Chapter 1

Monitoring The Weather

Weather StudiesIntroduction to Atmospheric Science

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

blizzard of 1888

– A similar storm in December 2000 Still lacked data on sea

surface temperatures –overestimated snowfall

– Today, though not exact, we are much better at monitoring and forecasting weather Satellites

Radiosondes

Surface observationsThe Blizzard of 1888

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Driving Question

What are some basic characteristics of the

atmosphere and weather?

– This chapter will introduce and describe some

of the tools and basic understandings that will

guide your investigation of the atmosphere

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Weather and Climate

Weather is the state of the atmosphere at some place and time

– Described with quantitative variables Temperature, humidity, cloudiness, precipitation, wind speed,

wind direction

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

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

last 30-years and updated every decade

– A locale’s climate also includes weather extremes

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Sources of Weather Information

Television

– The Weather Channel and local newscasts

Radio– NOAA Weather Radio

Continuous broadcasts repeated every 4 - 6 minutes

Interrupted with warnings

and watches

The Internet

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Weather Systems and Weather Maps

Weather info received via TV,

radio, or the Internet includes

– Weather maps

National

Regional

– Satellite/radar images

– Data on current/past

conditions

– Weather forecasts

Short-term

– 24 – 48 hours

Long-term

– Up to 7 days or longer Automated Surface Observing

System (ASOS)

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Pressure Systems2 Types

High Pressure

Systems, or

“Anticyclones”

Low Pressure

Systems, or

“Cyclones”

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Pressure Systems, Continued High and low refer to air pressure

– High pressure area is relatively high compared to surrounding air

– Low pressure area is relatively low compared to surrounding air

Highs– Fair weather

– Clockwise rotation of sinking air (in Northern Hemisphere)

– Generally track toward the east and southeast

Lows– Stormy weather

– Counterclockwise rotation of rising air (in Northern Hemisphere)

– Generally track toward the east and northeast

– Lows tracking across the northern U.S. or southern Canada produce less moisture than lows tracking across the southern U.S.

– Weather to the west and north – usually cold

– Weather to the south and east – usually warm

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Pressure Systems, ContinuedHigh and Low Pressure Centers

Arrows indicate surface horizontal winds

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Air Masses

Huge volume of air covering thousands of square kilometers

Horizontally relatively uniform in characteristics– Temperature

– Humidity

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

continents

– Cold, humid air masses form at higher latitudes over maritime surfaces

– Warm, dry air masses form over continents in subtropical regions

– Warm, humid air masses form near the equator or in the subtropics over maritime surfaces

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FrontsWarm Front

© American Meteorological Society

Cold Front

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

summer

Tropical air masses have less seasonal variation, due to nearly uniform sunlight duration and intensity

Air masses modify as they move away from source region– Starts taking on the characteristics of the area it is passing over

Fronts are 3-dimensional– Where the front intersects the earth surface is shown with the front

symbol on the map

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

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

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Characteristics of Air Masses &

Fronts, Continued Wind directions are different on the two sides of a

front

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

temperature/humidity changes

In summer, temperature can be nearly the same on both sides of a cold front– Difference will be humidity

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

together to form fronts

Thunderstorms/severe weather often occur in the warm, humid air mass located between the cold and warm front

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Other Weather Characteristics In coastal areas

– Lake/sea breezes may lower summer temperatures

– May push 10-50 km (6 -30 miles) inland from the coast/shore

Heavy lake-effect snow on Great Lakes or Great Salt Lake– Late fall and winter

– Downwind (eastern and southern shores)

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

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

Rockies

– Rare along Pacific Coast and in HI

Tropical storms and hurricanes impact Atlantic and Gulf coasts– Mostly August through October

– Rare on West Coast

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Describing the State of the AtmosphereWhat do Forecasters Tell Us?

Maximum Temperature– Usually occurs in early to mid-afternoon

Minimum temperature– Usually occurs around sunrise

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

to become saturated with water vapor and for dew (or frost) to form

Relative humidity– A percentage; the ratio of the actual concentration of the water

vapor component of air compared to the concentration the air would have if saturated with water vapor

Relative humidity will change throughout the day as the temperature varies

Generally highest around sunrise and lowest when warmest

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

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

Air Pressure– And its tendency (rising or falling)

– Falling may indicate approaching cold front

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

Example; a west wind is blowing from the west, toward the east

Sky cover– Fraction of the sky covered in clouds

NWS Weather watch – issued when hazardous weather is considered possible

NWS Weather warning – issued when hazardous weather is imminent or actually taking place

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Weather Satellite Imagery

Two major types of satellite orbits

– Geostationary

High orbits

– 36,000 km (22,300 miles) high

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

eastward direction

Currently 2 of these provide a complete view of much of N.

America and adjacent oceans to latitudes of about 60 degrees

– Positioned over equator at 750 W longitude, 1350 W longitude

– Polar orbiting

Low orbits

– 800-1000 km (~500-600 miles) high

Provides overlapping north-south strips of images

Passes over the same point twice every 24 hours

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Orbit of a Geostationary Satellite

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Orbit of a

Polar-

Orbiting

Satellite

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Visible– Black and

white photograph of the planet

– Only available during daylight hours

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

Weather Satellite Imagery

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Infrared– Available

anytime, not just during daylight

– Provides temperature comparison of features within image

– Whiter = colder Higher cloud

tops appear whiter, because they are colder

Weather Satellite Imagery

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Water vapor imagery

– Enables tracking of plumes of moisture

– Shades of white = increasing moisture

– Upper-level clouds appear milky to bright white

Weather Satellite Imagery

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Weather Radar Complements

satellite

surveillance

Doppler radar

detects

movement

– Excellent tool

to forecast

tornadoes

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Combination of Images Composite of

IR and Radar

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Sky Watching You can determine much about the weather by

watching the sky

Clouds are aggregates of tiny water droplets, ice

crystals, or some combination of both

– A cloud in contact with the ground is fog

– Cloud forms:

Stratiform clouds are sheet-like clouds formed in horizontal layers

– Form where air ascends gradually over a broad region

Cumuliform clouds are puffy, like cotton balls

– More vigorous ascent of air over a smaller area

– Under the right conditions can build vertically into a

cumulonimbus (thunderstorm) cloud

The appearance of high, wispy, feather-like clouds (composed of

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

warm front

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Cloud Forms These high thin cirrus clouds appear fibrous

because they are composed of mostly tiny

ice crystals

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Cloud Forms These relatively low clouds are composed of

tiny water droplets and have more sharply

defined edges than ice-crystal clouds

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Cloud Forms Fair weather cumulus clouds are most

common during the warmest time of day and

then vaporize after sunset

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Cloud Forms Clouds of vertical development

– Merging and vertically-growing cumulus clouds

Can become Cumulonimbus clouds

– Nimbo, nimbus prefix or suffix = rain producing

– These clouds always produce lightning and sometimes

heavy rain, hail, or strong and gusty surface winds

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Cloud Forms

Clouds may move in different directions at

different altitudes

– Indicates horizontal wind shifts with altitude

Essay: Weather Maps, Historical

Perspective

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