THUNDERSTORMSUnit 4 – Atmospheric Processes

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 ThunderstormsAir 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 fronthttp://www.nebraskastorms.com/video.htm