The Nature of Storms

Chapter 13Earth Science

2013-2014

Thunderstorms

• At any given moment, there are nearly 2,000 thunderstorms occurring around the world.

• Some are capable of producing hail, swirling tornadoes, and high surface winds.

• Can also provide energy for tropical storms and hurricanes.

How Thunderstorms Form

• Must have abundant moisture in the atmosphere.

• Some mechanism must lift the air so that the moisture can condense and release latent heat.

• The portion of the atmosphere through which the cloud grows must be unstable.

Air-Mass Thunderstorms

• A thunderstorm that occurs if the air rose because of unequal heating of Earth’s surface.

• The unequal heating of Earth’s surface reaches its maximum during mid-afternoon.

• Two main types:– Mountain thunderstorms– Sea-breeze thunderstorms

Air-Mass Thunderstorms

• Mountain thunderstorms– Occur when an air mass rises as a result of

orographic lifting (air moving up the side of a mountain).

• Sea-breeze thunderstorms– Local air-mass thunderstorms.– Caused in part by extreme temperature differences

between the air over land and the air over water.

Frontal Thunderstorms

• Produced by advancing cold fronts (and rarely warm fronts).

• Produces a line of thunderstorms along the leading edge of a cold front (and rarely warm front).

• Can be hundreds of kilometers long.

Stages of Development

• A thunderstorm usually has three stages:– Cumulus– Mature– Dissipation

• Classified according to the direction the air is moving.

Stages of Development

• Cumulus stage– Air starts to rise nearly vertically upward.– Creates updrafts that transport moisture to the

upper reaches of the cloud.– Moisture condenses and releases latent heat.– Larger droplets form and fall to Earth.– Leads to mature stage.

Stages of Development

• Mature stage– As the precipitation falls, it cools the air around it.– Newly cooled air is more dense, so it sinks.– Creates downdrafts.– Updrafts and downdrafts form a convection cell

that produces the gusty surface winds.– Nearly equal amounts of updrafts and downdrafts

in the mature stage.

Stages of Development

• Dissipation stage– Production of downdrafts is what ends

thunderstorms.– Convection cell can exist only if there is a steady

supply of warm, moist air at Earth’s surface.– When the supply runs out, the updrafts slow and

then stop.– This stage lasts until the cloud runs out of

previously formed raindrops.

Severe Weather

• Storms vary in severity.– Severe thunderstorms– Lightning– Wind– Hail– Floods– tornadoes

Severe Thunderstorms• With an increase in temperature difference, the instability of air

increases.

• This increases the strength of the storm’s updrafts and downdrafts.

• Of all annual thunderstorms, less than 10% are considered severe and even fewer are supercells.

• Supercells– Self-sustaining, extremely powerful storms.– Characterized by intense, rotating updrafts.– Can last for several hours and have updrafts as strong as 240 km/hr.

Lightning

• Electricity caused by the rapid rush of air in a cumulonimbus cloud.

• Lightning bolt forms when friction between the updrafts and downdrafts separate electrons from some of their atoms.

• Some become positively charged and others become negatively charged.

• Return stroke from the ground up to the cloud illuminates the channel.

Lightning

• Heats the surrounding air to 30,000°C.– About five times the temperature of the Sun.

• Thunder is the sound heard from lightning.

• May see lightning much sooner than you hear thunder, even though they begin at the same time.

• Accounts for about 7500 forest fires every year.

• Cause a yearly average of 300 injuries and 93 deaths.

Wind

• Sometimes the downward energy is not dispered, but becomes concentrated.

• Downbursts– Violent downdrafts.– Resulting winds have speed of more than 160 km/hr.– Macrobursts – path of destruction up to 5 km wide with

wind speeds of 200+ km/hr and can last up to 30 minutes.– Microbursts – less than 3 km wide with wind speeds over

250 km/hr lasting less than 10 minutes.

Hail

• Precipitation in the form of balls or lumps of ice.

• Most common during the spring growing season and can do significant damage to crops.

Hail

• Two characteristics help hail form.– Water droplets exists in liquid state where the

temperatures are below freezing. Come in contact with ice pellets and help the ice pellet grow.

– Abundance of strong updrafts and downdrafts. Helps them come into contact with more water and grow larger.

Floods

• Occur when weather systems and resulting storms move slowly.

• Storm dumps rain on a limited location.

• Rain can fall faster than the ground can absorb it or rivers and streams can transport it out of the area.

Tornadoes

• A violent whirling column of air in contact with the ground.

• Called a funnel cloud before it hits the ground.

• Forms when wind speed and direction change suddenly with height and can go from a horizontal rotation to vertical.

• Rarely exceed 200 m in diameter.

• Usually on last a few minutes, but can be extremely destructive.

Tornadoes

• Tornado Classification– Fujita tornado intensity scale – ranks tornadoes

according to their path of destruction, wind speed, and duration.

– Scale ranges from F0 (winds up to 118 km/hr) to F5 (winds of more than 500 km/hr).

– Most do not exceed F1.– Only about one percent reach F4 or F5, but can lift

buildings from their foundations.

Tornadoes

• Tornado Distribution– Can occur at any time and at any place.– Some times and locations are more conducive:• Most form in spring.• Late afternoon/evening.• Central United States – “Tornado Alley” extends from

northern Texas through Oklahoma, Kansas, and Missouri.

Tornadoes

• Tornado Safety– Average of 80 deaths and 1500 injuries from

tornadoes each year.– NWS issues tornado watches and warnings.– Signs of an approaching or developing tornado:• Severe thunderstorm• Dark, greenish skies• Towering wall of clouds• Large hailstones• Roaring noise similar to freight train

Tropical Storms

• Tropical cyclones– Large, rotating, low-pressure storms.– During summer and fall.– Known in the United States and Atlantic Ocean as

hurricanes.

Tropical Cyclones

• Air rises, usually due to some existing weather disturbance.

• As they produce more precipitation, more energy is released.

• Coriolis effect causes the moving air to turn counterclockwise in the northern hemisphere.

Tropical Cyclones

• Formation of Tropical Cyclones– Require two basic conditions:• An abundant supply of very warm ocean water.• Some sort of disturbance to lift warm air and keep it

rising.

– Conditions exists in all tropical oceans except the South Atlantic Ocean and the Pacific west of the South American Coast.

Tropical Cyclones

• Movement of Tropical Cyclones– Move according to the wind currents that steer them.– Often caught up in the circulation of tropical high-pressure

systems.• Carries them west until they reach the far west end of the system,

then turn towards the pole.

• Eye – the development of a calm center of the storm.

• Eyewall – the strongest winds that are concentrated in a band immediately surrounding the eye.

Tropical Cyclones

• Stages of Tropical Cyclones– Tropical disturbance

• Common during the summer and early fall.• Only a small percentage ever become hurricanes.

– Tropical depression• When a disturbance acquires a cyclonic circulation around a center

of low pressure.• Wind speeds around the center up to 65 km/hr

– Tropical Storm• Winds of 65 km/hr to 120 km/hr.

– Hurricane• Winds above 120 km/hr.

Classifying Hurricanes

• Saffir-Simpson hurricane scale– Classifies according to wind speed, air pressure in

the center, and potential for property damage.– Category 3 and above are considered major

hurricanes.

Classifying Hurricanes

Scale Number (Category) Sustained Winds (mph) Damage

1 74-95 Minimal

2 96-110 Moderate

3 111-130 Extensive

4 131-155 Extreme

5 >156 catastrophic

Saffir-Simpson Hurricane Scale

Classifying Hurricanes

• Running Out of Energy– A hurricane will last until it can no longer produce

enough energy to sustain itself.– Usually when it makes landfall or cold water.– No longer has warm water to draw energy from.

Hurricane Hazards

• Storm Surges– Occurs when hurricane-force winds drive a mound

of ocean water toward coastal areas, where it washes over land.

– Can sometimes reach 6m above normal sea level.– Causes major damage along with high tide.– Also leads to flooding.

Hurricane Hazards

• Hurricane Advisories– NHC (www.nhc.noaa.gov) issues a hurricane

warning at least 24 hours before a hurricane strikes.

– Also issues regular advisories with a storm’s position, strength, and movement.

– Tracking greatly reduces death tolls over the past several years.

Recurring Weather

• Floods and droughts• Heat waves• Cold waves

Floods and Droughts

• Floods can occur with large storms, hurricanes, and when weather patterns cause storms to stall over the same area.

• Small amounts of rain per hour are not an issue, but can be if it rains for 18-24+ hours.

Floods and Droughts

• Droughts– Extended periods of well-below-normal rainfall.– Usually the result of shifts in global weather

patterns that allow large high-pressure systems to persist for weeks or months over continental areas.

Heat Waves• Extended periods of above-normal temperatures.

• Can be formed by the same high-pressure systems that cause droughts.

• Blocks cooler air masses from moving into the area.

• Humidity reduces the body’s ability to evaporate perspiration, and can lead to serious health problems such as heatstroke, sunstroke, and even death.

• See table 13-6 on page 349 for the heat index.

Cold Waves

• Extended period of below-normal temperatures.

• Also brought on by large, high-pressure systems.

• Caused by systems of continental polar or arctic origin.

• Wind-chill factor – Cold air is worsened by wind.– Measured by the wind-chill index, which estimates the heat loss

from human skin caused by the combination of cold air and wind.