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NAS 125: Meteorology
Monitoring Weather
Ecclesiastes 1:4-7
• One generation passeth away, and another generation
cometh: but the earth abideth for ever.
• The sun also ariseth, and the sun goeth down, and
hasteth to his place where he arose.
• The wind goeth toward the south, and turneth about
unto the north; it whirleth about continually, and the
wind returneth again according to his circuits.
• All the rivers run into the sea; yet the sea is not full;
unto the place from whence the rivers come, thither
they return again.
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The Blizzard of ’88, part 1
• From March 12 to March 14, 1888, a blizzard swept
up the East Coast from Washington, D.C., to southern
New England.
– Snowfall totals reached 53 cm in New York City.
– 65 km winds whipped up snowdrifts to 6 m.
– Update New York received snowfalls up to 125 cm with
drifts to 12 m.
– About 300 persons died on land.
– Nearly 200 ships were sunk or damaged with 100 lives lost.
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The Blizzard of ’88, part 2
• U.S. Army Signal Corps, which handled weather
forecasts at the time, weather forecasters at the time
did not have technology nor sufficient understanding
of the weather to predict the storm.
– They had 17 years of observations.
– They had 154 weather stations telegraphing daily
observations to the headquarters in Washington, D.C.
– They did not have weather balloons or other aircraft,
weather satellites, radar, radio communications with ships
at sea, computers, or instantaneous electronic
communications.
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The Blizzard of 2000
• Another storm threatened the same region with heavy
snow, but forecasters, armed with everything the
1888 Signal Corps lacked, were better prepared.
– Weather warnings were issued well in advance.
– School systems were closed more than 12 hours in
advance.
– People stocked up on supplies.
• The blizzard did not arrive.
• It goes to show we still have a long way to go before
we can say we understand the weather.
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What can we do?
• Learn some of the basic concepts regarding the
atmosphere and weather.
– We will observe weather events as they happen.
– We will become familiar with the basic scientific principles
behind our current understanding of the atmosphere.
– We will combine observations with theory to understand a
variety of atmospheric phenomena.
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Weather information, part 1
• We learn about weather in many ways.
– Newspaper, radio, television, Internet
• In this class, you will:
– Check current weather data via the Internet several times a
week;
– Check a weather forecast every day
• On television – either on a local station or a cable station such as
The Weather Channel
• On the radio, although scope is often limited
• In a local or national newspaper
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Weather information, part 2
• Other sources of weather information:
– NOAA (National Oceanic and Atmospheric
Administration) Weather Radio
• NOAA is the parent agency of the National Weather Service
– NOAA Weather Radio operates 24 hours a day, broadcasting over
special FM frequencies (you need a radio designed to pick up those
frequencies)
– Routine messages are repeated every few minutes, but the routine
broadcasts are interrupted as warranted by warning or emergency
broadcasts.
– The World Wide Web, where numerous sources are
available as long as you have power to run your computers
and access the Internet
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Weather information, part 3
• Whatever the source, weather information distributed
via the mass media typically contains the following:
– National and regional weather maps
– Satellite or radar images (or video) that depicts cloud,
precipitation, and atmospheric circulation patterns
– Data on current and recent (typically 24-hour) weather
conditions
– Short-term and long-term weather forecasts
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Weather maps
• Weather maps typically plot:
–
–
–
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Temperature
Dewpoint
Wind
Air pressure
• By international agreement, weather observations are
made simultaneously several times of day at
established weather stations around the world.
• Special symbols are used to plot locations of major
weather makers – pressure systems and fronts.
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Pressure systems, part 1
• Air pressure is a measure of the weight of a column
of air pressing down upon a unit of area.
• There are two types of pressure systems, highs and
lows.
– Highs are like a mound of air, where the pressure at the
center is greater than in surrounding areas.
– Lows are like a depression in the atmosphere, where the
pressure at the center is less than in surrounding areas.
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Pressure systems, part 2
• Characteristics of highs
– Fair-weather systems
• They bring cold, dry weather in winter
• They bring cool, dry weather in summer
• Winds are light near the center of the high
– Clockwise flow away from center at surface in Northern
Hemisphere (but counterclockwise in Southern
Hemisphere)
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Pressure systems, part 3
• Characteristics of lows
– Stormy weather systems
• Lows often bring stormy, rainy, or snowy weather
• Lows that develop over arid or semiarid areas may bring more dry
weather, especially in summer
– Counterclockwise flow toward center at surface in
Northern Hemisphere (but clockwise in Southern
Hemisphere)
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Pressure systems, part 4
• Highs and lows typically move with the prevailing
winds aloft, several kilometers above the surface
– Prevailing movement in midlatitudes (between subtropical
and subpolar latitudes) is from west to east
– Tropical low-pressure systems, however, typically move
from east to west at first before making a turn back to the
east in the midlatitudes.
• In North America, storms on a more northerly track
are usually drier because they are farther from
sources of moisture.
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Pressure systems, part 5
• Northern and western quadrants of storms tend to be
colder than southern and eastern quadrants.
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Air masses
• An air mass is a large parcel of air – covering
hundreds of thousands of square kilometers – that is
distinct from surrounding air, that has relatively
uniform properties in the horizontal dimension, and
that moves as a unit.
• The characteristics of an air mass are determined by
the surface characteristics of the regions over which it
forms (the source region) and travels.
– The basic types of air masses are cold and dry, cold and
humid, warm and dry, and warm and humid.
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Fronts, part 1
• A front is a narrow zone of transition between two air
masses that differ in one or more characteristics.
• Winds on either side of the front move in different
directions.
• Air movements along fronts often generate cloudiness
and precipitation.
– Sometimes the passage of a front is marked only by a
change in temperature and/or humidity and wind direction.
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Fronts, part 2
• There are four basic types of fronts: stationary, warm,
cold, and occluded.
– Stationary fronts are those in which neither air mass
displaces the other. Winds on either side of the front blow
parallel to one another, but in opposite directions.
– Warm fronts are those in which a warm air mass displaces a
cold one.
• Warm air, which is less dense than cold air, overrides the colder air
mass.
• Warm fronts are broad, with a gentle slope; and cloudiness and
precipitation associated warm fronts is consequently spread over a
wide band.
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Fronts, part 3
• Four types of fronts (continued):
– Cold fronts are those in which a cold air mass displaces a
warm one.
• Cold air, which is more dense than warm air, slides under the
warmer air mass and pushes it up.
• Cold fronts are narrow, with a steep slope; and cloudiness and
precipitation associated with cold fronts is consequently spread
over a narrow band.
• In summer, air temperatures on either side of a cold front are nearly
the same, but the humidity of the air mass ahead of the front is
greater (thus the air mass is less dense) than that of the air mass
behind the front.
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Fronts, part 4
• Four types of fronts (continued):
– Occluded fronts are those in which a cold air mass
overtakes a warm one and forces the warm air aloft.
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Maps, masses, and fronts, part 1
• Fronts are denoted by special symbols on weather
maps.
– The symbols denote the line where the fronts intersect with
the ground.
• As an air mass moves from it source region, it is
modified in accordance with the characteristics of the
surfaces it passes over.
– The degree of air mass change is proportional to the
differences in characteristics between the surface of the
source region and those of the regions downstream.
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Maps, masses, and fronts, part 2
• Cold and warm fronts are often anchored at the center
of a low-pressure system as the convergent
circulation brings contrasting air masses together.
• Lows may develop along a stationary front and travel
rapidly like a ripple from west to east (in the
midlatitudes) along the front.
– Near the equator, along the Intertropical Convergence
Zone, such ripples, called tropical waves, move from east
to west and generate hurricanes.
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Maps, masses, and fronts, part 3
• The severest weather accompanying a low-pressure
systems is in the right, front quadrant.
• Local and regional effects to notice on weather maps:
– Land and sea breezes along coastal areas
– Lake-effect snows along the southern and eastern shores of
the Great Lakes.
– Severe thunderstorms and tornadoes in Tornado Alley.
– Frequent thunderstorms in Florida and the western High
Plains, but few along Pacific Coast and Hawai’i.
– Hurricanes along the Atlantic and Gulf Coasts.
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Weather parameters, part 1
• Maximum temperature: the highest temperature
recorded over a 24-hour period
• Minimum temperature: the lowest temperature
recorded over a 24-hour period
• Dewpoint or frost point temperature: temperature at
which air must be cooled (holding pressure constant)
so dew or frost can begin forming on cold surfaces
• Relative humidity: a measure of the amount of water
vapor held by a parcel of air relative to the amount
the parcel would hold if it was saturated
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Weather parameters, part 2
• Precipitation amount: the total rainfall or melted
snowfall over a 24-hour period (10 inches of snow
typically equate to 1 inch of rainfall)
• Wind direction and speed: wind direction is the
direction from which the wind blows
– A northeast wind blows from the northeast toward the
southwest
– Wind shifts from east to northeast to north are usually
accompanied by falling air temperature, while shifts from
east to southeast to south are usually accompanied by rising
air temperature
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Weather parameters, part 3
• Air pressure: the weight of a column of air over a unit
area of the Earth’s surface
– Pressure had traditionally been measured from the height
(inches or millimeters) of a column of mercury forced up a
narrow tube by atmospheric pressure, but now
meteorologists use a unit of pressure (millibars)
– Average pressure at sea level is 1013.25 mb, (29.93 in of
mercury)
– Rising pressure indicates approaching fair weather, while
falling pressure indicates oncoming stormy weather
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Weather parameters, part 4
• Air pressure: the weight of a column of air over a unit
area of the Earth’s surface
– Pressure had traditionally been measured from the height
(inches or millimeters) of a column of mercury forced up a
narrow tube by atmospheric pressure, but now
meteorologists use a unit of pressure (millibars)
– Average pressure at sea level is 1013.25 mb, (29.93 in of
mercury)
– Rising pressure indicates approaching fair weather, while
falling pressure indicates oncoming stormy weather
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Weather parameters, part 5
• Sky cover: the fraction of sky covered by clouds
– Cover categories
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Clear (no clouds)
Few clouds (1/8 to 2/8)
Scattered clouds (3/8 to 4/8)
Broken clouds (5/8 to 7/8)
Overcast (complete cloud cover)
• Weather watch: watches are issued by the National
Weather Service when severe weather is possible
• Weather warning: Warnings are issued when severe
weather is imminent or actually taking place
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Weather satellite imagery, part 1
• Satellite imagery (both still images and video) is a
routine component of televised and Internet-based
weather reports.
• Satellite images are obtained from space-based
platforms that measure components of the
electromagnetic spectrum, primarily in the infrared
and visible range.
– The information obtained provides measurements of
temperature and humidity, and it allows meteorologists to
locate and track weather systems.
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Weather satellite imagery, part 2
• Satellite platforms:
– Geostationary satellites orbit the Earth about 36,000 km
above the surface.
• The speed with which they orbit the Earth matches the speed of the
Earth’s rotation, thus they “sit” in the same spot above the surface.
• The subsatellite point – the location on the Earth’s surface directly
below the satellite – is located along the equator.
• Two geostationary satellites, at 75 degrees W longitude and at 135
degrees W longitude, provide a complete view of much of North
America and adjacent portions of the Atlantic and Pacific oceans up
to about 60 degrees N latitude.
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Weather satellite imagery, part 3
• Satellite platforms (continued):
– Polar-orbiting satellites orbit the Earth between about 800
and 1,000 km above the surface.
• The orbital track crosses both North and South polar regions.
• Each successive north-south track overlaps with the western edges
of previous tracks, thus providing overlapping images of the Earth’s
surface.
• Sun-synchronous satellites pass over the same area roughly twice
every 24 hours.
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Weather satellite imagery, part 4
• Satellite imagery:
– Sensors on weather satellites typically measure reflected
sunlight or emitted infrared (IR) radiation.
– Visible images are essentially black-and-white photographs
of the Earth, with highly reflective surfaces – such as ice
caps or clouds – appearing bright white, and less reflective
surfaces – such as boreal forests or oceans – appearing
much darker.
• Cloud patterns on visible images are of particular interest, as
meteorologists can determine the stage of development of a storm
system in addition to its location.
• Useful visible imagery can only be obtained during daytime hours.
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Weather satellite imagery, part 5
• Satellite imagery (continued):
– Infrared images are essentially a measure of heat radiation
emitted by objects.
• Useful infrared imagery can be obtained at any time, day or night.
• Images are calibrated to show temperatures of objects in the field of
view.
– In black-and-white images, the coldest objects are bright white, while
the hottest objects are dark gray.
– In color images, the coldest objects appear blue and violet, while the
hottest objects appear red and orange.
– Low clouds can be differentiated from high clouds, as high clouds are
colder.
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Weather satellite imagery, part 6
• Satellite imagery (continued):
– Water vapor satellite images use special infrared sensors to
measure the amount of water vapor in the air.
• Water vapor does not appear on visible or on conventional infrared
sensors.
• Such images allow meteorologists to track movement of plumes of
moisture through the atmosphere.
• Current platforms measure water vapor concentrations between
altitudes of about 5,000 m and 12,000 m.
• A gray scale is used, so that little or no water vapor appears black,
while high concentrations appear milky white. Upper-level clouds
appear as bright white blotches.
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Weather radar, part 1
• Weather radar measures the return of radio
(microwave) signals that are bounced off water
droplets in the atmosphere.
• Radars allow meteorologists to locate and track
movement of areas of precipitation and to observe
circulation within small-scale systems such as
thunderstorms.
• The heavier the precipitation, the more intense the
echo.
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Weather radar, part 2
• Echo intensity (continued):
– Echo intensity is calibrated on a color scale so that areas of
light precipitation appears light green and areas of heavy
precipitation appears dark red.
– Composite maps of data from a number of weather stations
allows meteorologists to track and analyze large-scale
weather systems.
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Weather radar, part 3
• By using specially designed radars that can take into
account the Doppler effect, meteorologists can
monitor the movement of raindrops, snowflakes, and
hailstones within a storm system.
– Hook echoes within a thunderstorm may provide early
warning of approaching tornados.
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Sky watching, part 1
• Observations of the sky are vital to the study of
meteorology.
– Sky watching makes us more aware of atmospheric
dynamics and may also provide clues to future weather.
• Clouds are aggregates of tiny water droplets, ice
crystals, or combinations of both.
– Clouds made of ice crystals are found at high elevations
and have a fibrous or wispy (cirriform) appearance.
– Clouds made of water droplets are found at lower
elevations and have sharply defined edges.
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Sky watching, part 2
• A cloud at or very near the surface is called fog.
– By definition, a fog reduces visibility to less than 1 km.
• Some clouds form horizontal layers (stratiform) while
others are puffy (cumuliform).
– Stratiform clouds develop where air ascends gently over a
broad region, such as along a warm front.
– Cumuliform clouds develop where there is vigorous uplift
over a relatively narrow area, such as along a cold front.
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Sky watching, part 3
• Clouds often serve as harbingers of the weather.
– High, thin stratiform clouds often signal an approaching
warm front.
• As the front gets nearer, the ceiling (elevation to the base of the
clouds) drops and the cloud layer thickens so that the cloud layer
can block out the light of sun or moon.
– Scattered, puffy clouds – cumulus clouds – often appear
during fair weather in the afternoons after several hours of
bright sunshine, but vaporize near sunset.
– Under certain conditions, cumulus clouds merge and grow
vertically to form cumulonimbus, or thunderstorm, clouds.
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Sky watching, part 4
• Harbingers of the weather (continued):
– Clouds at different altitudes may move in different
directions, thus indicating vertical changes in wind
direction in the atmosphere.
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Time keeping, part 1
• As pointed out earlier, weather observations are made
simultaneously at weather stations around the world.
• Such standardization of the measurement of weather
phenomena requires some standardization of the
measurement of time.
• In the past, timekeeping was based on local solar
time, but use of local solar time created increasing
problems with the advent of the telegraph and
railroad; the need to prevent train collisions prompted
development of a standardized time system.
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Time keeping, part 2
• The United States and Canada adopted a system of
civil time zones in 1883.
• The International Meridian Conference, held in
Washington, D.C., in 1884, expanded the concept of
standardized time around the world.
– Since the Royal Naval Observatory at Greenwich, England,
had been responsible for some of the most important
astronomical determinations, the meridian which ran
through Greenwich was established as the prime meridian,
from which all other longitudes are measured.
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Time keeping, part 3
• International Meridian Conference (continued):
– The location of solar noon migrates 15 degrees of longitude
every hour, so the conference divided the world into 24
standard time zones, each covering 15 degrees of longitude.
• Each time zone extends for 7.5 degrees of longitude on either side
of a specified meridian, such as the prime meridian.
– The time zone centered on the prime meridian is called Greenwich
Time or Zulu (for Z) time
• In international waters, time zone boundaries are defined
specifically and consistently by longitude.
• Over land areas, however, zone boundaries are adjusted for political
and economic convenience.
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Time keeping, part 4
• International Meridian Conference (continued):
– The system established in 1884 was called Greenwich
Mean Time (GMT).
– Greenwich Mean Time was replaced on January 1, 1972,
by a more precise system based on atomic clocks called
Coordinated Universal Time (UTC; also called Z time).
– Coordinated Universal Time and Greenwich Mean Time
are essentially the same, however.
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Time keeping, part 5
• Daylight Saving Time (DST) is a practice by which
clocks are set forward by an hour (or more) in the
summer so as to extend daylight into evening hours.
– Daylight Saving Time originated in Germany during World
War I as a measure to conserve electricity.
– The United States briefly adopted Daylight Saving Time in
1918 and again during World War II. The Universal Time
Act of 1966 made the implementation permanent, although
Arizona, Hawaii, and part of Indiana are exempted.
– Coordinated Universal Time is not affected by Daylight
Savings Time.
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Time keeping, part 6
• By international convention, surface weather
observations are made four times a day, at 0000 Z,
0600 Z, 1200 Z, and 1800 Z.
• Radiosonde observations (of the upper atmosphere)
are made twice a day at 0000 Z and 1200 Z.
• In the United States, surface observations are
obtained at the top of each hour, composite radar
maps are issued each hour at 35 minutes past, and
fronts are analyzed via weather maps every third hour
beginning at 0000 Z.
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