Understanding weather Chapter 7 Notes. Water in the Air Weather is the condition of the atmosphere at a certain time and place. Weather is the condition

Water in the AirWater in the Air

Weather is the condition of the atmosphere Weather is the condition of the atmosphere at a certain time and place.at a certain time and place.

The condition is affected by the amount of The condition is affected by the amount of water in the air.water in the air.

The Water CycleThe Water Cycle Water is Water is

constantly constantly being being recycled recycled through through the water the water cycle.cycle.

HumidityHumidity

As water evaporates As water evaporates from lakes oceans and from lakes oceans and plants it becomes plants it becomes water vapor. water vapor.

The amount of water The amount of water vapor in the air is vapor in the air is called humiditycalled humidity..

As the temperature increases, the amount of water vapor the air can hold increases.

Relative HumidityRelative Humidity

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

Factors that affect Relative Factors that affect Relative HumidityHumidity

Amount of Water VaporAmount of Water Vapor The more water vapor in The more water vapor in

the air the higher the the air the higher the relative humidity.relative humidity.

The water vapor drops The water vapor drops the relative humidity the relative humidity drops.drops.

TemperatureTemperature The relative humidity The relative humidity

decreases as the decreases as the temperature risestemperature rises and and increases as the increases as the temperature dropstemperature drops

Measuring Relative HumidityMeasuring Relative Humidity

A psychrometer is an instrument that is A psychrometer is an instrument that is used to measure relative humidityused to measure relative humidity..– It consists of two thermometersIt consists of two thermometers: one wet bulb : one wet bulb

thermometer and one dry bulb thermometer.thermometer and one dry bulb thermometer.– The difference in the temperature readings The difference in the temperature readings

between the two thermometers indicates the between the two thermometers indicates the amount of humidity.amount of humidity.

Measuring Relative HumidityMeasuring Relative Humidity

CondensationCondensation

Condensation: the Condensation: the process by which a process by which a gas, such as water gas, such as water vapor, becomes a vapor, becomes a liquid.liquid.

Condensation occurs Condensation occurs when saturated air (air when saturated air (air full of humidity) cools.full of humidity) cools.

Dew PointDew Point

Dew point: the temperature at which a gas Dew point: the temperature at which a gas condenses into a liquid.condenses into a liquid.

Air is saturated at dew pointAir is saturated at dew point

CloudsClouds

A cloud is a collection of millions of tiny A cloud is a collection of millions of tiny water droplets or ice crystals.water droplets or ice crystals.

Clouds form as warm air rises and coolsClouds form as warm air rises and cools

CloudsClouds

Cumulus CloudsCumulus Clouds Puffy, white clouds that tend to have flat Puffy, white clouds that tend to have flat

bottomsbottoms Form when warm air risesForm when warm air rises Indicate fair weatherIndicate fair weather When they get larger they produce When they get larger they produce

thunderstorms – cumulonimbus cloudsthunderstorms – cumulonimbus clouds Clouds with nimbus or nimbo will produce Clouds with nimbus or nimbo will produce

precipitationprecipitation

Stratus CloudsStratus Clouds

Stratus Clouds form in layersStratus Clouds form in layers Cover large areas and often block out the sunCover large areas and often block out the sun Caused by a gentle lifting of a large body of Caused by a gentle lifting of a large body of

air.air. Nimbostratus clouds are dark stratus clouds Nimbostratus clouds are dark stratus clouds

that produce continuous rain.that produce continuous rain. Fog is a stratus cloud found near theFog is a stratus cloud found near the ground. ground.

Cirrus CloudsCirrus Clouds

Cirrus Clouds are thin feathery white clouds Cirrus Clouds are thin feathery white clouds found at high altitudes found at high altitudes

Cirrus clouds form when the wind is strongCirrus clouds form when the wind is strong They can indicate a change in weather.They can indicate a change in weather.

Clouds and AltitudeClouds and Altitude

Clouds Clouds are are classified classified by the by the altitude altitude that they that they formform

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

PrecipitationPrecipitation

Precipitation is water in solid or liquid form Precipitation is water in solid or liquid form that falls from the air to the Earth that falls from the air to the Earth

There are four major formsThere are four major forms– RainRain– SnowSnow– SleetSleet– HailHail

RainRain

The most common form of precipitation. The most common form of precipitation. Rain is produced when the water droplets Rain is produced when the water droplets

become a certain size.become a certain size.

Sleet and SnowSleet and Snow

Sleet forms when rain falls through a layer Sleet forms when rain falls through a layer of freezing air. of freezing air.

Snow forms when temperatures are so cold Snow forms when temperatures are so cold that water vapor changes directly to a solid.that water vapor changes directly to a solid.

HailHail

Hail are balls or lumps of ice that fall from Hail are balls or lumps of ice that fall from clouds.clouds.

Hail forms in cumulonimbus cloudsHail forms in cumulonimbus clouds due to due to updrafts freezing the water droplets.updrafts freezing the water droplets.

Air Masses and FrontsAir Masses and Fronts

Changes in weather are caused by the Changes in weather are caused by the movement and interaction of air masses.movement and interaction of air masses.

An air mass is a large body of air where An air mass is a large body of air where temperature and moisture content are temperature and moisture content are similar throughout .similar throughout .

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

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

Cold Air MassesCold Air Masses

Cold air masses are Cold air masses are responsible for responsible for bringing extremely cold bringing extremely cold winters. winters.

Warm Air MassesWarm Air Masses

Warm air masses bring Warm air masses bring warm air weather warm air weather systems into the systems into the United States.United States.

This brings milder This brings milder temperatures temperatures

This can also bring This can also bring severe weather during severe weather during the summer months.the summer months.

Fronts Fronts

Front: the boundary between air masses of Front: the boundary between air masses of different densities and usually different different densities and usually different temperaturestemperatures

Four kinds of frontsFour kinds of fronts– Cold frontCold front– Warm frontWarm front– Occluded frontOccluded front– Stationary frontStationary front

Cold FrontsCold Fronts

A cold front forms A cold front forms where cold air moves where cold air moves under warm air under warm air

Move quickly and bring Move quickly and bring thunderstorms, heavy thunderstorms, heavy rain, or snowrain, or snow

Cooler weather follows Cooler weather follows a cold fronta cold front

Warm FrontsWarm Fronts

A warm front forms A warm front forms where warm air moves where warm air moves over colder denser air over colder denser air

Warm air replaces cold Warm air replaces cold airair

Bring drizzly rainBring drizzly rain Followed by clear and Followed by clear and

warm weatherwarm weather

Occluded FrontOccluded Front

An occluded front An occluded front forms when a warm air forms when a warm air mass is caught mass is caught between two colder air between two colder air massesmasses

This produces cold This produces cold temperatures and temperatures and large amounts of rain large amounts of rain and snow.and snow.

Stationary FrontsStationary Fronts

Stationary fronts form Stationary fronts form when a cold air mass when a cold air mass meets a warm air meets a warm air mass mass

Brings many days of Brings many days of cloudy wet weather.cloudy wet weather.

Air Pressure and WeatherAir Pressure and Weather

Areas that have lower pressure than the Areas that have lower pressure than the surrounding areas do are called Cyclones.surrounding areas do are called Cyclones.

These areas the air masses come together These areas the air masses come together and riseand rise


Areas that have high pressure are called Areas that have high pressure are called anticyclones.anticyclones.

Anticyclones are areas where the air moves Anticyclones are areas where the air moves apart and sinksapart and sinks


By keeping track of the cyclones and By keeping track of the cyclones and anticyclones, meteorologists can predict the anticyclones, meteorologists can predict the weather.weather.

Cyclones cause stormy weatherCyclones cause stormy weather Anticyclones bring dry clear weatherAnticyclones bring dry clear weather

Severe WeatherSevere Weather

ThunderstormsThunderstorms TornadoesTornadoes HurricanesHurricanes Severe weather safetySevere weather safety

ThunderstormsThunderstorms

Thunderstorms are small intense weather Thunderstorms are small intense weather systems that produce strong winds, heavy systems that produce strong winds, heavy rain, lightning, and thunder.rain, lightning, and thunder.

Two atmospheric conditions are needed to Two atmospheric conditions are needed to produce a thunderstorm:produce a thunderstorm:– Warm and moist air near the Earth’s surfaceWarm and moist air near the Earth’s surface– Unstable atmosphereUnstable atmosphere

ThunderstormsThunderstormsCumulonimbus Clouds

LightningLightning Lightning is an electric Lightning is an electric

discharge that occurs discharge that occurs between a positively between a positively charged area and a charged area and a negatively charged area.negatively charged area.

Can occurCan occur– Between two cloudsBetween two clouds

– Between Earth and a Between Earth and a cloudcloud

– Between two parts of the Between two parts of the same cloudsame cloud

Lightning Lightning

When lightning strikes, energy is released.When lightning strikes, energy is released. Energy is transferred to the air.Energy is transferred to the air. Thunder is the sound that results from the Thunder is the sound that results from the

rapid expansion of air along the lightning rapid expansion of air along the lightning strikes.strikes.

Severe ThunderstormsSevere Thunderstorms

Can produce:Can produce:– High windsHigh winds– Hail Hail – Flash floodsFlash floods– TornadoesTornadoes– Lightning that causes Lightning that causes

forest fires and burning forest fires and burning down homesdown homes

TornadoesTornadoes

Tornadoes occur in 1% of all thunderstorms.Tornadoes occur in 1% of all thunderstorms. A tornado is a small spinning column of air A tornado is a small spinning column of air

that has high wind speeds and low central that has high wind speeds and low central pressure and that touches the ground.pressure and that touches the ground.

It starts as a funnel cloud that pokes through It starts as a funnel cloud that pokes through a cumulonimbus cloud.a cumulonimbus cloud.

TornadoesTornadoes

TornadoesTornadoes

75% of the world’s tornadoes occur in the 75% of the world’s tornadoes occur in the United States.United States.

Occur in spring and early summerOccur in spring and early summer They average 120-180 km/h wind speedThey average 120-180 km/h wind speed

Tornado PicturesTornado Pictures



HurricanesHurricanes

Hurricanes are large rotating tropical Hurricanes are large rotating tropical weather systems that have wind speed of at weather systems that have wind speed of at least 120km/h least 120km/h

They are the most powerful storms on Earth.They are the most powerful storms on Earth. Pacific Ocean hurricanes are called: Pacific Ocean hurricanes are called:

typhoonstyphoons Indian Ocean hurricanes are called: Indian Ocean hurricanes are called:

cyclonescyclones

HurricaneHurricane

Hurricanes happen Hurricanes happen over warm tropical over warm tropical oceans.oceans.

Hurricanes vary in size Hurricanes vary in size from 160 to 1500 km in from 160 to 1500 km in diameter and can diameter and can travel for thousands of travel for thousands of kilometers.kilometers.

How Hurricanes formHow Hurricanes form

Begins as a group of thunderstorms moving Begins as a group of thunderstorms moving over tropical ocean watersover tropical ocean waters

Winds traveling in different directions cause Winds traveling in different directions cause the storm to spinthe storm to spin

It gets its fuel from the contact with the It gets its fuel from the contact with the warm ocean waterswarm ocean waters

The hurricane continues to grow as long as The hurricane continues to grow as long as it is over the moist warm source.it is over the moist warm source.

Hurricane Hurricane

Hurricane DamageHurricane Damage

Very destructiveVery destructive Average wind speed is Average wind speed is

120-150 km/h 120-150 km/h Can knock down trees Can knock down trees

and destroy buildingsand destroy buildings Flooding is the most Flooding is the most

destructive part.( storm destructive part.( storm surge).surge).

Weather SafetyWeather Safety

Thunderstorm SafetyThunderstorm Safety Lightning is attracted to Lightning is attracted to

tall objectstall objects Crouch down in open Crouch down in open

areasareas Stay away from waterStay away from water

Tornado SafetyTornado Safety Weather forecast: watch Weather forecast: watch

and warning systemand warning system Watch- tornado may Watch- tornado may

happenhappen Warning- tornado has Warning- tornado has

been spottedbeen spotted Find shelter: basement or Find shelter: basement or

cellarcellar



Flood SafetyFlood Safety Weather forecast: Weather forecast:

watches and warning watches and warning systemsystem

Find a high placeFind a high place Always stay out of flood Always stay out of flood

waterswaters

Hurricane SafetyHurricane Safety Watch weather updatesWatch weather updates Evacuate the areaEvacuate the area Have a disaster supply kit Have a disaster supply kit

available with food and available with food and waterwater

Board up windows with Board up windows with plywoodplywood

Stay indoors during the Stay indoors during the stormstorm


Forecasting the WeatherForecasting the Weather

A weather forecast is a prediction of weather A weather forecast is a prediction of weather conditions over the next 3 to 5 daysconditions over the next 3 to 5 days

A meteorologist is a person who observes A meteorologist is a person who observes and collects data on atmospheric conditions and collects data on atmospheric conditions to make weather predictionsto make weather predictions

Weather-Forecasting TechnologyWeather-Forecasting Technology

High in the skyHigh in the sky– Weather balloons Weather balloons

Measure weather conditions as high as 30 km above Measure weather conditions as high as 30 km above EarthEarth

Measures temperature, air pressure, and relative Measures temperature, air pressure, and relative humidityhumidity

Meteorologists can also measure wind speed and Meteorologists can also measure wind speed and direction by tracking the weather balloons.direction by tracking the weather balloons.



Measuring Air Temperature and PressureMeasuring Air Temperature and Pressure– Thermometer: a tool used to measure air Thermometer: a tool used to measure air

temperaturetemperature– Barometer: an instrument used to measure air Barometer: an instrument used to measure air

pressurepressure The mercury inside a barometer rises as the air The mercury inside a barometer rises as the air

pressure increasespressure increases


Measuring Wind DirectionMeasuring Wind Direction– Can be measured by using a windsock or wind Can be measured by using a windsock or wind

vanevane Windsock: is a cone shaped cloth bag open at both Windsock: is a cone shaped cloth bag open at both

endsends Wind vane: is shaped like an arrow with a large tail Wind vane: is shaped like an arrow with a large tail

and is attached to a poleand is attached to a pole


Measuring wind speedMeasuring wind speed– Anemometer: an instrument used to measure Anemometer: an instrument used to measure

wind speedwind speed Consists of four cups connected by spokes to a poleConsists of four cups connected by spokes to a pole The cups move The cups move The motion sends an electric current that measures The motion sends an electric current that measures

the wind speedthe wind speed



Radar and SatellitesRadar and Satellites– Radar is used to find the location movement Radar is used to find the location movement

and amount of precipitation and amount of precipitation – Doppler Radar systems on the weather stationDoppler Radar systems on the weather station– Weather Satellites provide the Earth images of Weather Satellites provide the Earth images of

weather systems you see on TV weather weather systems you see on TV weather reportsreports


Weather MapsWeather Maps

National Weather Service (NWS) and National Weather Service (NWS) and National Oceanic and Atmospheric National Oceanic and Atmospheric Administration (NOAA) use information from Administration (NOAA) use information from about 1000 weather stations in the USabout 1000 weather stations in the US

This is information from one weather station.This is information from one weather station.

Reading Weather MapsReading Weather Maps

Documents

Understanding weather Chapter 7 Notes. Water in the Air Weather is the condition of the atmosphere at a certain time and place. Weather is the condition