1. Understanding weather
Chapter 7 Notes

2. Water in the Air
Weather is the condition of the atmosphere at a certain time and place.
The condition is affected by the amount of water in the air.

3. The Water Cycle
Water is constantly being recycled through the water cycle.

5. Humidity
As water evaporates from lakes oceans and plants it becomes water vapor.
The amount of water vapor in the air is called humidity.
As the temperature increases, the amount of water vapor the air can hold increases.

6. Relative Humidity
Relative Humidity is the amount of water vapor in the air compared with the maximum amount of water vapor that the air can hold at a certain temperature

7. Factors that affect Relative Humidity
Amount of Water Vapor
Temperature
The more water vapor in the air the higher the relative humidity.
The water vapor drops the relative humidity drops.
The relative humidity decreases as the temperature rises and increases as the temperature drops

8. Measuring Relative Humidity
A psychrometer is an instrument that is used to measure relative humidity.
It consists of two thermometers: one wet bulb thermometer and one dry bulb thermometer.
The difference in the temperature readings between the two thermometers indicates the amount of humidity.

9. Measuring Relative Humidity

10. Dew Point
Dew point: the temperature at which a gas condenses into a liquid.
Air is saturated at dew point

11. Condensation
Condensation: the process by which a gas, such as water vapor, becomes a liquid.
Condensation occurs when saturated air (air full of humidity) cools.

12. Explain how water condenses on this glass.

13. How would you explain dew on grass in the morning?
The ice in the glass of water causes the air surrounding the glass to cool to its dew point. Water vapor then condenses on the outside of the glass.
How would you explain dew on grass in the morning?

15. Clouds
A cloud is a collection of millions of tiny water droplets or ice crystals.
Clouds form as warm air rises and cools ( condensation)

16. Clouds

18. Cumulus Clouds Puffy, white clouds that tend to have flat bottoms
Form when warm air rises
Indicate fair weather
When they get larger they produce thunderstorms – cumulonimbus clouds (AKA Thunderheads)
Clouds with nimbus or nimbo will produce precipitation

19. Cumulus Clouds
Cumulonimbus Cloud 

20. Stratus Clouds Stratus Clouds form in layers
Cover large areas and often block out the sun
Caused by a gentle lifting of a large body of air.
Nimbostratus clouds are dark stratus clouds that produce continuous rain.
Fog is a stratus cloud found near the ground.

21. Stratus Clouds

22. Cirrus Clouds
Cirrus Clouds are thin feathery white clouds found at high altitudes
Cirrus clouds form when the wind is strong
They can indicate a change in weather.

23. Cirrus Clouds

25. Clouds and Altitude
Clouds are classified by the altitude that they form

26. Cloud Classification Low clouds – have no prefix
Ex. Cumulus, Stratus
Middle clouds – Have the prefix alto
Ex. Altocumulus, Altostratus
High clouds – Have the prefix Cirro
Ex. Cirrostratus, Cirrus

27. 1 2 4 3 5
cirrus - Thin, wispy, curly-shaped clouds. cumulo-nimbus- Large, dense, towering clouds that cause rain and thunderstorms. cumulus - Puffy clouds. fog - Ground-hugging clouds. stratus - Layered, horizontal clouds with a flat base.

28. Precipitation
Precipitation is water in solid or liquid form that falls from the air to the Earth
There are four major forms
Rain
Snow
Sleet
Hail

29. Rain The most common form of precipitation.
Rain is produced when the water droplets in a cloud become a certain size.

30. Sleet and Snow
Sleet forms when rain falls through a layer of freezing air.
Snow forms when temperatures are so cold that water vapor changes directly to a solid.

31. Hail Hail are balls or lumps of ice that fall from clouds.
Hail forms in cumulonimbus clouds due to updrafts freezing the water droplets.

32. Match the correct definition with the correct term.
a.precipitation
b.condensation
c.weather
d.humidity
a.relative humidity
b.cloud
1.amount of water vapor in the air compared to the maximum it can hold
2.process in which water vapor cools and turns to liquid
3.collection of water droplets suspended in the air
4.amount of water vapor in the air
5.water that falls to the Earth’s surface as rain, snow, sleet, or hail
6.condition of the atmosphere at a certain time and place

33. 7. What role does runoff play in the water cycle?
a. It is the process in which liquid turns to water vapor.
b. It carries water from precipitation into oceans.
c. It takes water out of the water cycle.
D. It is not part of the water cycle.
8. What happens when air reaches its dew point?
a.Condensation occurs.
b. Relative humidity is 100%.
c. The humidity decreases.
D Clouds form.
9. How does sleet differ from snow?
a. It is not a form of precipitation.
b. It is liquid and not ice.
c. It starts as rain and freezes in the air.
d. It starts as water vapor and changes to a solid.

34. Air Masses and Fronts
Changes in weather are caused by the movement and interaction of air masses.
An air mass is a large body of air where temperature and moisture content are similar throughout .

35. Air Masses
Air Masses are characterized by their moisture content and temperature which is determined by the area over which the air mass forms (Source Region)

36. Types of air masses Maritime (m) Forms over water; wet
Continental ( c ) Forms over land; dry
Polar ( P) Forms over polar regions; cold
Tropical (T) develops over the tropics: warm

37. The first letter represents the moisture content
The first letter represents the moisture content. The second letter represents the temperature that is a characteristic of the air mass.

38. Cold Air Masses
Most of our cold weather comes from the three polar air masses.
If it forms over land it is cold and dry.
If is forms over the ocean it is cold and wet.
In the winter this means rain or snow.
In the summer it is cool and foggy.

39. Warm Air Masses
There are four warm air masses that influence weather in the U.S.
they bring hot, humid weather in the summer.
They can also produce hurricanes and thunderstorms.
In the winter they bring mild, cloudy weather.

40. Fronts
When two air masses meet, the air within them does not easily mix. That is to say that the air in one air mass will not easily mix with the air from another air mass. Instead, the air stays within its own air mass. Because of this phenomena, a border forms between two clashing air masses as they rub together. This border is called a front.

41. Fronts
The term ‘front’ was first used by Norwegian scientists who compared two air masses colliding to two armies as they clash on the battle front. Just like one army taking over another in a battle, one air mass ultimately takes over, pushing the other one out the away.
Fronts should not be thought of as a vertical wall. This is because they are actually sloped, like a hill. Often the slope of a front can be extremely gradual, taking hundreds of miles across the surface of the Earth to reach an altitude of just one mile.

42. Fronts
Front: the boundary between air masses of different densities and usually different temperatures
Four kinds of fronts
Cold front
Warm front
Occluded front
Stationary front

43. Cold Fronts
Cold fronts move faster than warm fronts. This causes the cold air mass to push the warm air mass up quickly.
The fast movement of a cold front brings thunderstorms, heavy rain, or snow followed by cooler weather.

44. Before Passing While Passing After Passing Winds south-southwest
from Meteorology Today by D. Ahrens
Before Passing
While Passing
After Passing
Winds
south-southwest
gusty; shifting
west-northwest
Temperature
warm
sudden drop
steadily dropping
Pressure
falling steadily
minimum, then sharp rise
rising steadily
Clouds
increasing: Ci, Cs and Cb Cb Cu
Precipitation
short period of showers
heavy rains, sometimes with hail, thunder and lightning
showers then clearing
Visibility
fair to poor in haze
poor, followed by improving
good, except in showers
Dew Point
high; remains steady
sharp drop
lowering

45. Warm Fronts
A warm front forms where warm air moves upward above colder denser air
The warm air gradually rises it cools and the water vapor condensates.
Warm fronts bring light rain followed by warmer weather

47. Occluded Front
An occluded front forms when a warm air mass is caught between two colder air masses
This produces cold temperatures and large amounts of rain or snow.

48. Stationary Fronts
Stationary fronts form when a cold air mass meets a warm air mass and neither displaces the other. This may happen because there is not enough wind to keep the air masses pushing against each other.
Brings many days of cloudy wet weather.

49. Locate the 4 types of fronts on this weather map.

51. Be a Weather Forecaster
You are planning to travel to Alabama in 2 days. The high temperature there for today is 68º F.
Use the map to help you predict whether the temperature in Alabama will increase, decrease, or stay the same. Explain why you think so.
Weather Fronts 52

52. Be a Weather Forecaster
There is a cold front approaching. The temperatures will probably be cooler behind the front. 53

53. Practice
1. What type of weather is associated with a continental polar air mass in the summer?
2. What is the area over which an air mass forms called?
3. Explain how a cold front develops.
4. What kind of weather is associated with a stationary front?

54. 1.Which of the following describes an air mass with the symbol
cT?
a. cold and wet c.warm and wet
b. cold and dry d.warm and dry
2.How does a warm front form?
a.Warm air becomes caught between cold air masses.
b.Two air masses meet and stay separated.
c.Warm air moves over cold air and replaces it.
d.Cold air moves under warm air and pushes it up.
3.What kind of weather would an occluded front likely bring?
a.sunny and warm c.thunderstorms
b. cold and much precipitation d.cold and dry

55. Air Pressure and Weather
Areas that have LOWER pressure than the surrounding areas do are called Cyclones. (AKA Low Pressure Systems)
These areas the air masses come together and RISE

56. Air Pressure and Weather
Areas that have HIGH pressure are called anticyclones. ( AKA High Pressure Systems)
Anticyclones are areas where the air moves apart and SINKS

57. Air Pressure and Weather
By keeping track of the low pressure (cyclones) and High Pressure (anticyclones), meteorologists can predict the weather.
Low pressure cause cloudy possibly stormy weather
High pressure brings dry clear weather

58. Air travels from areas of high pressure towards areas of low pressure.

59. a. They spiral toward the center and rise
.4. How do winds behave in a low pressure system?
a. They spiral toward the center and rise
b. They spiral out toward low pressure areas and sink
c. They are very calm.
d. They travel in two different directions.
5.Which of the following statements describes an anticyclone?
a. It is an area of low pressure.
b. It is an area of high pressure.
c. It has air masses that meet and rise.
d. It moves in the direction of the Earth’s rotation.
6.What kind of weather would an high pressure system likely bring?
a. stormy c. dry and clear
b. cool and wet d. changeable
7 .What kind of weather would an low pressure system likely bring?
a. cloudy or stormy c. dry and clear
b. hot and wet d. changeable

62. Severe Weather Thunderstorms Tornadoes Hurricanes
Severe weather safety

63. Thunderstorms
Thunderstorms are small intense weather systems that produce strong winds, heavy rain, lightning, and thunder.
Two atmospheric conditions are needed to produce a thunderstorm:
Warm and moist air near the Earth’s surface
Unstable atmosphere

64. Thunderstorms
Cumulonimbus Clouds

65. Lightning
Lightning is an electric discharge that occurs between a positively charged area and a negatively charged area.
Can occur
Between two clouds
Between Earth and a cloud
Between two parts of the same cloud

66. Lightning When lightning strikes, energy is released.
Energy is transferred to the air.
Thunder is the sound that results from the rapid expansion of air along the lightning strikes.

67. Severe Thunderstorms Can produce: High winds Hail Flash floods
Tornadoes
Lightning that causes forest fires and burning down homes

68. Tornadoes Tornadoes occur in 1% of all thunderstorms.
A tornado is a small spinning column of air that has high wind speeds and low central pressure and that touches the ground.
It starts as a funnel cloud that pokes through a cumulonimbus cloud.

69. Tornadoes

70. Tornadoes 75% of the world’s tornadoes occur in the United States.
Occur in spring and early summer
They average km/h wind speed

71. Tornado Pictures

72. Tornado Pictures

73. Tornado Pictures

74. Hurricanes
Hurricanes are large rotating tropical weather systems that have wind speed of at least 120km/h
They are the most powerful storms on Earth.
Pacific Ocean hurricanes are called: typhoons
Indian Ocean hurricanes are called: cyclones

75. Hurricane Hurricanes happen over warm tropical oceans.
Hurricanes vary in size from 160 to 1500 km in diameter and can travel for thousands of kilometers.

76. How Hurricanes form
Begins as a group of thunderstorms moving over tropical ocean waters
Winds traveling in different directions cause the storm to spin
It gets its fuel from the contact with the warm ocean waters
The hurricane continues to grow as long as it is over the moist warm source.

77. Hurricane

78. Hurricane Damage Very destructive Average wind speed is 120-150 km/h
Can knock down trees and destroy buildings
Flooding is the most destructive part.( storm surge).

79. Weather Safety Thunderstorm Safety Tornado Safety
Lightning is attracted to tall objects
Crouch down in open areas
Stay away from water
Weather forecast: watch and warning system
Watch- tornado may happen
Warning- tornado has been spotted
Find shelter: basement or cellar

80. Weather Safety

81. Weather Safety Flood Safety Hurricane Safety
Weather forecast: watches and warning system
Find a high place
Always stay out of flood waters
Watch weather updates
Evacuate the area
Have a disaster supply kit available with food and water
Board up windows with plywood
Stay indoors during the storm

82. Weather Safety

83. Forecasting the Weather
A weather forecast is a prediction of weather conditions over the next 3 to 5 days
A meteorologist is a person who observes and collects data on atmospheric conditions to make weather predictions

84. Weather-Forecasting Technology NOT ON NOTES YOU NEED TO WRITE
High in the sky
Weather balloons
Measure weather conditions as high as 30 km above Earth
Measures temperature, air pressure, and relative humidity
Meteorologists can also measure wind speed and direction by tracking the weather balloons.

85. Weather-Forecasting Technology

86. Weather-Forecasting Technology
Measuring Air Temperature
Thermometer: a tool used to measure air temperature

87. Measuring Air Pressure
Barometer: an instrument used to measure air pressure
The mercury inside a barometer rises as the air pressure increases
A rising barometer reading means higher pressure and possibly clear fair weather .
Decreasing barometer reading means a lower pressure and possibly cloudy or stormy weather.

89. Weather-Forecasting Technology
Measuring Wind Direction
Can be measured by using a windsock or wind vane
Windsock: is a cone shaped cloth bag open at both ends
Wind vane: is shaped like an arrow with a large tail and is attached to a pole

90. Wind Sock and Wind Vane

91. Weather-Forecasting Technology
Measuring wind speed
Anemometer: an instrument used to measure wind speed
Consists of four cups connected by spokes to a pole
The cups move
The motion sends an electric current that measures the wind speed

92. Anemometer

93. Weather-Forecasting Technology
Radar and Satellites
Radar is used to find the location movement and amount of precipitation
Doppler Radar systems on the weather station
Weather Satellites provide the Earth images of weather systems you see on TV weather reports

94. Weather-Forecasting Technology

95. Weather Maps
National Weather Service (NWS) and National Oceanic and Atmospheric Administration (NOAA) use information from about 1000 weather stations in the US
This is information from one weather station.

96. Reading Weather Maps

98. 1. Which statement best describes the driving force behind all the weather on Earth, from local weather systems to large-scale storms?
A.The Coriolis Effect curves the motion of wind due to the Earth's counter-clockwise rotation.
B.The Greenhouse Effect ensures that some of the Sun's heat is always trapped close to the Earth's surface.
C.The Sun heats the atmosphere unevenly, so temperatures and pressures are constantly equalizing.
D.The ocean holds the heat of the Sun more efficiently and for longer periods than land.
2.. Air masses move from areas with high pressure (such as the poles of the Earth) to areas with low pressure (the equator). The temperature and moisture of an air mass depend on where it is formed. A continental tropical air mass is _______ and _______.
A. warm, moist
B. cold, moist
C. cold, dry
D. warm, dry
3. The amount of water vapor in the air is referred to as
A.precipitation.
B.condensation.
C.humidity.
D.temperature.

99. 1. When warm air moves over cold air, a warm front forms
1. When warm air moves over cold air, a warm front forms. The warm air tends to rise along a gentle slope above the cold air and form layers of clouds. Given this information, which of the following cloud types and weather conditions would most likely result from an incoming warm front?
A.stratus clouds and drizzly rain
B.cumulonimbus clouds and thunderstorms
C.cumulus clouds and clear weather
D.isolated cirrus clouds and clear weather
2. Wispy, thin, white clouds can be seen high in the sky on a pleasant day. Which type of clouds are these?
A.stratus
B.cumulonimbus
C.cirrus
D.altostratus
3. Sharon woke up on a sunny morning and ate breakfast. Then she looked outside and saw tall, quickly-forming clouds. The clouds looked ready to rain. When she turned on the TV, she saw just what she thought—a forecast for sudden rains. What most likely caused the change in weather?
A.a warm front overtaking a cold air mass
B.lack of wind
C.a cold front overtaking a warm air mass
D.an expanding warm air mass

100. 1. If the collected information showed that the temperature was 25°C, there was little pressure, and the wind speed was 25 mph, what type of weather would it probably be outside?
A. warm and windy
B. warm and sunny
C. cold and wet
D. cold and windy
2. A swirling, high-speed windstorm begins over the ocean. It contains heavy rains. The wind and rain rotate around a center of low pressure. When the winds inside this tropical cyclone reach 74 mph, which of the following occurs?
A. hurricane
B. tropical depression
C. tropical storm
D. thunderstorm
3. A local barometer reading on a local weather forecast was 29.8 mm and falling. What type of weather conditions are most likely to occur?
A. clear and cool
B. clear and warm
C. fair skies
D. cloudy skies