Download Thunderstorms - MrNagribianko

Unit 4 – Atmospheric Processes
THUNDERSTORMS
Necessary Atmospheric
Conditions
1. Water vapour must be available in the lower
atmosphere to feed clouds and precipitation
as the storm forms
2. A temperature gradient must exist so that
rising air cools off rapidly with height
3. An updraft must force moist air up to colder
levels of the atmosphere
The 2 Kinds of Thunderstorms
Air mass thunderstorms
 Usually created by convective uplift of warm,
moist, and unstable air.
 Have you ever been surprised by a sudden
downpour of thunderous rain on what was up to
that point a pretty nice day?
 Air mass thunderstorms typically do not have
very high winds, hail, or much lightning
associated with them.
Severe thunderstorms
 Have very high winds, hail, or much lightning
associated with them
 May even spawn tornadoes
 Tend to form along strong cold fronts where the
air on either side is very different, the
atmosphere is very unstable, and wind shear
aloft is prevalent.
 Wind Shear – the change in wind speed or direction
with height in the atmosphere over a relatively short
horizontal distance
Stages of Thunderstorm
Development
CUMULUS STAGE
 During this stage warm, moist,
and unstable air is lifted from
the surface.
 In the case of an air mass
thunderstorm, the uplift
mechanism is convection.
 As the air ascends, it cools and
upon reaching its dew point
temperature begins to
condense into a cumulus
cloud.
 Near the end of this stage
precipitation forms.
MATURE STAGE
 Warm, moist updrafts continue to
feed the thunderstorm while cold
downdrafts begin to form.
 As rain falls through the air it drags
the cool, dry air that surrounds the
cloud into it.
 As dry air comes in contact with
cloud and rain droplets they
evaporate cooling the cloud.
 The falling rain drags this cool air to
the surface as a cold downdraft.
 In severe thunderstorms the region
of cold downdrafts is separate from
that of warm updrafts feeding the
storm. As the downdraft hits the
surface it pushes out ahead of the
storm.

Sometimes you can feel the
downdraft shortly before the
thunderstorm reaches your location
as a cool blast of air.
DISSIPATING STAGE
 Thunderstorm dissolves away.
 By this point, the entrainment
of cool air into the cloud helps
stabilize the air.
 In the case of the air mass
thunderstorm, the surface no
longer provides enough
convective uplift to continue
fueling the storm.
 As a result, the warm updrafts
have ceased and only the cool
downdrafts are present.
 The downdrafts end as the rain
ceases and soon the
thunderstorm dissipates.
Severe Thunderstorms
Conditions necessary:
 Winds blowing in different directions
producing wind shear
 high water-vapour content in the lower
troposphere
 uplift of air
 the existence of a dry air mass above a moist
air mass
Supercell Storms
 Most damaging of all severe thunderstorms
 Smaller than the other types of thunderstorms




(mesoscale convective complexes (MCCs) and squall
lines) they are extremely violent and the breeding
ground for most large tornadoes
Range from 20-50km in diameter
Last from 2-4 hours
Can bring high or low amounts of precipitation, create
strong downbursts, flash floods, large hail and tornadoes
MCCs
 Most common; very large clusters of self-propagating storms;
downdraft in one cell leads to the formation of a new cell nearby
 Squall Lines
 Average 500km in length; long lines of individual storm cells;
parallel to cold fronts approx 300-500km ahead of the front
http://www.nebraskastorms.com/video.htm