© 1994-2013 David L. Martin. All rights reserved.See complete copyright statement
Where to see migrating birds on weather radar:
Go to any site on the web that shows current radar images from the National Weather Service's NEXRAD (WSR-88D) radar. The radar images look different on these sites, but they are all showing the same data. Choose the one you like best.
Choose the base reflectivity (a.k.a. 0.5 degree reflectivity) image or loop. Other types of images are not as useful for following bird migration.
|National Weather Service||National and regional image loops.|
|National Weather Service||Images and loops from individual radar stations. Link is for Albany, NY, but you can easily move to another city.|
|NCAR||Images kept for 5 days. Loops up to 12 hours long (requires Java).|
|College of DuPage||Has hydrometeor classifications. Loops up to 200 images long. Images have range circles (50NM).|
|Plymouth State||Images kept for 15 days. Two-hour loops. Images have range circles (30NM).|
How to know you are looking at migrating birds:
The heaviest migration is at night in April and May and from late August through early October. The "bird" signal will be easy to see on a clear night during migration.
- Look at a loop that includes the first two hours after sunset. On a good migration night, there will be a "bloom" of reflectivity around the radar station like a flower opening. This signal is probably mostly birds along with bats and insects.
- See if the hydrometeor classification is biological.
- Check the extent of the signal. The bird signal is usually confined within 40 miles of the radar station. (It may extend a little farther on very good migration nights.)
- Check the reflectivity. The bird signal is generally 30dBz or less. Much higher values are unlikely to be echoes from birds.
Note: Precipitation often looks patchy, extends over wide areas, and moves in bands from the direction of approaching weather fronts.
How to interpret the radar signal:
The intensity of the radar echo is called the reflectivity and is given the sumbol ’z’. It is related to the number, size and type of objects reflecting the beam. Radar images show reflectivity in units called dBz (decibels of z), which are color coded. The Cornell Lab of Ornithology uses following terms to describe migration based on the radar signal.
|5 - 10||Light Migration||59 - 71|
|10 - 20||Moderate Migration||71 - 227|
|20 - 30||Heavy Migration||227 - 1788|
|>30||Extreme Migration (rare)||>1788|
*Bird density is the number of birds per cubic kilometer. These values were originally determined by Gauthereaux and Belser (1999).
How the radar works:
As the radar antenna rotates, it emits very short pulses of microwave energy and then detects the echos of these pulses from objects in the sky. The echos are produced by all types of precipitation, birds, bats, insects, dust and aircraft.
The radar beam is a 1-degree wide cone. Although the beam may seem to be very narrow, it is more than 2700 feet wide at 30 miles from the radar station and more than a mile wide at 60 miles.
The beam can be tilted upwards at various angles from 0.5 to 19.5 degrees. The base radar beam, which is tilted only 0.5 degree, is the one that is used to detect migrating birds, because it passes the greatest distance through the altitudes where the birds fly. The higher tilt angles are not useful for looking at bird migration.
The elevation of the radar beam increases with distance due to the tilt of the beam and curvature of the earth. At 30 miles from the radar station the center of the base level beam is roughly 2000 feet above the antenna. (The elevation may be more or less than 2000 ft depending on the amount the beam is refracted by atmospheric conditions.) As illustrated in the drawing below, the beam will be underneath the birds close to the station and above the birds far away from the station. By 40 miles from the station, the base level beam will usually be above most of the birds. Because the beam near the station passes under the birds, there is often a doughnut hole of low reflectivity around the station.
As stated above radar images show reflectivity in units called dBz (decibels of z), which are logarithmic. Thus, the reflectivity at higher dBz values is exponentially greater than at lower values. For example, the reflectivity of a 30 dBz signal is 10 times greater than that of a 20 dBz signal, 100 times greater than a 10 dBz signal, and 1000 times greater than a 0 dBz signal. (0 dBz = 1 z, because the log of 1 is 0.)
A single complete scan can take from 4 to 10 minutes depending on how the radar is being operated. The Weather Service has several different protocols that use the base scan and four or more of the higher tilt scans to monitor different types of weather. The radar system collects the data from each tilt angle separately before it starts the series again. The protocol called clear air mode is the most sensitive, because the radar rotates more slowly and gathers more data.
Other types of radar images:
There are several other types of radar images, but they aren't very useful for following bird migration.
The composite reflectivity image is composed of the highest reflectivities taken from all the tilt angle scans in a complete scan.
The velocity image shows the velocity of the objects toward or away from the radar station (i.e. the radial velocity along the radar beam.) Negative and positive velocities indicate objects are moving toward and away from the radar, respectively. In principle, the velocity image could be used to distinguish between migrating birds and dust or insects, because birds flying with the wind are moving faster than the wind, and birds can fly against the wind whereas dust just goes with the wind. However, one would have to know the actual velocity of the wind, and that is not easy to know accurately.
Where to find more about how the National Weather Service radar works:
JetStream - Online School for Weather has an extensive non-technical explanation of the system.