Weather Studies
Introduction to Atmospheric Science
American Meteorological Society
Chapter 1
Monitoring The Weather
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants
Case-in-Point
 There have been
monumental advances
in monitoring of
weather
– Case-in-point – the
blizzard of 1888
– A similar storm in
December 2000
 Still lacked data on sea
surface temperatures –
overestimated snowfall
– Today, though not
exact, we are much
better at monitoring and
forecasting weather
The Blizzard of 1888
 Satellites
 Radiosondes
 Surface observations
2
Driving Question
 What are some basic characteristics of the
atmosphere and weather?
– This chapter will introduce and describe some
of the tools and basic understandings that will
guide your investigation of the atmosphere
3
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
4
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
5
Weather Systems and Weather 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
6
System (ASOS)
Pressure Systems
2 Types
High Pressure
Systems, or
“Anticyclones”
Low Pressure
Systems, or
“Cyclones”
7
Pressure Systems, Continued
 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
8
Pressure Systems, Continued
High and Low Pressure Centers
Arrows indicate surface horizontal winds
9
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
Fronts
Warm Front
Cold Front
© American Meteorological Society
11
Characteristics of Air Masses & Fronts
 Polar air masses are colder in winter, and 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
– Starts taking on the characteristics of the area it is passing over
 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, and may occur over 1 to 2 days
– Usually light precipitation
 Cold front clouds and precipitation occur over a narrow
band, and may occur over minutes to hours
– May be very heavy precipitation
12
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
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and warm front
Other Weather Characteristics
 In coastal areas
– Lake/sea breezes may lower summer temperatures
– May push 10-50 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 U.S.
– 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
14
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
15
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
16
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
– 800-1000 km (~500-600 miles) high
 Provides overlapping north-south strips of images
 Passes over the same point twice every 24 hours
17
Orbit of a Geostationary Satellite
18
Orbit of a
PolarOrbiting
Satellite
19
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
20
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
21
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
22
Weather Radar
 Complements
satellite
surveillance
 Doppler radar
detects
movement
– Excellent tool
to forecast
tornadoes
23
Combination of Images
 Composite of
IR and Radar
24
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
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warm front
Cloud Forms
 These high thin cirrus clouds appear fibrous
because they are composed of mostly tiny
ice crystals
26
Cloud Forms
 These relatively low clouds are composed of
tiny water droplets and have more sharply
defined edges than ice-crystal clouds
27
Cloud Forms
 Fair weather cumulus clouds are most
common during the warmest time of day and
then vaporize after sunset
28
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
29
Cloud Forms
 Clouds may move in different directions at
different altitudes
– Indicates horizontal wind shifts with altitude
30
Essay: Weather Maps, Historical
Perspective
U.S. Army Signal Corps weather map dated 1 September 1872