4/5/2017

1

Chapter 13

Section 13-1: Thunderstorms

Main Idea: The intensity and duration of thunderstorms depend on the local conditions that create them. Air-mass thunderstorm

Mountain thunderstorm

Sea-breeze thunderstorm

Frontal thunderstorm

Stepped leader

Return stroke

3 Things Needed for a

Thunderstorm

Abundant moisture in the lower

atmosphere.

A lift mechanism

Unstable atmosphere: the

atmosphere must be cooler with

altitude, so the air will continue to

rise

Air Stability – Cause?

Stability: The ability of air to resist

rising.

4/5/2017

2

Atmosphere Stability –

Day vs. Night

The speed or rate at which air rises

& cools determines its stability.

Speed of rising depends on the

temperature change from ground

to upper troposphere

Stable atmosphere = high

stability = little rising

Air at surface is similar in temp and

density as the air above, resulting

in little to no rising.

Clouds are thin stratus, or absent.

Untable atmosphere = instability

= Air rises quickly

Air temperature quickly decreases with altitude.

Air at the surface is warm with low density compared to the air above it.

The low density causes it to rapidly rise

Causes cumulonimbus clouds and intense precipitation.

Untable atmosphere = instability

= Air rises quickly

4/5/2017

3

Untable atmosphere Cont…

Instability can be increased with a

high amount of humidity in the air.

Results:

Lots of condensation at the LDL

(Lifted Condensation Level) will

release “latent heat”

Latent heat warms the atmosphere,

air continues to rise, cloud expands.

Untable atmosphere Cont…

Instability is also increased when warm

air is forced quickly up at a cold front.

Instability is increased during

orographic lifting.

4/5/2017

4

Classification of Thunderstorms

Thunderstorms are classified

by type of lift mechanism.

Air mass thunderstorms

Mountain

Sea breeze

Frontal thunderstorms

Air Mass Thunderstorms

Occurs in the middle of a single air mass due to convection in the middle.

Due to unequal surface heating within the air mass during the day.

Most common in late afternoon around 4pm.

Mountain Thunderstorms

Occur when an air mass rises

by orographic lifting, which

involves air moving up the side

of a mountain.

Sea-Breeze Thunderstorms

Caused by heating and temperature differences between land and water.

mT air is warm and humid – perfect for thunderstorm formation.

Do not produce severe storms.

Happen in late afternoon.

4/5/2017

5

Sea-Breeze Thunderstorms Frontal Thunderstorms

Produced mainly by cold fronts.

Steep front forces warm air up

quickly.

T-Storm develops at leading edge

(front edge)

Frontal Thunderstorms

Differences from air mass T-Storms

Last longer

Can occur day or night

More severe

Sometimes produced by warm fronts,

but not as often and not as severe.

Stages of Thunderstorm

Development

T-Storm Development:

3 stages classified by direction of

air movement (up and/or down)

4/5/2017

6

1. Cumulus Stage – “Baby” Cumulus

Warm wet air rises, creating updrafts.

As air rises, it begins to cool until it reaches the dew point.

Water vapor condenses, releasing latent heat.

Latent heat causes air to warm and rise.

1. Cumulus Stage – “Baby” Cumulus

Warm wet air rises, creating updrafts.

As air rises, it begins to cool until it reaches the dew point.

Water vapor condenses, releasing latent heat.

Latent heat causes air to warm and rise.

4/5/2017

7

2. Mature Stage – tall & active

cumulonimbus

Precipitation/rain begins, cooling surrounding air.

Cooled, dense air sinks to the ground with rain, producing downdrafts

Equal amounts of updrafts and downdrafts form convection cells with strong winds.

Heavy rain and/or hail is possible

3. Dissipation Stage

Downdrafts predominate

Cool air predominates. Little

warm air left, so updrafts stop.

Very light rain, cloud

evaporates and disappears.

4/5/2017

8

3 Signs of Severe T-Storms

Strong winds of

50mph or more

Large hailstones

measuring 0.75” or more

Tornado present

Severe Thunderstorm Hazards

3 Hazards that accompany

severe T-storms responsible for

the most property damage and

deaths.

Lightning

Tornadoes

Flash Flooding

4/5/2017

9

Lightning

Caused by electricity generated by the rapid rushes of air inside a cumulonimbus cloud.

Friction rubs/knocks electrons off.

Charges separate: - charge builds up on bottom, + charge on top of cloud.

- charge at bottom of cloud attracts and pulls positive charges on the ground to the tops of trees.

Lightning

Lightning

When charges build up, a giant

electrical spark/current jumps

from negative area to positive

area.

Lightning

A branched channel of partially charged air forms between the cloud and the ground, called a stepped leader.

When the stepped leader reaches the ground, it connects with the return stroke, a branch of positively charged particles.

4/5/2017

10

Lightning Lightning

Lightning

Lightning causes an instant “heat”, 5x hotter than the surface of the Sun.

Thunder is due to rapid expansion followed by immediate contraction of the air.

Responsible for 300 injuries and 90 deaths annually.

Lightning

Lightning causes an instant “heat”, 5x hotter than the surface of the Sun.

Thunder is due to rapid expansion followed by immediate contraction of the air.

Responsible for 300 injuries and 90 deaths annually.

4/5/2017

11

13-2: Supercells

Supercells are self-sustaining,

extremely powerful storms.

Characterized by:

Intense, rotating updrafts

Spawning long-lived tornadoes

240km/h winds

Less than 10% of thunderstorms in

the U.S. reach supercell proportions

13-2: Strong Winds

Violent downdrafts of 200km/h or greater concentrated in a local area are called downbursts.

Macrobursts: Can affect an area 5km wide, and lasts for up to 30 minutes

Microbursts: Can affect an area of 3km wide, and lasts fewer than 10 minutes.

4/5/2017

12

13-2: Strong Winds

Downbursts can be as

destructive or more

destructive than tornadoes.

Extremely difficult to

predict and detect.

13-2: Tornadoes

Violent, whirling column of air

in contact with the ground.

Prior to contact, called a

funnel cloud.

Formed when wind speed and

direction change suddenly.

13-2: Tornadoes

Begins with air moving

horizontally on the surface.

Storm updrafts flip the air

up.

Air begins to stretch out.

More stretching – faster air.

4/5/2017

13

13-2: Tornadoes

Begins with air moving

horizontally on the surface.

Storm updraft picks this air

up.

Air begins to stretch out.

More stretching – faster air.

13-2: Tornadoes (simplified)

Measuring/Classifying

Fujita tornado intensity scale

Based on estimated wind

speed and damage.

Determined AFTER the

tornado occurred – damage

must be assessed

Measuring/Classifying

4/5/2017

14

Measuring/Classifying

Enhanced Fujita Scale

Type of structure combined

with damage is evaluated,

because building more poorly

made will have more damage.

Locations Occur most often in spring in

the central U.S. where cold cP collide with mT (Large temp. differences)

As warm, moist mT air moves N from Feb to May, tornado occurrences also move N.

Usually occur in late afternoon.

13-3 Tropical Storms

Normally peaceful, tropical oceans are capable of producing one of Earth’s most violent weather system.

During summer and fall, the tropics experience conditions ideal for the formation of large, rotating, low-pressure tropical storms called tropical cyclones.

4/5/2017

15

Tropical Cyclones

Large, rotating, low-pressure storms.

Require warm ocean water.

Occur in late summer / early fall.

Called hurricanes in the U.S. and

Atlantic ocean.

Called typhoons in the Pacific ocean.

Latent Heat & Pressure

Counter-clockwise spin in the

Northern hemisphere.

Warm, moist air rises, condenses,

releasing latent heat, and continuing

to rise.

Continued rising increases energy and

spinning speed.

Overall Result

Low-pressure in the center of the

cyclone.

Spiraling surface wind speed

increases.

The lower the pressure, the stronger

the storm.

4/5/2017

16

Formation – 2 things needed

Abundant supply of warm water.

Disturbance causing lift.

Movement

Moves with wind currents.

(trade winds)

Lasts until no warm ocean water is

available to sustain it.

Energy no longer available when it

reaches:

Land (loses moisture)

Cold Water (loses warmth)

4/5/2017

17

Tropical Cyclone Stages

1. Tropical Disturbance

2. Tropical Depression

3. Tropical Storm

4. Hurricane

Tropical Disturbance

Starts as a weak low-pressure

system where air rises

because it is warm or because

it is lifted by the ITCZ

(Intertropical Convergence

Zone)

Tropical Disturbance Tropical Disturbance

May be a band of

thunderstorms in the tropics.

4/5/2017

18

Tropical Depressions and Storms

Tropical Depressions form

when spinning around the low

pressure center begins

Tropical Storms form when

cyclonic winds around the

center are 40mph or more.

Hurricanes

Hurricanes form when winds are >= 75mph (120kmph) and the eye is present.

The eye is calm and an area of extremely low pressure

The eyewall has the strongest/fastest winds.

Measuring Hurricanes

Saffir-Simpson hurricane

scale: Based on CURRENT

wind speed, pressure, and

storm surge damage potential

(NOT actual damage like the

Fujita Tornado Scale)

Saffir-Simpson hurricane scale

A hurricane can move up and

down through the different

levels throughout its life.

1-5 scale: lowest to highest

intensity

4/5/2017

19

Hurricane Destruction:

3 Main Hazards

1. Storm Surge

2. Wind Damage

3. Flash Flooding

Storm Surge

Huge wall of water that is

pushed over the land by the

hurricane winds.

Causes 90% of hurricane

deaths.

Storm Surge

4/5/2017

20

Storm Surge

Storm Surge

Wind Damage

75-150 mph winds

These change in intensity

depending on your location

and the path of the hurricane.

Most violent winds are in the

eye wall.

Wind Damage

4/5/2017

21

Flash Flooding

Caused by heavy rainfall in a

small duration of time.

Worse if combined with a

storm surge.

13-4: Recurrent Weather

Relatively mild weather can

become destructive if it

persists for long periods of

time.

Too much of any type of

weather can have serious

consequences.

4/5/2017

22

Droughts: Lack of Rainfall

Extended period with no rain,

or less than average rainfall.

Result of _______ pressure

system?

HIGH

Heat Waves:

Extended periods above

normal temperature.

With a HIGH-pressure system,

there are little to no clouds.

Air is rising or sinking?

Sun is intense since it does not

reflect off of cloud cover.

Heat Waves:

Usually accompany droughts.

Winds blowing out away from

center (H-Pressure) prevent

cool air masses from moving

in.

Health problems can occur

(heat stroke)

Heat Index

Compares combined heat and

humidity to the body’s ability

to sweat and cool off

4/5/2017

23

Cold Wave

Extended periods below

normal temperature.

Also due to high pressure, but

cP in origin.

Influenced by the jet stream.

Wind-Chill Factor

How cold the air feels based on

the estimated heat loss from

the skin due to cold

temperatures and WIND.