Weekly Radar Images

Look at these radar images along with weather maps from corresponding days to determine whether there was nocturnal bird migration.  See Below for an overview on how to interpret the radar images.

September 13 - 19 
Central New Jersey

September 13 - 19
Southern New Jersey

September 20 - 26
Central New Jersey

September 20 - 26
Southern New Jersey

September 27 - October 3
Central New Jersey

September 27 - October 3
Southern New Jersey

October 4 - 10
Central New Jersey

October 4 - 10
Southern New Jersey

October 11 - 17
Central New Jersey

October 11 - 17
Southern New Jersey

IS IT RAIN OR IS IT BIRD MIGRATION?

The radar measures returned energy, or "reflectivity."  Reflectivity magnitude is related to the number and size of the objects encountered.  On these radar maps, the objects represent birds.  The amount of reflected energy is measured in decibels (dbz), which are a unit of expressing energy.  The higher the amount of reflected energy, the higher the dbz (decibels).  In this case the decibels are recording the density of birds flying. 

Rain density is measured in decibels (dBZ), a relative scale used to represent other forms of wave energy, like sound.  The color scale and corresponding dBZ values are shown in the information bar on the right side of the image. 

Regions of light and dark blue (5-10 dBZ) show areas of lighter precipitation, while areas of orange and red (45-50 dBZ) are strong, to occasionally severe thunderstorms.

The image has a "blocky," irregular appearance, typical of precipitation.  It also shows rapid changes in reflectance values over short distances, which is characteristic of rain events, especially when they contain thunderstorms.

The image above shows a fine stippling pattern typical of birds and insects, in contrast to the “blocky” appearance of precipitation. 

The transitions between high reflectivity areas at the center, and low reflectivity areas of towards the outer boundary, are smooth and somewhat circular.  This pattern is indicative of bird migration. 

Maximum values observed during bird or insect migration are usually no higher than 30-35 dBZ. 

SO HOW DO WE KNOW THEY ARE BIRDS?

To further help identify whether the objects in the air are birds, it is important to determine at what speed they are traveling.  Other airborne objects can be detected by the radar; pollen, dust, insects, etc. all tend to travel at or about the same velocity as the wind.  Birds typically sustain flight velocity of > 10 knots above or against the wind speed. 

Base velocity map:  Greens/blues represent objects moving toward the radar receiver while yellows/reds are objects moving away.  A migration event shows a “line” that clearly separates the two sets of colors.  This line is perpendicular to the movement of the objects.

Another piece to the puzzle is looking at surface weather maps for the day.  This will help determine what the direction and speed of the wind is for the day in question.  Compare the wind speed with the velocity of the objects – if it is >10 knots faster than the wind speed, the map is showing bird migration.