Chapter 1Chapter 1

Monitoring The WeatherMonitoring The Weather

Weather StudiesWeather Studies Introduction to Atmospheric ScienceIntroduction to Atmospheric Science

American Meteorological SocietyAmerican Meteorological Society

Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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Case-in-PointCase-in-Point There have been There have been monumental advances monumental advances in monitoring of weatherin monitoring of weather– Case-in-point – the Case-in-point – the

blizzard of 1888blizzard of 1888– A similar storm in A similar storm in

December 2000December 2000 Still lacked data on sea Still lacked data on sea

surface temperatures – surface temperatures – overestimated snowfalloverestimated snowfall

– Today, though not exact, Today, though not exact, we are much better at we are much better at monitoring and monitoring and forecasting weatherforecasting weather SatellitesSatellites RadiosondesRadiosondes Surface observationsSurface observations

The Blizzard of 1888

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Driving QuestionDriving Question

What are some basic characteristics of the What are some basic characteristics of the atmosphere and weather?atmosphere and weather?– This chapter will introduce and describe some This chapter will introduce and describe some

of the tools and basic understandings that will of the tools and basic understandings that will guide your investigation of the atmosphereguide your investigation of the atmosphere

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Weather and ClimateWeather and Climate

Weather is the state of the atmosphere at some Weather is the state of the atmosphere at some place and timeplace and time– Described with quantitative variablesDescribed with quantitative variables

Temperature, humidity, cloudiness, precipitation, wind speed, Temperature, humidity, cloudiness, precipitation, wind speed, wind directionwind direction

– Meteorology is the study of the atmosphere and the Meteorology is the study of the atmosphere and the processes that cause weatherprocesses that cause weather

Climate is weather conditions at some locality Climate is weather conditions at some locality averaged over a specified time periodaveraged over a specified time period– Climate is an average of the weather, figured over the Climate is an average of the weather, figured over the

last 30-years and updated every decadelast 30-years and updated every decade– A locale’s climate also includes weather extremesA locale’s climate also includes weather extremes

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Sources of Weather InformationSources of Weather Information

TelevisionTelevision– The Weather ChannelThe Weather Channel

and local newscastsand local newscasts RadioRadio

– NOAA Weather RadioNOAA Weather Radio Continuous broadcasts Continuous broadcasts

repeated every 4 - 6 repeated every 4 - 6 minutesminutes

Interrupted with warningsInterrupted with warningsand watchesand watches

The InternetThe Internet

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Weather Systems and Weather MapsWeather Systems and Weather Maps Weather info received via TV, Weather info received via TV,

radio, or the Internet includesradio, or the Internet includes– Weather mapsWeather maps

NationalNational RegionalRegional

– Satellite/radar imagesSatellite/radar images– Data on current/past Data on current/past

conditionsconditions– Weather forecastsWeather forecasts

Short-term Short-term – 24 – 48 hours24 – 48 hours

Long-term Long-term – Up to 7 days or longerUp to 7 days or longer Automated Surface Observing

System (ASOS)

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Pressure SystemsPressure Systems2 Types2 Types

High PressureSystems, or“Anticyclones”

Low Pressure Systems, or“Cyclones”

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Pressure Systems, ContinuedPressure Systems, Continued High and low refer to air pressureHigh and low refer to air pressure

– High pressure area is relatively high compared to surrounding airHigh pressure area is relatively high compared to surrounding air– Low pressure area is relatively low compared to surrounding airLow pressure area is relatively low compared to surrounding air

HighsHighs– Fair weatherFair weather– Clockwise rotation of sinking air (in Northern Hemisphere)Clockwise rotation of sinking air (in Northern Hemisphere)– Generally track toward the east and southeastGenerally track toward the east and southeast

LowsLows– Stormy weatherStormy weather– Counterclockwise rotation of rising air (in Northern Hemisphere)Counterclockwise rotation of rising air (in Northern Hemisphere)– Generally track toward the east and northeastGenerally track toward the east and northeast– Lows tracking across the northern U.S. or southern Canada Lows tracking across the northern U.S. or southern Canada

produce less moisture than lows tracking across the southern U.S.produce less moisture than lows tracking across the southern U.S.– Weather to the west and north – usually coldWeather to the west and north – usually cold– Weather to the south and east – usually warmWeather to the south and east – usually warm

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Pressure Systems, ContinuedPressure Systems, ContinuedHigh and Low Pressure CentersHigh and Low Pressure Centers

Arrows indicate surface horizontal winds

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Air MassesAir Masses

Huge volume of air covering thousands of square Huge volume of air covering thousands of square kilometerskilometers

Horizontally relatively uniform in characteristicsHorizontally relatively uniform in characteristics– TemperatureTemperature– HumidityHumidity

Gathers characteristics from its source regionGathers characteristics from its source region– Cold, dry air masses form at higher latitudes over Cold, dry air masses form at higher latitudes over

continentscontinents– Cold, humid air masses form at higher latitudes over Cold, humid air masses form at higher latitudes over

maritime surfacesmaritime surfaces– Warm, dry air masses form over continents in subtropical Warm, dry air masses form over continents in subtropical

regionsregions– Warm, humid air masses form near the equator or in the Warm, humid air masses form near the equator or in the

subtropics over maritime surfacessubtropics over maritime surfaces

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FrontsFrontsWarm Front

© American Meteorological Society

Cold Front

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Characteristics of Air Masses & FrontsCharacteristics of Air Masses & Fronts Polar air masses are colder in winter, and milder in Polar air masses are colder in winter, and milder in

summersummer Tropical air masses have less seasonal variation, due to Tropical air masses have less seasonal variation, due to

nearly uniform sunlight duration and intensitynearly uniform sunlight duration and intensity Air masses modify as they move away from source regionAir masses modify as they move away from source region

– Starts taking on the characteristics of the area it is passing overStarts taking on the characteristics of the area it is passing over Fronts are 3-dimensionalFronts are 3-dimensional

– Where the front intersects the earth surface is shown with the front Where the front intersects the earth surface is shown with the front symbol on the mapsymbol on the map

Warm front clouds and precipitation occur over a wide Warm front clouds and precipitation occur over a wide band, and may occur over 1 to 2 daysband, and may occur over 1 to 2 days– Usually light precipitationUsually light precipitation

Cold front clouds and precipitation occur over a narrow Cold front clouds and precipitation occur over a narrow band, and may occur over minutes to hoursband, and may occur over minutes to hours– May be very heavy precipitationMay be very heavy precipitation

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Characteristics of Air Masses & Characteristics of Air Masses & Fronts, ContinuedFronts, Continued

Wind directions are different on the two sides of a Wind directions are different on the two sides of a frontfront

Some fronts have no clouds or precipitation.Some fronts have no clouds or precipitation.– Passage indicated by wind shift, and Passage indicated by wind shift, and

temperature/humidity changestemperature/humidity changes In summer, temperature can be nearly the same In summer, temperature can be nearly the same

on both sides of a cold fronton both sides of a cold front– Difference will be humidityDifference will be humidity

Fronts are anchored to lows on a weather map.Fronts are anchored to lows on a weather map.– Counterclockwise flow brings contrasting air masses Counterclockwise flow brings contrasting air masses

together to form frontstogether to form fronts Thunderstorms/severe weather often occur in the Thunderstorms/severe weather often occur in the

warm, humid air mass located between the cold warm, humid air mass located between the cold and warm frontand warm front

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Other Weather CharacteristicsOther Weather Characteristics In coastal areasIn coastal areas

– Lake/sea breezes may lower summer temperaturesLake/sea breezes may lower summer temperatures– May push 10-50 km (6 -30 miles) inland from the coast/shoreMay push 10-50 km (6 -30 miles) inland from the coast/shore

Heavy lake-effect snow on Great Lakes or Great Salt LakeHeavy lake-effect snow on Great Lakes or Great Salt Lake– Late fall and winterLate fall and winter– Downwind (eastern and southern shores)Downwind (eastern and southern shores)

Central U.S.Central U.S.– Most common location of tornadoes in the springMost common location of tornadoes in the spring

ThunderstormsThunderstorms– Common in FL, western High Plains and eastern slopes of the Common in FL, western High Plains and eastern slopes of the

RockiesRockies– Rare along Pacific Coast and in HIRare along Pacific Coast and in HI

Tropical storms and hurricanes impact Atlantic and Gulf Tropical storms and hurricanes impact Atlantic and Gulf coastscoasts– Mostly August through OctoberMostly August through October– Rare on West CoastRare on West Coast

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Describing the State of the AtmosphereDescribing the State of the AtmosphereWhat do Forecasters Tell Us?What do Forecasters Tell Us?

Maximum TemperatureMaximum Temperature– Usually occurs in early to mid-afternoonUsually occurs in early to mid-afternoon

Minimum temperatureMinimum temperature– Usually occurs around sunriseUsually occurs around sunrise

Dewpoint (frost point)Dewpoint (frost point)– The temperature at which air must be cooled at constant pressure The temperature at which air must be cooled at constant pressure

to become saturated with water vapor and for dew (or frost) to formto become saturated with water vapor and for dew (or frost) to form Relative humidityRelative humidity

– A percentage; the ratio of the actual concentration of the water A percentage; the ratio of the actual concentration of the water vapor component of air compared to the concentration the air vapor component of air compared to the concentration the air would have if saturated with water vaporwould have if saturated with water vapor Relative humidity will change throughout the day as the Relative humidity will change throughout the day as the

temperature variestemperature varies Generally highest around sunrise and lowest when warmestGenerally highest around sunrise and lowest when warmest

Precipitation amountsPrecipitation amounts– General rule – 10” of snow = 1” of precipitationGeneral rule – 10” of snow = 1” of precipitation

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Describing the State of the AtmosphereDescribing the State of the AtmosphereWhat do Forecasters Tell Us? (Continued)What do Forecasters Tell Us? (Continued)

Air PressureAir Pressure– And its tendency (rising or falling)And its tendency (rising or falling)– Falling may indicate approaching cold frontFalling may indicate approaching cold front

Wind direction and speedWind direction and speed– Wind direction is the direction wind is blowing fromWind direction is the direction wind is blowing from

Example; a west wind is blowing from the west, toward the eastExample; a west wind is blowing from the west, toward the east Sky coverSky cover

– Fraction of the sky covered in cloudsFraction of the sky covered in clouds NWS Weather watch – issued when hazardous NWS Weather watch – issued when hazardous

weather is considered possibleweather is considered possible NWS Weather warning – issued when hazardous NWS Weather warning – issued when hazardous

weather is imminent or actually taking placeweather is imminent or actually taking place

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Weather Satellite ImageryWeather Satellite Imagery

Two major types of satellite orbitsTwo major types of satellite orbits– GeostationaryGeostationary

High orbitsHigh orbits– 36,000 km (22,300 miles) high36,000 km (22,300 miles) high

Orbits planet at same rate as Earth’s rotation and in same Orbits planet at same rate as Earth’s rotation and in same eastward directioneastward direction

Currently 2 of these provide a complete view of much of N. Currently 2 of these provide a complete view of much of N. America and adjacent oceans to latitudes of about 60 degreesAmerica and adjacent oceans to latitudes of about 60 degrees

– Positioned over equator at 75Positioned over equator at 750 0 W longitude, 135W longitude, 1350 0 W longitudeW longitude

– Polar orbiting Polar orbiting Low orbitsLow orbits

– 800-1000 km (~500-600 miles) high800-1000 km (~500-600 miles) high

Provides overlapping north-south strips of imagesProvides overlapping north-south strips of images Passes over the same point twice every 24 hoursPasses over the same point twice every 24 hours

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Orbit of a Geostationary SatelliteOrbit of a Geostationary Satellite

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Orbit of a Orbit of a Polar-Polar-

Orbiting Orbiting SatelliteSatellite

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VisibleVisible– Black and Black and

white white photograph of photograph of the planetthe planet

– Only available Only available during during daylight hoursdaylight hours

– Highly Highly reflective reflective surfaces surfaces appear bright appear bright white and less white and less reflective reflective surfaces are surfaces are darkerdarker

Weather Satellite ImageryWeather Satellite Imagery

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InfraredInfrared– Available Available

anytime, not anytime, not just during just during daylightdaylight

– Provides Provides temperature temperature comparison of comparison of features within features within imageimage

– Whiter = Whiter = coldercolder Higher cloud Higher cloud

tops appear tops appear whiter, whiter, because they because they are colderare colder

Weather Satellite ImageryWeather Satellite Imagery

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Water vapor Water vapor imageryimagery– Enables Enables

tracking of tracking of plumes of plumes of moisturemoisture

– Shades of Shades of white = white = increasing increasing moisturemoisture

– Upper-level Upper-level clouds clouds appear milky appear milky to bright to bright whitewhite

Weather Satellite ImageryWeather Satellite Imagery

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Weather RadarWeather Radar Complements Complements

satellite satellite surveillancesurveillance

Doppler radar Doppler radar detects detects movementmovement– Excellent tool Excellent tool

to forecast to forecast tornadoestornadoes

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Combination of ImagesCombination of Images Composite of Composite of

IR and RadarIR and Radar

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Sky WatchingSky Watching You can determine much about the weather by You can determine much about the weather by

watching the skywatching the sky Clouds are aggregates of tiny water droplets, ice Clouds are aggregates of tiny water droplets, ice

crystals, or some combination of bothcrystals, or some combination of both– A cloud in contact with the ground is fogA cloud in contact with the ground is fog– Cloud forms:Cloud forms:

Stratiform clouds are sheet-like clouds formed in horizontal layersStratiform clouds are sheet-like clouds formed in horizontal layers– Form where air ascends gradually over a broad regionForm where air ascends gradually over a broad region

Cumuliform clouds are puffy, like cotton ballsCumuliform clouds are puffy, like cotton balls– More vigorous ascent of air over a smaller areaMore vigorous ascent of air over a smaller area– Under the right conditions can build vertically into a Under the right conditions can build vertically into a

cumulonimbus (thunderstorm) cloudcumulonimbus (thunderstorm) cloud The appearance of high, wispy, feather-like clouds (composed of The appearance of high, wispy, feather-like clouds (composed of

ice) in the western sky is often the first sign of an approaching ice) in the western sky is often the first sign of an approaching warm frontwarm front

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Cloud FormsCloud Forms These high thin cirrus clouds appear fibrous These high thin cirrus clouds appear fibrous

because they are composed of mostly tiny because they are composed of mostly tiny ice crystalsice crystals

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Cloud FormsCloud Forms These relatively low clouds are composed of These relatively low clouds are composed of

tiny water droplets and have more sharply tiny water droplets and have more sharply defined edges than ice-crystal cloudsdefined edges than ice-crystal clouds

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Cloud FormsCloud Forms Fair weather cumulus clouds are most Fair weather cumulus clouds are most

common during the warmest time of day and common during the warmest time of day and then vaporize after sunsetthen vaporize after sunset

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Cloud FormsCloud Forms Clouds of vertical developmentClouds of vertical development

– Merging and vertically-growing cumulus cloudsMerging and vertically-growing cumulus clouds Can become Can become CumulonimbusCumulonimbus clouds clouds

– Nimbo, nimbus prefix or suffix = rain producingNimbo, nimbus prefix or suffix = rain producing– These clouds always produce lightning and sometimes These clouds always produce lightning and sometimes

heavy rain, hail, or strong and gusty surface windsheavy rain, hail, or strong and gusty surface winds

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Cloud FormsCloud Forms Clouds may move in different directions at Clouds may move in different directions at

different altitudesdifferent altitudes– Indicates horizontal wind shifts with altitudeIndicates horizontal wind shifts with altitude

Essay: Weather Maps, Historical Perspective

U.S. Army Signal Corps weather map dated 1 September 1872