Water in the AirWater in the AirAir Masses and FrontsAir Masses and Fronts

Severe WeatherSevere WeatherForecasting WeatherForecasting Weather

weatherweather air massair mass

water cyclewater cycle frontfront

Humidity Humidity thunderstormthunderstorm

relative humidity relative humidity lightninglightning

Condensation Condensation thunderthunder

dew point dew point tornado tornado

cloudcloud hurricanehurricane

Precipitation Precipitation

thermometer

barometer

windsock

wind vane

anemometer

isobars

Water in the AirWater in the AirAlthough the Earth’s atmosphere contains only about 0.001 percent of the total volume of water on the planet (about 1.46 x 109 km3), it is an essential link between land masses and bodies of water on Earth.

The rate at which water evaporates into Earth’s atmosphere is about 5.1 x 1017 L per year.

About 78% of all precipitation falls over Earth’s oceans. Of the 22% that falls on land, about 65% returns to the air by evaporation.

Rainbows are evidence that air contains water. Water in the air affects weather and weather is the condition of the atmosphere at a particular time and place.

The Water CycleThe Water Cycle

The water cycle is the continuous movement of water from water sources, such as lakes and oceans, into the air, onto and over land, into the ground.

Physical Science Physical Science ConnectionConnection

At the Earth’s surface and in the At the Earth’s surface and in the atmosphere, energy from the sun moves atmosphere, energy from the sun moves water thru the water cycle. Sunlight water thru the water cycle. Sunlight provides the energy for evaporation, provides the energy for evaporation, transpiration, condensation and transpiration, condensation and precipitation.precipitation.

Life Science Life Science ConnectionConnection

When air is humid, hair becomes When air is humid, hair becomes frizzy. Hair is made of a protein frizzy. Hair is made of a protein called keratin. Each hair has a scaly called keratin. Each hair has a scaly outer cuticle, which allow moisture outer cuticle, which allow moisture to enter the inner part of the hair to enter the inner part of the hair fiber. Humidity can cause hair length fiber. Humidity can cause hair length to change as much as 2.5%. to change as much as 2.5%.

CondensationCondensationCondensation occurs when water vapor Condensation occurs when water vapor cools and changes back into liquid cools and changes back into liquid droplets. This is how clouds form. It is droplets. This is how clouds form. It is the process by which a gas, such as the process by which a gas, such as water vapor, becomes a liquid. Before water vapor, becomes a liquid. Before condensation can occur the air must be condensation can occur the air must be saturated.saturated.

EvaporationEvaporation

Evaporation occurs when liquid Evaporation occurs when liquid water changed into a water vapor, water changed into a water vapor, which is a gas.which is a gas.

increases

which is a measure of

which condenses to

form

which condenses to

form

which condenses to

form

UNDERSTANDING WEATHER

Evaporation

Relative Humidity

Water Vapor

CloudsDew Fog

TranspirationTranspiration

Transpiration is the process by Transpiration is the process by which plants release water vapor which plants release water vapor into the air through their leaves.into the air through their leaves.

PrecipitationPrecipitation

Precipitation occurs when rain, Precipitation occurs when rain, snow, sleet, or hail falls from the snow, sleet, or hail falls from the clouds onto the Earth’s surface.clouds onto the Earth’s surface.

4 Types of Precipitation4 Types of Precipitation

Rain ( measured by a rain Rain ( measured by a rain gauge)gauge)

SnowSnow SleetSleet HailHail

RunoffRunoff

Runoff is water, usually from Runoff is water, usually from precipitation, that flows across precipitation, that flows across land and collects in rivers, land and collects in rivers, streams, and eventually the ocean.streams, and eventually the ocean.

HumidityHumidity Humidity is the amount of water vapor or Humidity is the amount of water vapor or

moisture in the air. It is the moisture in moisture in the air. It is the moisture in the air that makes for bad hair days! Air’s the air that makes for bad hair days! Air’s ability to hold water vapor depends on ability to hold water vapor depends on the temperature. As temperature the temperature. As temperature increasesincreases, the air’s ability to hold water , the air’s ability to hold water also also increases. increases. Page 423, figure 3 shows Page 423, figure 3 shows the relationship between air temperature the relationship between air temperature and air’s ability to hold water.and air’s ability to hold water.

Relative HumidityRelative Humidity Relative humidity is the amount of Relative humidity is the amount of

moisture the air contains compared with moisture the air contains compared with the maximum amount it can hold. When the maximum amount it can hold. When air holds all the water it can at a given air holds all the water it can at a given temperature, the air is said to be temperature, the air is said to be saturated. Saturated air has a relative saturated. Saturated air has a relative humidity of 100%. A humidity of 100%. A psychrometer psychrometer measures relative humidity.measures relative humidity.

Math ConceptMath Concept Relative humidity is the amount of moisture the air Relative humidity is the amount of moisture the air

is holding compared with the amount it can hold at is holding compared with the amount it can hold at a particular temperature. The relative humidity of a particular temperature. The relative humidity of air that is holding all the water it can at a given air that is holding all the water it can at a given temperature is 100%, meaning it is saturated. You temperature is 100%, meaning it is saturated. You can calculate it with the following equation:can calculate it with the following equation:

(present) g/m(present) g/m33 x 100 = relative humidity x 100 = relative humidity

(saturated) g/m(saturated) g/m33

How does Humidity How does Humidity relate to the water cycle?relate to the water cycle?

Evaporation occurs when liquid Evaporation occurs when liquid water changes into water vapor and water changes into water vapor and returns to the air. Humidity is the returns to the air. Humidity is the amount of water vapor in the air.amount of water vapor in the air.

Dew PointDew Point

Dew point is the temperature to Dew point is the temperature to which air must cool to be completely which air must cool to be completely saturated. It must also have a saturated. It must also have a surface to condense on. Have you surface to condense on. Have you ever noticed the dew on the grass?ever noticed the dew on the grass?

CloudsClouds A cloud is a collection of millions of A cloud is a collection of millions of

tiny water droplets or ice crystals. tiny water droplets or ice crystals. They form as warm air rises and cools They form as warm air rises and cools becoming saturated. At saturation the becoming saturated. At saturation the water vapor becomes liquid.water vapor becomes liquid.

Cumulus CloudsCumulus Clouds

Cumulus CloudsCumulus Clouds

Cumulus clouds form when Cumulus clouds form when warm air rises. They tend to warm air rises. They tend to have flat bottoms are white have flat bottoms are white and puffyand puffy

Cumulus CloudsCumulus Clouds Larger cumulus clouds form Larger cumulus clouds form

thunderstorms. A cumulus cloud thunderstorms. A cumulus cloud that produces thunderstorms is that produces thunderstorms is called a called a cumulonimbus cloud. When cumulonimbus cloud. When –nimbus or nimbo- is used as part of –nimbus or nimbo- is used as part of a clouds name, it means that it a clouds name, it means that it might rain.might rain.

Stratus CloudsStratus Clouds

Stratus clouds form in layers and Stratus clouds form in layers and cover large areas. These clouds are cover large areas. These clouds are formed by a gentle lifting of warm air formed by a gentle lifting of warm air into the atmosphere. into the atmosphere.

Cirrus CloudsCirrus Clouds

Cirrus clouds are thin, feathery Cirrus clouds are thin, feathery white clouds found at high altitudes white clouds found at high altitudes and form when the wind is strong. and form when the wind is strong. They may indicate approaching bad They may indicate approaching bad weather if they thicken and lower in weather if they thicken and lower in altitude.altitude.

Cirrus CloudsCirrus Clouds

Cloud Types Based on Form and Cloud Types Based on Form and AltitudeAltitude

High Clouds- made of ice crystals and the High Clouds- made of ice crystals and the prefix prefix cirro- cirro- is used to describe high clouds.is used to describe high clouds.

Middle Clouds- can be made of both water Middle Clouds- can be made of both water droplets and ice crystals. The prefix droplets and ice crystals. The prefix alto-alto- is is used to describe middle clouds.used to describe middle clouds.

Low Clouds- made of water droplets. The Low Clouds- made of water droplets. The prefix prefix strato- strato- is used to describe low clouds.is used to describe low clouds.

Air Masses and FrontsAir Masses and Fronts

An air mass is a large body of air An air mass is a large body of air that has similar temperature and that has similar temperature and moisture throughout. It gets it moisture throughout. It gets it moisture and temperature moisture and temperature characteristics from the area in characteristics from the area in which it forms. See page 430, figure which it forms. See page 430, figure 14.14.

Warm Air MassesWarm Air Masses

Four warm air masses influence Four warm air masses influence the weather in the United States. the weather in the United States. As shown in figure 14.As shown in figure 14.

Cold Air MassCold Air Mass

Most of the cold winter weather Most of the cold winter weather in the United States is influenced in the United States is influenced by the three polar air masses. by the three polar air masses. The cP is dry and the 2 mP’s are The cP is dry and the 2 mP’s are moist.moist.

Cold and Warm FrontsCold and Warm Fronts

Occluded and Stationary Occluded and Stationary FrontsFronts

QuizQuiz1.1. If a cP air mass moves over Ohio in the If a cP air mass moves over Ohio in the

summer, what would the weather be summer, what would the weather be like?like?

2.2. Why does the cT air mass that forms Why does the cT air mass that forms over northern Mexico bring clear, dry, over northern Mexico bring clear, dry, hot weather?hot weather?

3.3. Explain how a cold front develops.Explain how a cold front develops.4.4. What kind of weather is associated with What kind of weather is associated with

a stationary front?a stationary front?

AnswersAnswers1.1. Cool and DryCool and Dry

2.2. It forms over the desert, which is hot and It forms over the desert, which is hot and contains little moisture.contains little moisture.

3.3. A cold air mass moves under a warm air A cold air mass moves under a warm air mass, forcing the warmer air upwards.mass, forcing the warmer air upwards.

4.4. Will be cloudy and rainy as long as the Will be cloudy and rainy as long as the front lies over an area. After the front front lies over an area. After the front passes, the weather will usually be clearpasses, the weather will usually be clear..

Severe WeatherSevere Weather

Severe WeatherSevere Weather is weather that is weather that can cause property damage and can cause property damage and even death. Examples of severe even death. Examples of severe weather include thunderstorms, weather include thunderstorms, tornadoes and hurricanes.tornadoes and hurricanes.

ThunderstormsThunderstorms

Thunderstorms are small, intense Thunderstorms are small, intense weather systems that produce strong weather systems that produce strong winds, heavy rain and lightning. They winds, heavy rain and lightning. They occur when warm, moist air rises rapidly occur when warm, moist air rises rapidly in an unstable atmosphere. A typical in an unstable atmosphere. A typical thunderstorm produces approximately thunderstorm produces approximately 470 million liters of water and enough 470 million liters of water and enough electricity to power the electricity to power the “ENTIRE”“ENTIRE” United United States for 20 minutes.States for 20 minutes.

Trees sometimes explode when Trees sometimes explode when struck by lightning.struck by lightning. Why? Lightning Why? Lightning causes the sap in the tree to causes the sap in the tree to vaporize (turn from a liquid to a gas).vaporize (turn from a liquid to a gas). The steam expands rapidly as it is The steam expands rapidly as it is heated, causing the tree to explode.heated, causing the tree to explode.

Lightning and ThunderLightning and ThunderLightningLightning is a large electrical discharge is a large electrical discharge

that occurs between two oppositely that occurs between two oppositely charged surfaces. When lightning charged surfaces. When lightning strikes, energy is released. The energy strikes, energy is released. The energy is transferred to the air and causes the is transferred to the air and causes the air to expand rapidly and send out air to expand rapidly and send out sound waves. sound waves. Thunder Thunder is the sound is the sound that results from the rapid expansion of that results from the rapid expansion of air along a lightning strike.air along a lightning strike.

Math & MoreMath & More To find the distance of a To find the distance of a

thunderstorm in kilometers, count thunderstorm in kilometers, count the seconds between a lightning the seconds between a lightning flash and the thunder and divide the flash and the thunder and divide the number by 3. If you see a lightning number by 3. If you see a lightning flash and then hear the thunder 21 flash and then hear the thunder 21 seconds later, how far away is the seconds later, how far away is the storm? storm? 9 kms? 7 kms?9 kms? 7 kms?

TornadoesTornadoes

Tornadoes are produced in only 1% of all Tornadoes are produced in only 1% of all thunderstorms. A tornado is a small, thunderstorms. A tornado is a small, rotating column of air that has high wind rotating column of air that has high wind speeds and low central pressure and speeds and low central pressure and touches the ground. A tornado starts out touches the ground. A tornado starts out as a small funnel cloud and hangs in the as a small funnel cloud and hangs in the air. It is called a tornado when it makes air. It is called a tornado when it makes contact with the Earth’s surface.contact with the Earth’s surface.

Tornadoes contd.Tornadoes contd.

About 75% of tornadoes occur in About 75% of tornadoes occur in the United States. The average the United States. The average tornado has wind speeds of 120-tornado has wind speeds of 120-180 km/hr, but rarer, more 180 km/hr, but rarer, more violent tornadoes have spinning violent tornadoes have spinning winds up to 500 km/hr.winds up to 500 km/hr.

HurricanesHurricanes Hurricanes are large, rotating tropical Hurricanes are large, rotating tropical

weather systems with wind speeds of weather systems with wind speeds of at least 119 km/hr. at least 119 km/hr. They are the They are the most powerful storms on Earth!most powerful storms on Earth! They have different names depending They have different names depending on where they form.on where they form. Western Pacific- Western Pacific- TyphoonsTyphoons. . Indian Ocean- Indian Ocean- Cyclones.Cyclones.

Hurricanes generally form in the area between 50 and 200 north and south latitude over warm, tropical oceans. At higher latitudes, the water is too cold for hurricanes to form. They vary in size from 160 km to 1500 km in diameter, and they can travel thousands of miles.

Hurricane FormationHurricane Formation A hurricane begins as a group of A hurricane begins as a group of

thunderstorms moving over tropical thunderstorms moving over tropical ocean waters. Winds traveling in two ocean waters. Winds traveling in two different directions collide, causing the different directions collide, causing the storm to rotate over the area of low storm to rotate over the area of low pressure. Because of the Coriolis effect, pressure. Because of the Coriolis effect, the storm turns counterclockwise in the the storm turns counterclockwise in the Northern Hemisphere and clockwise in Northern Hemisphere and clockwise in the Southern Hemisphere.the Southern Hemisphere.

Hurricane EnergyHurricane Energy Hurricanes get their energy from the Hurricanes get their energy from the

condensation of water vapor. Once condensation of water vapor. Once formed, the hurricane is fueled through formed, the hurricane is fueled through contact with the warm ocean waters. As contact with the warm ocean waters. As warm moist air rises, water vapor warm moist air rises, water vapor condenses, releasing large amounts of condenses, releasing large amounts of energy. As long as the hurricane is over energy. As long as the hurricane is over it’s source of warm water, it will continue it’s source of warm water, it will continue to grow. to grow.

Hurricane DamageHurricane Damage High winds cause a great deal of High winds cause a great deal of

damage but most damage is caused by damage but most damage is caused by flooding associated with the storm flooding associated with the storm surge. A surge. A storm surgestorm surge is a wall of water is a wall of water that builds up over the ocean due to that builds up over the ocean due to the heavy winds and low atmospheric the heavy winds and low atmospheric pressure. A storm surge can be 1-8 m pressure. A storm surge can be 1-8 m high and 65-160km long.high and 65-160km long.

Forecasting WeatherForecasting WeatherAir temperature is measured by a Air temperature is measured by a

thermometerthermometer..A A barometer barometer is used tois used to measure air measure air

pressure.pressure.Wind direction can be measure by using a Wind direction can be measure by using a

windsock or wind vane.windsock or wind vane.Wind speed is measured by an Wind speed is measured by an

anemometer.anemometer.

Eyes in the SkyEyes in the Sky Weather Balloons-Weather Balloons- carry electronic carry electronic

equipment that can measure conditions equipment that can measure conditions as high as 30 km.as high as 30 km.

Radar-Radar- is used to find the location, is used to find the location, movement and intensity of precipitation.movement and intensity of precipitation.

Weather Satellites-Weather Satellites- provide images of provide images of swirling clouds, and can measure wind swirling clouds, and can measure wind speeds, humidity, and temperatures at speeds, humidity, and temperatures at various altitudes.various altitudes.

Weather MapsWeather MapsMeteorologists base their forecasts on information gathered from many sources. In the United States, the National Weather Service (NWS) and the National Oceanic and Atmospheric Administration collect and analyze weather data. The NWS produces maps based on information from about 1000 weather stations across the country. This information is presented on a map as a station model.

IsobarsIsobars Isobars are lines on a weather map, Isobars are lines on a weather map,

similar to contour lines on a similar to contour lines on a topographical map. Isobars are lines topographical map. Isobars are lines that connect points of air pressure that connect points of air pressure rather that elevation. These areas rather that elevation. These areas are usually marked with an H or L. are usually marked with an H or L. Fronts are also labeled on weather Fronts are also labeled on weather maps. See page 443, figure 32.maps. See page 443, figure 32.