1. AMS Weather Studies Introduction to Atmospheric Science, 5th Edition
Chapter 1
Monitoring the Weather
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2. Driving Question
What are some basic characteristics of the atmosphere and weather?
This chapter covers:
Difference between weather and climate
Time keeping
Overview of weather systems and weather maps
Introduction to satellite imagery and radar
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3. Case-in-Point Blizzards Then and Now
The Blizzard
of 1888
There have been monumental advances in monitoring of weather
The blizzard of 1888
4 nor’easters hit the mid-Atlantic during the winter of
Today, though not exact, much better at monitoring and forecasting weather
Understanding of coastal winter storms
Satellites/Radar
Radiosondes
Surface observations
February 2010 nor’easter
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4. 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, processes that cause weather, and the life cycle of weather systems.
Climate is weather conditions at some locality averaged over a specified time period
Climate is an average of weather, computed for the last 30-years and updated every decade
A locale’s climate also includes weather extremes
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5. Accessing Weather Information
Internet
Television
The Weather Channel and local newscasts
Radio
NOAA Weather Radio
Continuous broadcasts repeated every 4-6 min
Interrupted with warnings and watches
Weather Apps
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6. Time Keeping Time zones UTC
Measured degrees east/west of the prime meridian (0 degrees longitude)
UTC
Based on atomic clock
Expressed as a 24-hour clock
7:45 a.m.=0745
2:20 p.m.=1420
Does not shift to Daylight Savings Time and therefore must adjust local time
0700 CDT=0600 CST=1200 Z
Surface observations are taken at least 4 times per 24 hours (U.S. takes hourly observations).
Upper air takes measurements at Z and 1200 Z
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7. Weather Systems & Weather Maps
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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8. Pressure Systems 2 Types
High Pressure
Systems or
“Anticyclones”
Low Pressure
Systems or
“Cyclones”
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9. Weather Systems & Weather Maps
A. clockwise and outward flow from a high-pressure system.
B. counterclockwise and inward into a low-pressure system.
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10. Weather Systems & Weather Maps
Air Masses
Huge volume of air covering hundreds of 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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11. Weather Systems & Weather Maps
Air Masses
Polar air masses are colder in winter, 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
Taking on the characteristics of the area passing over
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12. Fronts
Warm Front
Cold Front
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13. Weather Systems & Weather Maps
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
May occur hours or longer
Usually light precipitation
Cold front clouds and precipitation occur over a narrow band,
May occur over minutes to hours
May be very heavy precipitation
Wind directions are different on the two sides of a front
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14. Weather Systems & Weather Maps
Some fronts have no clouds or precipitation
Passage indicated by wind shift, temperature/humidity changes
In summer, temperature can be nearly the same on both sides of a cold front
Difference will be humidity
Fronts 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 fronts
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15. Weather Systems & Weather Maps
In coastal areas
Lake/sea breezes may lower summer temperatures
May push 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 US
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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16. Describing the State of the Atmosphere
Maximum Temperature
Usually occurs in early to mid-afternoon
Minimum temperature
Usually occurs around sunrise
Dewpoint (frost point)
Temperature when air is cooled, at constant pressure, so it is saturated with water vapor; dew (frost) forms
Relative humidity
Percentage/ratio of actual water vapor concentration of air vs water vapor concentration if air was saturated
Relative humidity changes through the day as temperature varies
Generally highest around sunrise and lowest when warmest
Precipitation amounts
General rule: 10 in. of snow = 1 in. of precipitation
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17. Describing the State of the Atmosphere
Air Pressure
Tendency (rising or falling)
Falling may indicate approaching cold front
Wind direction and speed
Wind direction is the direction wind is blowing from
Example: west wind is blowing from the west, toward the east
Sky cover
Fraction of the sky covered in clouds
Weather watch
Issued by NWS when hazardous weather is considered possible
Weather warning
Issued by NWS when hazardous weather is imminent or happening
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18. Weather Satellite Imagery
Geostationary
High orbits
36,000 km (22,300 miles) high
Orbits planet at same rate as Earth’s rotation, in same direction
Currently, 2 provide a view of North America and adjacent oceans, to latitudes of ~60°
Positioned over equator at 75° W and 135° W longitude
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19. Weather Satellite Imagery
Polar orbiting
Low orbits
km (~ mi) high
Provides overlapping north-south strips of images
Passes over the same point twice every 24 hrs
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20. Weather Satellite Imagery
Visible Satellite Image
Black and white photograph of the planet
Only available during daylight hours
Highly reflective surfaces appear bright white and less reflective surfaces are darker
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21. Weather Satellite Imagery
Infrared Satellite Image
Available anytime, not just during daylight
Provides temperature comparison of features within image
Higher cloud tops appear whiter, because they are colder
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22. Weather Satellite Imagery
Water vapor satellite images
Enables tracking of plumes of moisture
Shades of white = increasing moisture
Upper-level clouds appear milky to bright white
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23. Weather Radar Complements satellite surveillance
Doppler radar detects movement
Excellent tool to forecast tornadoes
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24. Weather Radar Composite of infrared satellite image and radar echoes.
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25. Sky Watching
Clouds
Aggregates of tiny water droplets, ice crystals or both
High thin cirrus clouds appear fibrous because they are composed of tiny ice crystals.
Relatively low clouds composed of tiny water droplets with more sharply defined edges.
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26. Sky Watching Fog Cloud forms:
Cloud in contact with the ground
Cloud forms:
Stratiform clouds
Sheet-like clouds formed in horizontal layers
Form where air ascends gradually over a broad region
Cumuliform clouds
Puffy, like cotton balls
Form with a more vigorous ascent of air over smaller area
Under the right conditions can build vertically into a cumulonimbus (thunderstorm) cloud
High, wispy feather-like clouds in western sky
First sign of approaching warm front
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27. Sky Watching Cumulus clouds Fair weather
Most common during warmest time of day
Vaporize after sunset
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28. Sky Watching Clouds of vertical development
Merging and vertically-growing cumulus clouds
Cumulonimbus clouds
Nimbo, nimbus prefix or suffix = rain producing
Produce lightning, heavy rain, hail, strong, gusty surface winds
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29. Sky Watching
Clouds move in different directions at different altitudes
Indicates horizontal wind shifts with altitude
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