NAS 125: Meteorology Monitoring Weather. Rev. 25 January 2007Monitoring Weather2 Ecclesiastes 1:4-7...

NAS 125: Meteorology

Monitoring Weather

Rev. 25 January 2007 Monitoring Weather 2

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.


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.


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.


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.


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.


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



• 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



• 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


Weather maps

• Weather maps typically plot:– 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.


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.



• 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)



• 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)



• 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.



• Northern and western quadrants of storms tend to be colder than southern and eastern quadrants.


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.


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.


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.


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.


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.


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.



• 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.



• 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.


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



• 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



• 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



• Sky cover: the fraction of sky covered by clouds– Cover categories

• 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


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.



• 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.



• 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.



• 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.



• 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.



• 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.


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.



• 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.



• 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.


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.



• 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.



• 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.



• Harbingers of the weather (continued):– Clouds at different altitudes may move in different

directions, thus indicating vertical changes in wind direction in the atmosphere.


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.



• 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.



• 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.



• 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.



• 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.



• 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.

NAS 125: Meteorology Monitoring Weather. Rev. 25 January 2007Monitoring Weather2 Ecclesiastes 1:4-7...

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Transcript of NAS 125: Meteorology Monitoring Weather. Rev. 25 January 2007Monitoring Weather2 Ecclesiastes 1:4-7...