Why is there a stationary blob on the weather radar?

Question

If you follow this link... http://radar.weather.gov/radar.php?rid=LOT&product=N0Z&overlay=11101111&loop=yes

...you will be taken to the National Weather Service's radar loop for Chicago. Centered around Joliet is a blue blob that always seems to change, but never seems to move from that location. No matter what the weather, this blob will always be there. What exactly is this?

Answer 1

This is called clutter or radar interference.  It can be the result of a number of things that are getting picked up by the radar's transmissions.  

The little moving blob that always sticks around is a result of a long radar waveguide between the radar transceiver and the antenna. The rotating antenna will usually be seen as a sunburst in the centre of the display as the receiver responds to echoes from dust particles and misguided reflections. Some other issues include reflections from birds/bats, buildings, atmopheric turbulence, dust, elevation increases, etc..