Chapter 13 – Severe Weather

At any given moment, nearly 2000 thunderstorms are in progress around the world.

Both geography and air mass movements make thunderstorms most common in the southeastern United States.

For a thunderstorm to form, three conditions must exist:

•An abundant source of moisture in the lower levels of the atmosphere

•lifting of the air mass in order for condensation to occur (necessary for cloud formation & growth)

•unstable atmosphere.

Overview of Thunderstorms

How thunderstorms form

Lightning is the transfer of electrical charge caused by the rapid rushes of air in a cumulonimbus cloud.

Friction between the updrafts and downdrafts within a cumulonimbus cloud removes electrons from some of the atoms in the cloud.

Atoms that lose electrons become positively charged ions, and atoms that receive the extra electrons become negatively charged ions.

Eventually, the differences in charges break down, and a branched channel of partially charged air, called a stepped leader, is formed between the positive and negative regions.

When the stepped leader nears the ground, a branched channel of positively charged particles, called the return stroke, rushes upward to meet it and illuminates the connecting channel with about 100 million volts of electricity.

Stepped leader moving in ~50m steps.

© 1969 Martin Uman -- From: Uman, Lightning

A lightning bolt heats the surrounding air to about 30,000 C, about five times hotter than the surface of the Sun.

The thunder you hear is the sound produced as this superheated air rapidly expands and contracts.

Thunder

Each year in the United States, lightning causes about 7500 forest fires and an average of 300 injuries and 93 deaths to humans.

Thunderstorm and lightning safety

Assume LSP. Crouch with feet as

close together as possible. Have heels touch. Place hands over ears.

-REMEMBER- DO NOT LIE FLAT ON THE GROUND

30 Seconds

◦ MOVE INDOORS if thunder is heard within 30 seconds of seeing Lightning

30 Minutes

◦ SUSPEND ACTIVITIES for 30 minutes after the last observed lightning or thunder.

30-30 RULE

By referencing the time in seconds from seeing the lightning (the FLASH) to hearing the accompanying thunder (the BANG), we can range lightning's distance.

A "Flash" to "Bang" of five seconds equals lightning distance being one mile away.

A "Flash" to "Bang" of ten = two miles; etc…..

Severe thunderstorms can develop into self-sustaining, extremely powerful storms called supercells.

These furious storms can last for several hours and can have updrafts as strong as 240 km/h.

An anvil-shaped cumulonimbus cloud is characteristic of many severe thunderstorms.

Supercells

Precipitation in the form of balls or lumps of ice.

It forms because of two characteristics common to thunderstorms.

1. water droplets rise to the heights of a cumulonimbus cloud where the temperature is below freezing, encounter ice pellets, and freeze on contact with the pellets, which causes the ice pellets to grow larger.

2. There needs to be an abundance of strong updrafts and downdrafts moving side by side within a cloud.

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

Location

Favorable conditions for cyclone formation exist in all tropical oceans except the South Atlantic Ocean and the Pacific Ocean off the west coast of South America.

Tropical cyclones require two basic conditions to form

1) an abundant supply of warm ocean water

2) some sort of mechanism to lift warm air and keep it rising.

Cyclone formation

The first indication of a building tropical cyclone is a moving tropical disturbance.

When a disturbance over a tropical ocean acquires a cyclonic circulation around a center of low pressure, it has reached the developmental stage and is known as a tropical depression.

Cyclone formation

When wind speeds around the low-pressure center of a tropical depression exceed 65 km/h, the system is called a tropical storm.

If air pressure continues to fall and winds around the center reach at least 120 km/h, the storm is officially classified as a cyclone.

Cyclone formation

Tropical Storms

Section 13.3

Once winds reach at least 120 km/h, another phenomenon occurs—the development of a calm center of the storm called the eye.

The eye of the cyclone is a span of 30 to 60 km of calm weather and blue sky.

Cyclone formation

Cyclone formation

The strongest winds in a hurricane are usually concentrated in the eyewall—a tall band of strong winds and dense clouds that surrounds the eye.

Tropical Storms

Section 13.3

Tropical Storms

Section 13.3

Cyclone formation

A hurricane will last until it can no longer produce enough energy to sustain itself. This usually happens when the storm has moved either over land or over colder water.

Like most storms, cyclones begin with warm moist air rising.

Tropical Storms

Section 13.3

Tropical Storms

Section 13.3

The Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, potential for flooding due to the effect on the height of sea level, and potential for property damage.

Damage

Hurricanes can cause extensive damage, particularly along coastal areas, which tend to be where human populations are the most dense.

Winds Much of the damage caused by hurricanes is associated with violent winds.

A storm surge occurs when hurricane-force winds drive a mound of ocean water toward coastal areas where it washes over the land.

Storm surge

The National Hurricane Center issues a hurricane warning at least 24 hours before a hurricane is predicted to strike.

Awareness, combined with proper safety precautions, has greatly reduced death tolls associated with hurricanes in recent years.

Hurricane advisories and safety

A tornado is a violent, whirling column of air in contact with the ground.

Before a tornado reaches the ground, it is called a funnel cloud.

A tornado forms when wind speed and direction change suddenly with height, a phenomenon associated with wind shear.

Although tornadoes rarely exceed 200 m in diameter and usually last only a few minutes, they can be extremely destructive

Severe Weather

Section 13.2

The Fujita tornado intensity scale, which ranks tornadoes according to their path of destruction, wind speed, and duration, is used to classify tornadoes.

Tornado classification

Most tornadoes—especially violent ones—form in the spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air are the greatest.

This type of large temperature contrast occurs most frequently in the central United States.

Many of the more than 700 tornadoes that touch down in the United States each year occur in a region called “Tornado Alley,” which extends from northern Texas through Oklahoma, Kansas, and Missouri.

Tornado distribution

If you are caught in a tornado

Indoors: take shelter in the southwest corner of a basement, a small downstairs room or closet, or a tornado shelter.

Outdoors: find lowest spot & lie flat and face-down, protecting the back of your head with your arms. Avoid seeking shelter under bridges. Stay away from objects that might become debris, i.e. cars.

Car: Drive away from storm if possible. If not, park the car quickly & off road. Find shelter if possible. If not, follow outdoor guidelines.

An individual thunderstorm can unleash enough rain to produce floods, and hurricanes also cause torrential downpours, which result in extensive flooding.

Floods can also occur when weather patterns cause even mild storms to persist over the same area.

Low-lying areas are most susceptible to flooding, making coastlines particularly vulnerable to storm surges during hurricanes.

Rivers in narrow-walled valleys and streambeds can rise rapidly, creating high-powered and destructive walls of water.

Droughts are extended periods of well-below-average rainfall.

Droughts are usually the result of shifts in global wind patterns that allow large, high-pressure systems to persist for weeks or months over continental areas.

Because the sinking air prevents humid air from rising, condensation cannot occur, and drought sets in until global patterns shift enough to move the high-pressure system.

Recurrent Weather

Section 13.4

An unpleasant side effect of droughts often comes in the form of heat waves, which are extended periods of above-average temperatures.

Heat waves can be formed by the same high-pressure systems that cause droughts.

Heat waves

Recurrent Weather

Section 13.4

Because of the dangers posed by a combination of heat and humidity, the National Weather Service (NWS) routinely reports the heat index.

The heat index assesses the effect of the body’s difficulty in regulating its internal temperature as relative humidity rises.

Heat waves

Recurrent Weather

Section 13.4

The opposite of a heat wave is a cold wave, which is an extended period of below-average temperatures.

Cold waves are also brought on by large, high-pressure systems. However, cold waves are caused by systems of continental polar or arctic origin.

Because of the location and the time of year in which they occur, winter high-pressure systems are much more influenced by the jet stream than are summer high-pressure systems.

Recurrent Weather

Section 13.4

The winter location of the jet stream can remain essentially unchanged for days or even weeks. This means that several polar high-pressure systems can follow the same path and subject the same areas to continuous numbing cold.

The effects of cold air on the human body are magnified by wind. Known as the wind-chill factor, this phenomenon is measured by the wind-chill index.

Recurrent Weather

Section 13.4

Wind-chill index