Yet More Stormwatcher Images!

How Doppler Radar Works

NEXRAD (Next Generation Radar) can measure both precipitation and wind. The radar emits a short pulse of energy, and if the pulse strike an object (raindrop, snowflake, bug, bird, etc), the radar waves are scattered in all directions. A small portion of that scattered energy is directed back toward the radar.

This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned radar waves, time it took to travel to the object and back, and frequency shift of the pulse. The ability to detect the "shift in the frequency" of the pulse of energy makes NEXRAD a Doppler radar. The frequency of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.). This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. You have most likely experienced the "Doppler effect" around trains.

As a train passes your location, you may have noticed the pitch in the train's whistle changing from high to low. As the train approaches, the sound waves that make up the whistle are compressed making the pitch higher than if the train was stationary. Likewise, as the train moves away from you, the sound waves are stretched, lowering the pitch of the whistle. The faster the train moves, the greater the change in the whistle's pitch as it passes your location.

The same effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes an object and is reflected back toward the radar. The radar's computers measure the frequency change of the reflected pulse of energy which then convert that change to a velocity of the object, either toward or from the radar. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to "see" the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice.

The National Weather Service's Doppler radars can detect most precipitation within approximately 90 mi of the radar, and intense rain or snow within approximately 155 mi. However, light rain, light snow, or drizzle from shallow cloud weather systems are not necessarily detected. Echoes from objects like buildings and hills appear in almost all radar reflectivity images. This "ground clutter" generally appears within a radius of 25 miles of the radar as a roughly circular region with a random pattern.

Precipitation intensity is measured by a ground-based radar that bounces radar waves off of precipitation. The Local Radar base reflectivity product is a display of echo intensity (reflectivity) measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). "Reflectivity" is the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. Base reflectivity images are available at several different elevation angles (tilts) of the antenna; the base reflectivity image currently available on this website is from the lowest "tilt" angle (0.5�).

The maximum range of the base reflectivity product is 143 miles (230 km) from the radar location. This image will not show echoes that are more distant than 143 miles, even though precipitation may be occurring at these greater distances. To determine if precipitation is occurring at greater distances, link to an adjacent radar. In addition, the radar image will not show echos from precipitation that lies outside the radar's beam, either because the precipitation is too high above the radar, or because it is so close to the Earth's surface that it lies beneath the radar's beam.

Clear Air Mode

In this mode, the radar is in its most sensitive operation. This mode has the slowest antenna rotation rate which permits the radar to sample a given volume of the atmosphere longer. This increased sampling increases the radar's sensitivity and ability to detect smaller objects in the atmosphere than in precipitation mode. A lot of what you will see in clear air mode will be airborne dust and particulate matter. Also, snow does not reflect energy sent from the radar very well. Therefore, clear air mode will occasionally be used for the detection of light snow. In clear air mode, the radar products update every 10 minutes.

Precipitation Mode

When rain is occurring, the radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning signals. In Precipitation Mode, the radar products update every 6 minutes.

The dBZ Scale

The colors on the legend are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used.

The dBZ values increase as the strength of the signal returned to the radar increases. Each reflectivity image you see includes one of two color scales. One scale represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). The other scale represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75).

The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rainrate. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rain rates which are associated to the dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.

Under highly stable atmospheric conditions (typically on calm, clear nights), the radar beam can be refracted almost directly into the ground at some distance from the radar, resulting in an area of intense-looking echoes. This "anomalous propagation " phenomenon (commonly known as AP) is much less common than ground clutter. Certain sites situated at low elevations on coastlines regularly detect "sea return", a phenomenon similar to ground clutter except that the echoes come from ocean waves.

Radar returns from birds, insects, and aircraft are also rather common. Echoes from migrating birds regularly appear during nighttime hours between late February and late May, and again from August through early November. Return from insects is sometimes apparent during July and August. The apparent intensity and areal coverage of these features is partly dependent on radio propagation conditions, but they usually appear within 30 miles of the radar and produce reflectivities of <30 dBZ.

However, during the peaks of the bird migration seasons, in April and early September, extensive areas of the south-central U.S. may be covered by such echoes. Finally, aircraft often appear as "point targets" far from the radar.

A portion of this explanation was supplied by The National Weather Service.

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Q. What is the Saffir-Simpson Hurricane Scale?

A. The Saffir-Simpson Hurricane Scale is used in public hurricane releases to classify hurricanes according to their potential for generating property damage and flooding in coastal areas. The following are the five classifications assigned to hurricanes and a discussion of each:

Category I: A Category One hurricane produces winds of 74 to 95 mph and/or a storm surge 4 to 5 feet above normal. No real damage to buildings is likely. Some damage may be expected to unanchored mobile homes, shrubbery, and trees. Some coastal road flooding and minor pier damage may be expected.

Category II: A Category Two hurricane produces winds of 96 to 110 mph and/or a storm surge 6 to 8 feet above normal. Buildings will receive some roof, door and window damage. Considerable damage to vegetation, mobile homes and piers will occur. Coastal and low-lying escape routes likely will flood 2 to 4 hours before arrival of the hurricane center. Small craft in unprotected anchorages will lose moorings.

Category III: A Category Three hurricane generates winds of 111 to 130 mph and/or a storm surge 9 to 12 feet above normal. Structural damage to residences and utility buildings will occur and mobile homes frequently are destroyed. Flooding near the coast destroys small structures and larger structures are damaged by floating debris. Terrain lower than 5 feet above sea level is flooded 8 or more miles inland.

Category IV: A Category Four hurricane produces winds of 131 to 155 mph and/or a storm surge 13 to 18 feet above normal. Extensive outside wall failure with complete roof failure on small residences will occur. Major erosions of beaches and major damage to the lower floors of buildings near the shore is likely. Terrain continuously lower than 10 feet above sea level may be flooded and evacuation of residential areas as far inland as 6 miles may be required.

Category V: A Category Five hurricane produces winds greater than 155 mph and/or a storm surge greater than 18 feet above normal. Complete roof failure will occur on many residences and industrial buildings and some complete destruction of small utility buildings can be expected. Major damage is likely to lower floors of structures located less than 15 feet above sea level and within 500 yards of the shoreline. Evacuation of residential areas on low ground within 10 miles of the shoreline may be required.