1. Understanding Weather
Chapter 16
Pages 422 – 449

2. Page 424
Water in the Air
Water can exist in the air as a solid, liquid, or gas.
Ice = a solid, found in clouds as snowflakes
Water droplets = Liquid water that exists in clouds
Water vapor = water in gaseous form
Weather = The condition of the atmosphere at a particular time & place.
The Water Cycle
Water cycle = the continuous movement of water from water sources, such as lakes & oceans, into the air, onto & over land, into the ground, & back to the water source
Rainbows are evidence that there is water in the air.

4. Pages 425 – 426
Humidity
Humidity = the amount of water vapor or moisture in the air.
As water evaporates, the humidity increases. But the air’s ability to hold water depends on air temperature. (warmer air can hold more water)
Relative Humidity
Relative humidity = the amount of moisture the air contains compared with the maximum amount it can hold at a particular temperature.
Calculate by: (Present humidity)/(saturated humidity at the given temperature)
Relative humidity is given as a percentage.
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Relative humidity is affected by how much vapor is in the air & by the temperature

5. Measuring Relative Humidity
Page 426
Measuring Relative Humidity
Psychrometer = an instrument used to measure relative humidity.
It has a wet-blub thermometer (measures the rate of evaporation) & a dry thermometer (measures the air temperature).
The larger the difference between the two readings indicates that there is less water vapor in the air and thus lower humidity.
If you want look up “How to Use a Sling Psychrometer”

6. The Process of Condensation
Page 427
The Process of Condensation
Condensation = the process by which a gas, such as water vapor, becomes a liquid. It occurs when saturated air cools.
Or it can happen if air cools to a point of saturation for the water vapor present in it.
Dew point = the temperature to which air must cool to be completely saturated.
It must also have a surface to condense on.
Before condensation can occur the air must become saturated.

7. Page 428
Clouds
Cloud = a collection of millions of tiny water droplets or ice crystals.
They form as warm air rises & cools. In order for water vapor to change physical state, it needs a surface on which to change.
Condensation nuclei – are small particles, such as dust, smoke, & salt, suspend in the air.
At higher temperatures, water vapor condenses on these small particles as water droplets.
At temperatures below freezing water vapor changes directly to a solid, forming ice crystals.

8. Kinds of Clouds Pages 428 – 429 There are three basic types of clouds
Cumulus clouds = puffy, white clouds that tend to have flat bottoms; form when warm air rises. Generally indicate fair weather. Cumulonimbus cloud – larger clouds that produce thunderstorms
Stratus clouds = layers of clouds; cover large areas; caused by a large body of air lifting into the atmosphere. Nimbostratus clouds – dark clouds that produce light-heavy rain. Fog = Near the ground
Cirrus clouds = thin, feathery, white clouds found at high altitudes; form in strong wind; indicate approaching bad weather if they thicken & lower
As a rule when –nimbus or nimbo- is part of a cloud’s name, it means that precipitation might fall from the cloud.
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Clouds are also classified by the altitude at which they form. NEXT SLIDE

9. The prefix cirro- is used to describe high clouds that form about 6,000 m. Because of the cold temperatures at high altitude, high clouds are made up of ice crystals.
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Middle clouds for between 2,000 m & 6,000 m. Middle clouds can be made up of both water droplets & ice cystals. The prefix alto- is used to describe clouds with this altitude range.
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Low clouds form below 2,000 m. These clouds are made up of water droplets. The prefix strato- is commonly used to describe these types of clouds.

10. Pages 430 – 431
Precipitation
Precipitation = water, in solid or liquid form, that falls from the air to the Earth.
Cloud droplets start out about the size of a period. They get larger by colliding & joining with other droplets. Eventually the droplets become too heavy to remain suspended & fall to the Earth.
Rain – liquid water that falls from the clouds.
Snow, Sleet, Hail
Snow – solid water that falls to the Earth.
Sleet (freezing rain) – forms when rain falls through a lay of freezing air.
Hail – solid water that falls as balls or lumps of ice.

11. Measuring Precipitation
Pages 430 – 431
Measuring Precipitation
Rain gauge – instrument used to measure the amount of rain that has fallen
Snow is measured by both depth & water content.
The snow’s depth is measured using a measuring stick.
The snow’s water content is determined by melting the snow & measuring the amount of water.
Dry snow = 20cm of snow yields 1 cm of water
Wet snow = 6 cm of snow yields 1 cm of water.

12. Air Masses & Fronts Page 432
Changes in weather are caused by the movement & interaction of air masses.
Air mass = a large body of air that has similar temperature & moisture throughout.
Air Masses
An air mass gets tis moisture & temperature characteristics from the area over which it forms. These areas are called source regions.
There are many types of air masses, each associated with a particular source region. Each of theses masses can be identified by its moisture & temperature characteristics.

13. Page 433
Cold Air Masses
There are 3 source regions that affect the US.
Canada – brings extremely cold weather in the winter & cool dry weather in the summer
North Pacific Ocean – brings rain & snow to the Pacific coast in the winter & cool, foggy weather in summer.
North Atlantic Ocean – produces cold, cloudy weather in winter, & cool weather with fog in summer.
North Pacific Ocean – very wet but not as cold as the weather coming down from Canada

14. Pages 433 – 434
Warm Air Masses
There are 4 warm air masses that influence the US
1Atlantic Ocean & 2Gulf of Mexico – In the summer these bring bring hot, humid weather, thunderstorms, & hurricanes. In the winter they bring mild, often cloudy weather.
3Pacific Ocean – has less moisture content & is weaker than the maritime polar mass = southern California has less rain than the rest of California.
4Desert region of Mexico & southwest US – only influences the weather in the summer brings dry, clear, & very hot weather.
Atlantic & Gulf of Mexico affect the East Coast & across the Midwest

16. Pages 434 – 435
Fronts
Air masses with different characteristics, such as temperatures & humidity, do not usually mix.
Front = a boundary where two different air masses meet
Weather at a front is usually cloudy & stormy. They occur in mild climates because both warm & cold air masses can be found there.
Google “weather fronts animation” an click on phschool.com

17. Cold front – forms when a cold air mass meets & displaces a warm air mass. Because the moving cold air is more dense, moves under the less dense warm air, pushing it up. Cold fronts can move fast, producing large cumulonimbus clouds with thunderstorms, heavy rain, or snow. Cooler weather usually follows a cold front because the warm air is pushed away from the Earth’s surface.
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Warm front – forms when a warm air mass meets & overrides a cold air mass. The warm, less-dense air moves over the cold, denser air. The warm air gradually replaces the cold air. Warm fronts generally bring nimbostratus clouds & drizzly precipitation. After the front passes, weather conditions are clear & warm.
Occluded front – forms when a faster-moving cold front overtakes a slower-moving warm front & forces the warm air up. The cold front then continues advancing until it meets a cold air mass that is warmer. The cold front then forces this air mass to rise. An occluded front has cool temperatures & large amounts of precipitation.
Stationary front – forms when a cold air mass meets a war air mass & little horizontal movement occurs. The weather associated with a stationary front is similar to that produced by a warm front.

18. Page 436
Severe Weather
Weather in the mid-latitudes can change from day to day. These changes result from the continual shifting of air masses.
Severe weather = weather that can cause property damage & even death.
Thunderstorms
Thunderstorms = are small, intense weather systems that produce strong winds, heavy rain, lightning, & thunder.
Two atmospheric conditions are required to produce thunderstorms: the air near the Earth’s surface must be warm & moist, & the atmosphere must be unstable.
Thunderstorms can occur along cold fronts.
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The atmosphere is unstable when the surrounding air is colder than the rising air mass. As long as the air surrounding the rising air mass is colder, the air mass will continue to rise.

19. Lightning Page 437 Thunderstorms are very active electronically.
Lightning = a large electrical discharge that occurs between two oppositely charged surfaces.
Thunder = the sound that results from the rapid expansion of air along the lightning strike.
Severe Thunderstorms
≈ 10% of thunderstorms are considered severe.
Severe thunderstorms produce one or more of the following conditions – high winds, hail, flash floods, & tornadoes.
Hailstorms damage crops, dent metal on cars, & break windows.
Sudden flash flooding due to heavy rains causes millions of dollars in property damage annually & is the biggest cause of weather-related deaths.
Lightning, which occurs will all thunderstorms, is responsible for thousands of forest fires each year in the US. It also kills or injures hundreds of people a year in the US.

20. Pages 438 – 439
Tornadoes
Tornadoes are produced in only 1% of all thunderstorms.
Tornado = a small, rotating column of air that has high wind speeds & low central pressure & that touches the ground.
The length of a tornado’s path of destruction can vary, but it is usually about 8 km long & m wide.
Tornadoes are capable of picking up heavy objects, such as houses, cars, & store signs, & hurling them through the air.
75% of the worlds tornadoes happen in the US.

21. Page 439
Hurricanes
Hurricane = a large, rotating tropical weather system with wind speeds of at least 119 km/h.
They generally form in the area between 5o & 20o north & south latitude over warm, tropical oceans.
Hurricanes vary in size from 160 km to 1,500 km in diameter, & they can travel for thousands of miles.
Hurricanes are the most powerful storms on Earth.
In the Pacific Ocean, they are called typhoons
In the Indian Ocean, they are called cyclones.

22. Formation of a Hurricane
Page 440
Formation of a Hurricane
A hurricane begins as a group of thunderstorms moving over tropical ocean waters. Winds traveling in two different directions collide, causing the storm to rotate over an area of low pressure.
Hurricanes get their energy from the condensation of water vapor. Once formed, the hurricane is fueled through contact with the warm ocean water.
As long as the hurricane is over warm water it continues to grow
Because of the Coriolis effect, the storms turns counterclockwise in the Northern Hemisphere & clockwise in the Southern Hemisphere.

23. Damage Caused by Hurricane
Page 441
Damage Caused by Hurricane
Hurricanes can cause a lot of damage when they move near or onto land.
Their winds knock down trees & telephone poles & can damage & destroy buildings & homes.
Most hurricane damage is done by heavy rains & the storm surge.
Depending on the strength of the hurricane, as storm surge can be 1 m to 8 m high.
Storm surge – a wall of water that builds up over the ocean due to the heavy winds & low atmospheric pressure.
The wall of water gets bigger & bigger as it nears the shore, reaching its greatest height when it crashes onto the shore.

24. Forecasting the Weather
Page 442
Forecasting the Weather
Weather forecast = a prediction of weather conditions over the next 3 to 5 days.
Weather Forecasting Technology
In order for meteorologists to accurately forecast the weather, they need to measure various atmospheric conditions, such as air pressure, humidity, precipitation, temperature, wind speed, & wind direction.
Measuring Air Temperature
Thermometer = a tool used to measure air temperature.
Toots used psychrometers (to measure relative humidity) rain gauge (used to measure precipitation)

25. Measuring Air Pressure
Pages 443 – 444
Measuring Air Pressure
Barometer = used to measure air pressure
Measuring Wind Direction & Wind Speed
Windsock or Wind vane = used to measure wind direction.
Anemometer = used to measure wind speed
Measuring Weather in the Upper Atmosphere
Weather balloons carry radiosondes to measure weather conditions 30 km above the Earth’s surface.
Radar is used to find the location, movement, & intensity of precipitation.
Weather satellites orbiting the Earth provide weather information that cannot be obtained from the ground.
radiosondes = measure temperature, air pressure, & relative humidity
Satellites take images of swirling clouds, it can measure wind speeds, humidity, & temperatures.

26. Pages 444 – 445
Weather Maps
Station model = a small circle, which shows the location of the weather station, with a set of symbols & numbers surrounding it, which represents the weather data.
isobars = lines that connect points of equal air pressure.