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.

4. 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.

5. 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

6. 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

7. 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.

8. Measuring Relative Humidity

9. 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.

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

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

12. Clouds

13. 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
Clouds with nimbus or nimbo will produce precipitation

14. 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.

15. 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.

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

17. 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

18. 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

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

20. 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.

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

22. 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 .

23. 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)

24. 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

25. Cold Air Masses
Cold air masses are responsible for bringing extremely cold winters.

26. Warm Air Masses
Warm air masses bring warm air weather systems into the United States.
This brings milder temperatures
This can also bring severe weather during the summer months.

27. 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

28. Cold Fronts A cold front forms where cold air moves under warm air
Move quickly and bring thunderstorms, heavy rain, or snow
Cooler weather follows a cold front

29. Warm Fronts
A warm front forms where warm air moves over colder denser air
Warm air replaces cold air
Bring drizzly rain
Followed by clear and warm weather

30. 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 and snow.

31. Stationary Fronts
Stationary fronts form when a cold air mass meets a warm air mass
Brings many days of cloudy wet weather.

32. Air Pressure and Weather
Areas that have lower pressure than the surrounding areas do are called Cyclones.
These areas the air masses come together and rise

33. Air Pressure and Weather
Areas that have high pressure are called anticyclones.
Anticyclones are areas where the air moves apart and sinks

34. Air Pressure and Weather
By keeping track of the cyclones and anticyclones, meteorologists can predict the weather.
Cyclones cause stormy weather
Anticyclones bring dry clear weather

35. Severe Weather Thunderstorms Tornadoes Hurricanes
Severe weather safety

36. 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

37. Thunderstorms
Cumulonimbus Clouds

38. 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

39. 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.

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

41. 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.

42. Tornadoes

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

44. Tornado Pictures

45. Tornado Pictures

46. Tornado Pictures

47. 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

48. 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.

49. 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.

50. Hurricane

51. 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).

52. 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

53. Weather Safety

54. 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

55. Weather Safety

56. 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

57. Weather-Forecasting Technology
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.

58. Weather-Forecasting Technology

59. Weather-Forecasting Technology
Measuring Air Temperature and Pressure
Thermometer: a tool used to measure air temperature
Barometer: an instrument used to measure air pressure
The mercury inside a barometer rises as the air pressure increases

60. 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

61. 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

62. Weather-Forecasting Technology

63. 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

64. Weather-Forecasting Technology

65. 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.

66. Reading Weather Maps