The word "radar," which is an acronym for "radio detection and ranging," is an important tool in detecting precipitation. Radar has been in use detecting precipitation here in the United States since the late 1950s. The U. S. Weather Bureau developed their first network of vacuum tube radars in 1957.. with the individual radar unit referred to as "WSR57".. with the WSR standing for "weather surveillance radar." Weather radars were made more solid state with the advent of the "WSR74" in the mid 1970s. With tremendous strides in technology, the WSR57 and the WSR74 quickly out grew their usefulness and became harder to maintain during the 1980s.

It was in the late 1980s that the National Weather Service, in a tri-agency effort with the U. S. Departments of Defense and Transportation, developed a network of new doppler weather radars (the WSR88D) across the United States. These computer aided radars are far superior to the earlier radars mainly due to the fact that the radar beam itself was about 2 to 3 times as sensitive as the previous radars when considering signal strength and how we were able to view precipitation. In addition, these doppler radars, based on volume scans of the atmosphere, use algorithms to create additional radar products that include storm total rainfall estimates, vertically integrated liquid (to determine the possibility of hail) as well as radial velocity and storm relative motion which is particularly important in detecting the presence of mesocyclones which can contain tornadoes. In all, about 50 different products are produced in each WSR88D volume scan.. which is performed about every six minutes.

So how does radar work? Weather radars transmit a pulse of energy into the atmosphere and if targets are present, then as the signal becomes scattered, a much smaller part of the pulse energy is reflected back to the radar. The resultant reflected signals, which we refer to as return signals, are indicated on radar displays as "reflectivity" echoes and generally show us areas of precipitation (rain, snow, etc). The larger the target, in this case the precipitation element, the stronger the return signal. Accordingly, not only does the location of these color coded echoes indicate where precipitation is occurring (even though, at times, the precipitation may not reach the ground), but the color indicates the intensity of the precipitation.. with blues and greens indicating lighter precipitation while yellows and reds depict precipitation of heavier intensities. It should be noted that very high reflectivities (red colors) can indicate the presence of hail in more violent thunderstorms.

It should be pointed out that how sensitive a radar happens to be is based upon the wavelength of the pulses of energy that it transmits. While shorter wavelength radars (for example, 3-5 centimeter wavelength radars) can detect smaller precipitation elements and, even, cloud droplets, close in to the radar, longer wavelengths are generally preferred in operational weather forecasting. The longer wavelength radars, such as the National Weather Service's WSR88D, operate at 10 centimeter wavelengths, are much better able to detect precipitation at much greater distances and are able to distinguish smaller atmospheric features throughout the sampled atmosphere. It should be added, as well, that the weaker, generally shorter wavelength radars, such as used at some television stations, are much less expensive than the longer wavelength radars that are used by the National Weather Service.

A very important facet of the new National Weather Service doppler weather radars is the ability, through the understanding of the principle of the Doppler Shift, to not only detect the precipitation (what we have referred to as "reflectivity") but to also detect the motion of the precipitation elements toward or away from the radar, something we refer to as "radial velocity." For the first time ever, by color coding the motion of these targets/echoes, we are able to see wind motion within the precipitation field when that motion is toward or away from the radar. When using the radial velocity and storm relative motion products from the doppler weather radar, it is critical that you understand that "cool" colors (green) indicate motion toward the radar, while "hot" colors (red) indicate wind motion away from the radar.

It goes without saying that this information is extremely important when watching for developing straight line thunderstorm winds, downburst winds, mesocyclones as well as tornadoes. There are three important factors to consider when radar detects a precipitation target. Those considerations, that the radar takes into account when looking at a target, are (1) azimuth angle of the echoes from the radar antenna (when considering that north is at 360 degrees, east is at 90 degrees, south is at 180 degrees and west is located at 270 degrees), (2) elevation angle of the echoes from the radar antenna (ranging from near 0 degrees at the horizon to 90 degrees directly overhead) and (3) judging distance (in miles or kilometers) of the echo from the radar antenna.

What about those pesky "false echoes" we sometime see on radar. Sometimes, the radar energy pulses will be intercepted by vegetation and buildings close to the radar (especially if those things are wet) and the return signal will become contaminated and a wide "ground clutter" pattern is noted. In addition, temperature inversions in the atmosphere can return trapped radar energy which will produce false echoes. These false echoes are referred to as anomalous propagation or, for short, "AP." The newer doppler weather radars are not as prone to this "AP" as the earlier National Weather Service radars or the shorter wavelength radars sometimes used, as mentioned earlier, by television stations. It is important to note, however, that there are times, in clear skies, that the National Weather Service will increase the sensitivity of the WSR88D from "Precipitation Mode" to "Clear Air Mode." This supersensitive reflectivity mode can detect wind shifts in clear air (cloud and precipitation free skies!) and by "targeting" insects, migrating birds and even the bats that fly from under the Congress Street bridge that might be caught in wind pattern.

Now that you've grasped the basic of weather radar, let's take a look at a few radar images:
(1) Image in Precipitation Mode (Light Pcpn)
(2) Image in Precipitation Mode (T'Storms - Squall Line)
(3) Image in Velocity Mode (Tornado)
(4) Image in Clear Air Velocity Mode
(5) Image in Storm Total Precipitation Mode




(BYX RADAR) In reflectivity (precipitation) mode, this WSR88D image loop, from the NWS Weather Forecast Office in Key West, Florida, indicates mainly scattered light to moderate (blues and greens) rain and rain showers mainly over the southeastern Gulf of Mexico as well as the Florida Straights and southern Florida. Note isolated heavier (yellows and reds) rain showers and thunderstorms over inland south Florida (northwest of Miami) as well as just east of Key West as well as distant northwest of Key West over the open Gulf of Mexico.




(FTG RADAR) In reflectivity (precipitation) mode, this WSR88D image loop, from the NWS Weather Forecast Office in Denver, Colorado, shows light to moderate rain over the eastern plains of Colorado with smaller pockets of heavier rain south of Limon and Akron. There are smaller areas of rainfall over the Rockies west through north of Denver. Notice a small northward moving thunderstorm (yellow and red) north northwest of Denver that diminishes in intensity quickly during the lifetime of the image loop.


(GRR RADAR) In reflectivity (precipitation) mode, this WSR88D image loop, from the NWS Weather Forecast Office in Grand Rapids, Michigan, shows a developing eastward moving line of strong thunderstorms extending southward across central Michigan.


(OUN RADAR) In velocity mode, this WSR88D image (with the radar being located at the "KTLX" ID southeast of Oklahoma City) depicts velocity data that shows us two mesocyclones (counterclockwise circulations) that are producing tornadoes. Remember that warmer colors (reds) depict air motion away from the radar while cooler colors (greens) indicate air motion toward the radar. Locate the radar's location, then look for the red and green couplet near Shawnee. This couplet indicates a significant counterclockwise rotation and is where a tornado is present (but is dissipating at the time). Look further north and eastward, northwest of Prague, where a second significant tornado is developing within a very strong counterclockwise rotation.

(IMAGE COURTESY: NWS WSR88D Operational Support Facility)

(LRX RADAR) In reflectivity (clear air) mode, this WSR88D image loop, from the NWS Weather Forecast Office in Elko. Nevada, represents a "super sensitive" appraisal of the atmosphere. The skies over northern Nevada are clear at the time of this radar sampling. The small "echoes" that we are seeing on the radar are most likely produced by biological "targets" such as migrating birds and insects.


(RAX RADAR) This WSR88D radar product, from the Raleigh, North Carolina NWS Weather Forecast Office, represents storm total rainfall associated with Hurricane Floyd. This image depicts estimated rainfall for the period from 1430 UTC on 14 September 1999 through 1938 UTC on 16 September 1999. Notice several large pockets of estimated 15 inch rainfall totals over central and eastern North Carolina south through east and northeast of Raleigh. It should be pointed out that the WSR88D precipitation products sometimes underestimate rainfall associated with tropical cyclones; in this case, rain gauges picked up closer to 21 inches of rain in some of the heavier areas of rainfall. In any case, this landfalling hurricane caused historic flooding over central and eastern North Carolina. (IMAGE COURTESY: NOAA/National Climatic Data Center)




Operating the country's system of environmental ( weather ) satellites is one of the major responsibilities of the National Oceanic and Atmospheric Administration's (NOAA's) National Environmental Satellite, Data, and Information Service (NESDIS). NESDIS operates the satellites and manages the processing and distribution of the millions of bits of data and images theses satellites produce daily. The primary customer is NOAA's National Weather Service, which uses satellite data to create forecasts for the public, television, radio, and weather advisory services. Satellite information is also shared with various Federal agencies, such as the Departments of Agriculture, Interior, Defense, and Transportation; with other countries, such as Japan, India, and Russia, and members of the European Space Agency (ESA) and the United Kingdom Meteorological Office; and with the private sector.

NOAA's operational weather satellite system is composed of two types of satellites: geostationary operational environmental satellites (GOES) for short-range warning and "now-casting" and polar-orbiting satellites for longer-term forecasting. Both kinds of satellite are necessary for providing a complete global weather monitoring system.

A new series of GOES and polar-orbiting satellites has been developed for NOAA by the National Aeronautics and Space Administration (NASA). The new GOES-I through M series provide higher spatial and temporal resolution images and full-time operational soundings. The polar-orbiting meteorological satellites (beginning with NOAA-K in 1998) will provide improved atmospheric temperature and moisture data in all weather situations. This new technology will help provide the National Weather Service the most advanced weather forecast system in the world.

Geostationary Operational Environmental Satellites (GOES) GOES satellites provide the kind of continuous monitoring necessary for intensive data analysis. They circle the Earth in a geosynchronous orbit, which means they orbit the equatorial plane of the Earth at a speed matching the Earth's rotation. This allows them to hover continuously over one position on the surface. The geosynchronous plane is about 35,800 km (22,300 miles) above the Earth, high enough to allow the satellites a full-disc view of the Earth. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop the GOES satellites are able to monitor storm development and track their movements.

GOES satellite imagery is also used to estimate rainfall during the thunderstorms and hurricanes for flash flood warnings, as well as estimates snowfall accumulations and overall extent of snow cover. Such data help meteorologists issue winter storm warnings and spring snow melt advisories. Satellite sensors also detect ice fields and map the movements of sea and lake ice.

NASA launched the first GOES for NOAA in 1975 and followed it with another in 1977. Currently, the United States is operating GOES-8 and GOES-10, launched in 1997. GOES-9 (which malfunctioned in 1998) is being stored in orbit to replace either GOES-8 or GOES-10, should either fail.


The United States normally operates two meteorological satellites in geostationary orbit over the equator. Each satellite views almost a third of the Earth's surface: one monitors North and South America and most of the Atlantic Ocean, the other North America and the Pacific Ocean basin. GOES-8 (or GOES-East) is positioned at 75 W longitude and the equator, while GOES-10 (or GOES-West) is positioned at 135 W longitude and the equator. The two operate together to produce a full-face picture of the Earth, day and night. Coverage extends approximately from 20 W longitude to 165 E longitude. This figure shows the coverage provided by each satellite.

The main mission is carried out by the primary instruments, the Imager and the Sounder. The imager is a multichannel instrument that senses radiant energy and reflected solar energy from the Earth's surface and atmosphere. The Sounder provides data to determine the vertical temperature and moisture profile of the atmosphere, surface and cloud top temperatures, and ozone distribution. Other instruments on board the spacecraft are a Search and Rescue transponder, a data collection and relay system for ground-based data platforms, and a space environment monitor. The latter consists of a magnetometer, an X-ray sensor, a high energy proton and alpha detector, and an energetic particles sensor. All are used for monitoring the near-Earth space environment or solar "weather."

GOES-10 Characteristics
Main body: 2.0m (6.6 ft) by 2.1m (6.9 ft) by 2.3m (7.5 ft)
Solar array: 4.8m (15.8 ft) by 2.7m (8.9 feet)
Weight at liftoff: 2105 kg (4641 pounds)
Launch vehicle: Atlas I
Launch date: April 25, 1997 Cape Canaveral Air Station, FL
Orbital information: Type: Geosynchronous
Altitude: 35, 786 km (22, 236 statute miles)
Period: 1,436 minutes
Inclination: 0.41 degrees
Sensors: Imager
Space Environment Monitor (SEM)
Data Collection System (DCS)
Search and Rescue (SAR) Transponder

The United States reaps many benefits from the new series of GOES satellites as they aid forecasters in providing better advanced warnings of thunderstorms, flash floods, hurricanes, and other severe weather. The GOES-I series provide meteorologists and hydrologists with detailed weather measurements, more frequent imagery, and new types of atmospheric soundings. The data gathered by the GOES satellites, combined with that from new Doppler radars and sophisticated communications systems make for improved forecasts and weather warnings that save lives, protect property, and benefit agricultural and a variety of commercial interests.

For users who establish their own direct readout receiving station, the GOES satellites transmit low resolution imagery in the WEFAX service. Highest resolution Imager and Sounder data is found in the GVAR primary data user service.


Complementing the geostationary satellites are two polar-orbiting satellites known as Advanced Television Infrared Observation Satellite (TIROS-N or ATN), constantly circling the Earth in an almost north-south orbit, passing close to both poles. The orbits are circular, with an alitude between 830 (morning orbit) and 870 (afternoon orbit) km, and are sun synchronous. One satellite crosses the equator at 7:30 a.m. local time, the other at 1:40 p.m. local time. The circular orbit permits uniform data acquisition by the satellite and efficient control of the satellite by the NOAA Command and Data Acquisition (CDA) stations located near Fairbanks, Alaska and Wallops Island, Virginia. Operating as pair, these satellites ensure that data for any region of the Earth are no more than six hours old.

A suite of instruments is able to measure many parameters of the Earth's atmosphere, its surface, cloud cover, incoming solar protons, positive ions, electron-flux density, and the energy spectrum at the satellite altitude. As a part of the mission, the satellites can receive, process and retransmit data from Search and Rescue beacon transmitters, and automatic data collection platforms on land, ocean buoys, or aboard free-floating balloons. The primary instrument aboard the satellite is the Advanced Very High Resolution Radiometer or AVHRR.


NOAA-15 Characteristics
Main body: 4.2m (13.75 ft) long, 1.88m (6.2 ft) diameter
Solar array: 2.73m (8.96 ft) by 6.14m (20.16 ft)
Weight at liftoff: 2231.7 kg (4920 pounds) including 756.7 kg of expendable fuel
Launch vehicle: Lockheed Martin Titan II
Launch date: May 13, 1998  Vandenburg Air Force Base, CA
Orbital information: Type: sun synchronous
Altitude: 833 km
Period: 101.2 minutes
Inclination: 98.70 degrees
Sensors: Advanced Very High Resolution Radiometer (AVHRR/3)
Advanced Microwave Sounding Unit-A (AMSU-A)
Advanced Microwave Sounding Unit-B (AMSU-B)
High Resolution Infrared Radiation Sounder (HIRS/3)
Space Environment Monitor (SEM/2)
Search and Rescue (SAR) Repeater and Processor
Data Collection System (DCS/2)

The polar orbiters are able to monitor the entire Earth, tracking atmospheric variables and providing atmospheric data and cloud images. They track weather conditions that eventually affect the weather and climate of the United States. The satellites provide visible and infrared radiometer data that are used for imaging purposes, radiation measurements, and temperature profiles. The polar orbiters' ultraviolet sensors also provide ozone levels in the atmosphere and are able to detect the "ozone hole" over Antarctica during mid-September to mid-November. These satellites send more than 16,000 global measurements daily via NOAA's CDA station to NOAA computers, adding valuable information for forecasting models, especially for remote ocean areas, where conventional data are lacking.

Currently, NOAA is operating two polar orbiters: NOAA-14 launched in December 1994 and a new series of polar orbiters, with improved sensors, which began with the launch of NOAA-15 in May 1998. NOAA-12 continues transmitting APT and HRPT data as a stand-by satellite.

For users who want to establish their own direct readout receiving station, low resolution imagery data is available in the Automatic Picture Transmission (APT) service, while the highest resolution data is transmitted in the High Resolution Picture Transmission (HRPT) service.


NOAA assigns a letter to the satellite before it is launched, and a number once it has achieved orbit. For example, GOES-H, once in orbit, was designated GOES-7, GOES-G, which was lost at launch, was never assigned a number. The same system is used for polar orbiters; for example, NOAA-11, still in orbit, was designated NOAA-H before launch. NOAA-J became NOAA-14.