January History
Great Flood of 1937
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70% of Louisville was submerged
3.3 billion in damages
Crest - 85.4 ft. (Flood Stage – 55 ft.)
15 inches of rain in 12 days
Chapter 1: Monitoring the Weather (Basics)
One of the Deepest Extratropical Cyclones Ever Recorded
Difference between 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 30years and updated every decade
– A locale’s climate also includes weather extremes
Look at Climatological Information
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 (Ag Weather)
• What about now?
Primary Source of Weather
Information
Survey of Farmers at the
2013 National Farm
Machinery Show
Television
Smart Phone
Multi
3%
29%
18%
Personal Computer
24%
26%
Radio
Retrieving Weather Information & 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
Two Types of Pressure Systems
High Pressure
Systems, or
“Anticyclones”
Low Pressure
Systems, or
“Cyclones”
Pressure Systems Cont.
• 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
Pressure Systems Cont.
(High and Low Pressure Centers)
Arrows indicate surface horizontal winds
Pressure Systems Cont.
(What’s the weather like?)
1. Tallahassee, FL
2. Greenville, NC
3. Duluth, MN
4. Scranton, PA
• Wind
Direction?
• Cloudy/Wet,
Clear/Dry?
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
Air Masses Across North America
Old Saying in the
Ohio Valley:
“Don’t like the
weather today?
It will change
tomorrow!”
Fronts
“Transition Zones between Air Masses”
Warm Front
Warm Air Rising
Cold Front
Fronts – 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
e.
Sharp Temperature Change
Fronts – Boundary Between Air Masses
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
Fronts – Boundary Between Air Masses
Right - 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.
Left - 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
2. Cloud Cover
3. Wind Shift
4. Temperature
Difference
5. Dew Point
Difference
Other Types of Frontal Boundaries
1. Stationary – a non-moving
front where winds on either
side blow in opposite
directions. Can become a
cold or warm front based on
advection.
2. Occluded – when the air
behind the cold front
overtakes the air ahead of the
warm front
Characteristics of Air Masses & Fronts
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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?
• 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
Example: What do forecasters tell us?
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%
Livestock Cold
Stress?
None
Danger
Emergency
Describing the State of the Atmosphere Cont.
What do Forecasters Tell Us?
• 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
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
– Low angle in polar regions
– Polar orbiting
• Low orbits
– 800-1000 km (~500-600 miles) high (Much more detailed info)
• Provides overlapping north-south strips of images
• Passes over the same point twice every 24 hours
Orbits of Each Type of Satellite
Geostationary Satellite
Polar Orbiting Satellite
Weather Satellite Imagery
•
Visible
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•
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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
Weather Satellite Imagery
• 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
• Water vapor
imagery
• Enables
tracking of
plumes of
moisture
• Shades of
white =
increasing
moisture
• Upper-level
clouds appear
milky to bright
white
Weather Radar
•
•
Complements
satellite
surveillance
Doppler radar
detects
movement
•
Excellent tool
to forecast
tornadoes
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
Cloud Forms
• These high thin cirrus clouds appear fibrous
because they are composed of mostly tiny ice
crystals
Cloud Forms
• These relatively low clouds are composed of tiny
water droplets and have more sharply defined
edges than ice-crystal clouds
Clouds Forms
Fog, stratus clouds in
contact with the ground,
reduces visibility
Cloud Forms
• Fair weather cumulus clouds are most common
during the warmest time of day and then
vaporize after sunset
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
Cloud Forms
• Clouds may move in different directions at
different altitudes
– Indicates horizontal wind shifts with altitude
Understanding UTC Time
• Weather observations are
taken across the world
based on a standard time.
• In doing so, a 24 hour
clock is used, similar to
military time.
• UTC (Coordinated
Universal Time or
Universal Time
Coordinated)
• Also called “Z Time”
• To get local time in the
United States, you have to
subtract a certain number
of hours based on time
zone.
• Daylight Savings Time does
make a difference
Understanding UTC Time
UTC Time = 1200 UTC
Daylight Savings Time?
(Not until March 8th)
No
-5 Hours for EST
= 7 AM
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18 UTC = 18Z = ?
00 UTC = 0Z = ?
06 UTC = 6Z = ?