1. Storms
Chapter 13
Section 4

2. Essential Questions Chap 13 Sec 4
What are the main kinds of storms, and how do they form?
What measures can you take to ensure safety in a storm?

3. Storms

4. Storms A storm is a violent disturbance in the atmosphere
It is involves sudden changes in air pressure, which can cause rapid air movements
There are several types of storms

5. Types of Storms
Thunderstorms
Tornadoes
Snow Storms
Hurricanes

6. Thunderstorms

7. Thunderstorms
Thunderstorms are small storms with heavy precipitation and frequent thunder & lightning
Many thunderstorms form in the spring and summer in southern states and on the Western Plains
Thunderstorms form in large cumulonimbus clouds (either on hot, humid afternoons or when cold fronts pass through)

8. How thunderstorms form:
Warm, humid air rises rapidly
The air cools, forming dense thunderheads (cumulonimbus clouds)
Heavy rain and sometimes hail falls
The clouds contain strong upward and downward winds (called updrafts and downdrafts)
Thunderstorms contain thunder & lightning

10. Stages in the Development of a Thunderstorm
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11. Thunder & lightning
Lightning is a sudden spark (an electrical discharge) as positive and negative charges jump between parts of a cloud, between nearby clouds, or between a cloud and the ground
Thunder is caused by the superfast heating of the air by lightning
Lightning heats the air around it to 30,000° C (much hotter than the surface of the sun)
The rapidly heated air expands suddenly and explosively
Thunder is the sound of the explosion
Because light travels much faster than sound, you see lightning before you hear thunder

12. Calculate the distance of lightning:
Count the number of seconds between the moment you see the lightning and when you hear the thunder
Divide that number by 5 to get the approximate distance in miles
Example: 15 seconds divided by 5 = 3 miles
Do this inside!!!!!!!

17. Damage & Danger from Thunderstorms:
Heavy rains can cause flooding
Lightning can strike and damage trees, homes, electronics in our homes, animals, and people
Lightning can start fires
When it strikes people and animals, it can cause unconsciousness, serious burns, or even heart failure

18. Floods
Floods are caused by too much water pouring into streams and rivers at once
In cities floods can be caused by lack of ground to absorb the water (covered with roads, buildings, etc)
If the ground is already soaked because of previous rains it is more likely to flood
Floods can destroy human structures and wildlife habitats
Floods can be good! They can provide rich new soil for agriculture

19. Safety during Thunderstorms
Go inside
Avoid touching telephones, electrical appliances, or plumbing fixtures (all conduct electricity)
Cars are generally safe if they have a hard top because the electricity will flow along the metal surface of the car to the ground
Avoid metal objects, tall structures, and bodies of water such as:
Trees, poles, high hills, fences, being in pools or lakes (even in a boat)
Get in a low area as close to the ground as possible

20. Tornadoes

21. All About Tornadoes A frightening and destructive storm
A rapidly whirling, funnel-shaped cloud that reaches down from a storm cloud to touch Earth’s surface
A waterspout is a tornado over a lake or ocean
Tornadoes usually touch the ground for 15 minutes or less and are usually only a few hundred meters across
Winds in the tornado reach up to 500 kilometers per hour (311 mph)

22. How Tornadoes Form
Tornadoes usually develop in cumulonimbus clouds just like thunderstorms, but can develop in any situation that produces severe weather
They are most likely to occur in the spring and early summer, often late in the afternoon when the ground is warm (just like thunderstorms)

23. Formation continued
A special situation on the Great Plains creates perfect weather for tornadoes to form
A warm, humid air mass moves north from the Gulf of Mexico into the lower Great Plains
A cold, dry air mass moves south from Canada
When the air masses meet, the cold air moves under the warm air, forcing it to rise
A squall line (narrow band of thunderstorms) is formed and the storms travel from southwest to northeast
A single squall line can produce ten or more tornadoes

24. Tornado Alley
The US has more tornadoes per year than any other country (about 800)
Tornadoes occur in nearly every part of the US
Because of the warm air from the Gulf of Mexico and the cold air from Canada, there is Tornado Alley
It includes Texas, Oklahoma, Kansas, Nebraska, and Iowa (and a very small area of South Dakota). See page 453 for a map.

26. Tornado Safety
The safest place to be is in a storm shelter or the basement of a well-built building
Stay away from windows and doors and avoid flying debris
Lie on the floor under a sturdy piece of furniture (such as a strong table)
Outside – lie flat in a ditch
Don’t take cover under an overpass or bridge (the forces are actually strengthened here)
There is usually little warning of a tornado – take shelter immediately if you hear of a warning

27. 20.3 Severe Storms Tornadoes  Tornado Intensity  Tornado Safety
• Because tornado winds cannot be measured directly, a rating on the Fujita scale is determined by assessing the worst damage produced by the storm.
 Tornado Safety
• Tornado watches alert people to the possibility of tornadoes in a specified area for a particular time.
• A tornado warning is issued when a tornado has actually been sighted in an area or is indicated by weather radar.

28. Fujita Tornado Intensity Scale
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30. Snowstorms

31. Snowstorms
All year, most precipitation begins in clouds as snow!! If the air is colder than 0° C all the way to the ground, the precipitation falls as snow.
A snowstorm has high winds blowing the snow and even picking up snow from the ground and blowing it back into the air
It is easy to become lost in a snowstorm because of poor visibility
Strong winds cool a person’s body rapidly

32. Snowstorm Safety Try to find shelter from the wind
Cover exposed parts of your body and try to stay dry
If in a car, only run the engine if the exhaust pipe is clear of snow

35. Hurricanes

37. Satellite View of Hurricane Floyd
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38. Hurricane
A tropical cyclone (remember from last week) that has winds of 119 km/h (kilometers per hour) or (74 mph)
A typical hurricane is about 600 km (373 miles) across
Hurricanes form in the Atlantic, Pacific, and Indian oceans
Called typhoons in the western Pacific Ocean
Called cyclones in the Indian Ocean

39. How Hurricanes Form
A hurricane begins over warm ocean water as a low-pressure area (called a tropical disturbance)
If the tropical disturbance grows in size and strength it becomes a tropical storm, then can become a hurricane
Hurricanes that strike the US usually form in the Atlantic Ocean north of the equator near Africa in August, September, or October

40. Formation Continued
Warm, humid air at the ocean’s surface rises and forms clouds
The rising air draws more warm, moist air into the system
Inside the storm are bands of very high winds and heavy rain
Winds spiral inward toward the area of lowest pressure at the center
The lower the air pressure at the center of a storm, the faster the winds blow toward the center
Hurricane winds may be as strong as 320 km/h (199 mph)

41. Structure of the Hurricane
The hurricane has bands of very high winds and heavy rains
The narrow band around the center of the storm has the strongest winds
At the center of the storm is a ring of clouds called the eyewall which encloses a quiet “eye”
The winds get stronger as the eye approaches
When the eye arrives, the weather suddenly changes to calm air and possibly clear skies
After the eye passes, the storm resumes, but the wind blows from the opposite direction

42. Cross Section of a Hurricane
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44. How Hurricanes Move
Hurricanes last much longer than other storms (usually a week or more)
They travel a far distance over the ocean before making “Landfall”
Hurricanes formed in the Atlantic Ocean are steered by trade winds toward the Caribbean Islands and the southeastern US (that is us!)
After a hurricane moves over land, it loses its supply of warm, humid air and gradually loses strenght
Even after losing strength, it can carry heavy rainfall for long distances over several days

45. Hurricane Damage
Hurricanes bring high waves, severe flooding, damaging winds, and even tornadoes as they come ashore
In the area of the hurricane while over the ocean, low pressure and high winds can raise water up to 6 meters (20 feet) above normal sea level
As the hurricane makes landfall, a huge dome of water called a storm surge sweeps across the coast where the hurricane lands
Storm surge is the most deadly part of a hurricane where it makes landfall
Storm surges can cause great damage, washing away beaches, destroying buildings along the coast, and eroding coastlines

46. Hurricane Safety
Prior to the 1950s people had little warning about hurricanes because of a lack of technology
Today, we watch hurricanes move across the ocean for days
Cities will issue hurricane warnings when hurricane conditions are expected with 24 hours
Citizens are often told to evacuate which means they should leave the area immediately

47. 20.3 Severe Storms Hurricanes  Hurricane Intensity
• The intensity of a hurricane is described using the Saffir-Simpson scale.
• A storm surge is the abnormal rise of the sea along a shore as a result of strong winds.

48. Saffir-Simpson Hurricane Scale
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50. Essential Questions Answered Chap 13 Sec 4
What are the main kinds of storms, and how do they form?
Thunderstorms – form when warm, humid air rises quickly
Tornadoes – form in cumulonimbus clouds
Hurricanes – form in low-pressure areas over the ocean
Snowstorms – form when it is really cold, windy, and snowing

51. Essential Questions Chap 13 Sec 4
What measures can you take to ensure safety in a storm?
Thunderstorm – stay inside, avoid water & electricity
Tornadoes – storm cellar, basement, avoid cars, windows, and doors
Hurricanes – evacuate
Snowstorms – find shelter, no exposed body parts

52. Section 5 – Predicting the Weather

53. Essential Questions (Ch 13 S 5)
How do weather forecasters predict the weather?

54. Collect Data
Collect data about clouds, air pressure, and other weather readings
Analyze the data to predict what will happen next
Meteorologists are scientists that study the causes of weather and try to predict it
They use maps, charts, and computers to analyze weather data and to prepare weather forecasts
Data is collected by local weather observers, instruments carried by balloons, satellites, and weather stations around the world
The National Weather Service owns much of the equipment and provides data to the meteorologists

55. You can collect data too:
Observe air pressure. Falling air pressure usually indicates an approaching low-pressure area, possibly bringing rain or snow
Observe clouds. Thin cirrus clouds high in the sky indicate a warm front may be approaching. Cumulus clouds growing taller and taller on a warm afternoon can become cumulonimbus clouds and produce thunderstorms
A red sky at sunrise might indicate an approaching low-pressure area and stormy weather
A red sky at sunset often indicates storms are moving away and clear weather is ahead

56. Weather Maps
The National Weather Service assembles weather maps using data from all over the country
Isobars are lines often found on weather maps. These lines show areas having the same air pressure. Iso means equal and bar means pressure. Measurement may be in inches or mercury, millibars, or both.
Isotherms show areas have the same temperature.
Weather maps have symbols to show fronts, areas of high and low pressure, types of precipitation, and temperatures
See the weather map on page 463

58. Limits of Weather Forecasts
Computers, satellites, and radar have made weather monitoring and predicting much better
However, we will probably never be able to predict the weather a month in advance because just a small change today can create big changes several days later

59. The Butterfly Effect
A scientist once suggested that even the flapping of a butterfly’s wings causes a tiny disturbance in the atmosphere. This tiny event might cause a larger disturbance that could grow into a large storm.

61. Essential Questions Answered (Ch 13 S 5)
How do weather forecasters predict the weather?
They make observations, collect weather data, and interpret the data to make their predictions

62. Review
What is the name of a scientist that studies and predicts weather?
Which type of storm develops over the ocean over warm, humid air?
Which type of storm usually develops inside a cumulonimbus cloud over land and has strong, swirling winds?
Which type of storm produces heavy rain, thunder, and lightning?