1. 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

2. Chapter 1: Monitoring the Weather (Basics)

3. 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
Seattle? Arizona? 3

4. Sources of Weather Information
Television
The Weather Channel and local newscasts
Radio
NOAA Weather Radio
Continuous broadcasts
repeated every minutes
Interrupted with warnings
and watches
The Internet 4

5. Weather Systems and 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) 5

6. 2 Types of Pressure Systems
High Pressure
Systems, or
“Anticyclones”
Low Pressure
“Cyclones” 6

7. Pressure Systems, Cont. High and low refer to air pressure Highs Lows
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 7

8. Pressure Systems, Continued High and Low Pressure Centers8
Arrows indicate surface horizontal winds

10. 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 10

11. AIR MASSES – large volume of air that has a relatively uniform temperature and humidity
Continental Polar – cP – cold and dry (Canada)
Maritime Polar – mP – cold and wet (Gulf of Alaska)
Continental Tropical – cT – hot and dry (Mexico)
Maritime Tropical – mT – hot and wet (Gulf of Mexico)
Arctic – A – very cold and dry (Northern Canada)

13. Fronts
Warm Front
Cold Front 13
© American Meteorological Society

14. Fronts – the boundary between air masses
Cold Front
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)
Precipitation can be severe
Boundary between advancing cold air and retreating warm air
Plotted on a map as a blue line with triangles pointed in the direction of motion

15. 2. Warm Front
Generally, a wide band of precipitation along or just ahead of the surface warm front, where precipitation can be persistent (12-24 hours)
Precipitation is generally light to moderate
Boundary between advancing warm air and retreating cold air
Plotted on a map as a red line with semi-circles pointed in the direction of motion

16. 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.

17. 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.

18. Ways to locate a front on a surface weather map:
Precipitation/Clouds
Wind Shift
Temperature Difference
Dew Point Difference (change in relative humidity)

19. 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

20. 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

21. 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 21

22. 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%

23. Describing the State of the Atmosphere What do Forecasters Tell Us
Describing the State of the Atmosphere What 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 23

24. 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
km (~ miles) high
Provides overlapping north-south strips of images
Passes over the same point twice every 24 hours 24

25. Orbit of a Geostationary Satellite25

26. Orbit of a Polar-Orbiting Satellite26

27. 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 27

28. 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 28

29. 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
GOES Imagery 29

30. Weather Radar Complements satellite surveillance
Doppler radar detects movement
Excellent tool to forecast tornadoes 30

31. 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 31

32. Cloud Forms
These high thin cirrus clouds appear fibrous because they are composed of mostly tiny ice crystals 32

33. Cloud Forms
These relatively low clouds are composed of tiny water droplets and have more sharply defined edges than ice-crystal clouds 33

34. Fog, stratus clouds in contact with the ground, reduces visibility
Cloud Forms
Fog, stratus clouds in contact with the ground, reduces visibility

35. Cloud Forms
Fair weather cumulus clouds are most common during the warmest time of day and then vaporize after sunset 35

36. 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 36

37. Cloud Forms
Clouds may move in different directions at different altitudes
Indicates horizontal wind shifts with altitude 37