January HistoryGreat Flood of 1937

• 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 30-

years 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?

24%

26%18%

29% 3%

Primary Source of Weather Informa-tion

Television Smart Phone Multi Personal Computer RadioSurvey of Farmers at the 2013 National Farm Machinery Show

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 PressureSystems, 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, FL2. Greenville, NC3. Duluth, MN4. 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 Air Rising

Warm Front

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

• 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

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

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?

NoneDangerEmergency

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• 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 TimeUTC Time = 1200 UTC

Daylight Savings Time?(Not until March 8th)

No

-5 Hours for EST

= 7 AM

• 18 UTC = 18Z = ?• 00 UTC = 0Z = ?• 06 UTC = 6Z = ?