Transcript Chapter 1: Monitoring the Weather

January History
Great Flood of 1937
• 70% of Louisville was submerged
• 3.3 billion in damages
• Crest was 85.4 feet and flood stage
is only 55 feet.
• 15 inches of rain in 12 days
Chapter 1: Monitoring the
Weather (Basics)
Difference between Weather
and Climate
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Weather is the state of the atmosphere at some
place and time
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Described with quantitative variables
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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
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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
Seattle? Arizona?
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Sources of Weather
Information
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Television
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The Weather Channel and local newscasts
Radio
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NOAA Weather Radio
 Continuous
broadcasts
repeated every 4 - 6 minutes
 Interrupted with warnings
and watches
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The Internet
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Weather Systems and Maps
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Weather info received via TV,
radio, or the Internet includes
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Weather maps
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National
Regional
Satellite/radar images
Data on current/past
conditions
Weather forecasts
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Short-term
 24 – 48 hours
Long-term
 Up to 7 days or longer
Automated Surface Observing
System (ASOS)
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2 Types of Pressure Systems
High Pressure
Systems, or
“Anticyclones”
Low Pressure
Systems, or
“Cyclones”
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Pressure Systems, Cont.
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High and low refer to air pressure
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Highs
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High pressure area is relatively high compared to surrounding air
Low pressure area is relatively low compared to surrounding air
Fair weather
Clockwise rotation of sinking air (in Northern Hemisphere)
Generally track toward the east and southeast
Lows
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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, Continued
High and Low Pressure Centers
Arrows indicate surface horizontal winds
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Air Masses
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Huge volume of air covering thousands of square
kilometers
Horizontally relatively uniform in characteristics
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Temperature
Humidity
Gathers characteristics from its source region
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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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AIR MASSES – large volume of air that has a relatively
uniform temperature and humidity
1. Continental Polar – cP – cold and dry (Canada)
2. Maritime Polar – mP – cold and wet (Gulf of Alaska)
3. Continental Tropical – cT – hot and dry (Mexico)
4. Maritime Tropical – mT – hot and wet (Gulf of Mexico)
5. Arctic – A – very cold and dry (Northern Canada)
Fronts
Warm Front
Cold Front
© American Meteorological Society
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Fronts –
the boundary between air masses
1. Cold Front
a.
Generally, a narrow band of
precipitation along or just ahead
of the surface front, where
precipitation is brief (couple of
minutes to a few hours)
b.
Precipitation can be severe
c.
Boundary between advancing cold
air and retreating warm air
d.
Plotted on a map as a blue line
with triangles pointed in the
direction of motion
2. Warm Front
a.
Generally, a wide band of precipitation
along or just ahead of the surface warm
front, where precipitation can be
persistent (12-24 hours)
b.
Precipitation is generally light to
moderate
c.
Boundary between advancing warm air
and retreating cold air
d.
Plotted on a map as a red line with
semi-circles pointed in the direction of
motion
A cyclone with the warm
and cold fronts extending
outward from the low
pressure center. Showers
generally form along the
warm front, while more
severe weather can occur
along the cold front.
This shows how the
warm and cold fronts
act as boundaries
between different air
masses. Notice how the
wind directions are
different on either side
of the fronts, and that
the flow is
counterclockwise and
convergent.
Ways to locate a front on a surface weather map:
1. Precipitation/Clouds
2. Wind Shift
3. Temperature Difference
4. Dew Point Difference (change in relative humidity)
Other Fronts
1. Stationary – a non-moving
front where winds on either
side blow in opposite directions
2. Occluded – when the air
behind the cold front overtakes
the air ahead of the warm front
Characteristics of Air Masses &
Fronts, Continued
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Wind directions are different on the two sides of a
front
Some fronts have no clouds or precipitation.
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In summer, temperature can be nearly the same
on both sides of a cold front
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Difference will be humidity
Fronts are anchored to lows on a weather map.
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Passage indicated by wind shift, and
temperature/humidity changes
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
Describing the State of the Atmosphere
What do Forecasters Tell Us?
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Maximum Temperature
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Minimum temperature
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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
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Usually occurs around sunrise
Dewpoint (frost point)
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Usually occurs in early to mid-afternoon
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
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General rule – 10” of snow = 1” of precipitation
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Based on Temperature of 90 F
Dew Point Temp F Human Perception
R. Humidity
75 +
Extremely uncomfortable,
oppressive
62%
70-74
Very Humid, quite
uncomfortable
52-60%
65-69
Somewhat uncomfortable
for most people
44-52%
60-64
OK for most
37-46%
55-59
Comfortable
31-41%
50-54
Very comfortable
31-37%
49 or lower
Feels like the western US
30%
Describing the State of the Atmosphere
What do Forecasters Tell Us? (Continued)
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Air Pressure
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And its tendency (rising or falling)
Falling may indicate approaching
cold front
Wind direction and speed
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Wind direction is the direction wind
is blowing from
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Sky cover
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Example; a west wind is blowing from the west, toward the east
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
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Two major types of satellite orbits
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Geostationary
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High orbits
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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
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36,000 km (22,300 miles) high
Positioned over equator at 750 W longitude, 1350 W longitude
Polar orbiting
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Low orbits
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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
PolarOrbiting
Satellite
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Weather Satellite Imagery
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Visible
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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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Weather Satellite Imagery
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Infrared
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Available
anytime, not
just during
daylight
Provides
temperature
comparison of
features within
image
Whiter =
colder
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Higher cloud
tops appear
whiter,
because they
are colder
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Weather Satellite Imagery
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Water vapor
imagery
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Enables
tracking of
plumes of
moisture
Shades of
white =
increasing
moisture
Upper-level
clouds
appear milky
to bright
white
GOES Imagery
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Weather Radar
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Complements
satellite
surveillance
Doppler radar
detects
movement
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Excellent tool
to forecast
tornadoes
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Sky Watching
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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
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A cloud in contact with the ground is fog
Cloud forms:
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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
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warm front
Cloud Forms
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These high thin cirrus clouds appear fibrous
because they are composed of mostly tiny
ice crystals
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Cloud Forms
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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
Fog, stratus clouds in contact with the
ground, reduces visibility
Cloud Forms
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Fair weather cumulus clouds are most
common during the warmest time of day and
then vaporize after sunset
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Cloud Forms
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Clouds of vertical development
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Merging and vertically-growing cumulus clouds
 Can
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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
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Clouds may move in different directions at
different altitudes
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Indicates horizontal wind shifts with altitude
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