorms that is not readily

circumnavigable

• Normally occur 50 to 300 miles ahead of a

cold front

• Produce high winds

• Hail and tornadoes are possible

Lightning

• A discharge of electricity which occurs

during the mature stage of thunderstorms

• Types of lightning

– In cloud

– Cloud to cloud

– Cloud to air

– Cloud to ground

• Lightning

hazards

– Flash

blindness

– Puncture

aircraft skin

– Shock and

burns

– Fuel ignition

– Damage to

equipment

Downburst

• Strong concentrated downdraft with a great

potential for damaging surface winds

• Usually located behind the gust front

• Wind speeds are greater than 40 knots at the

surface

Microburst

• Concentrated downburst

• 4 kilometers or less

• Change in velocity of 50 knots or greater

Questions?