A while ago in a post discussing Frontal Weather, I spliced in a number of NEXRAD images including one that showed the situation within a minute of the time I touched down at KVKX.
I commented that, "...these NEXRAD images show composite reflectivity and ... the surface weather was not as evil as the depiction just above would have you think." Let's take a closer look at that.
Below are two archival NEXRAD images showing returns seen by the Sterling, VA site at 2338Z on Friday, the 26th of June. I had landed at KVKX one minute earlier. The image on the left shows Composite Reflectivity; the one on the right shows Base Reflectivity.
If you enlarge the Composite (left) image by clicking on it, you'll see pretty intense returns very close to the location of the airport. You'd be thinking that I had cut things awfully close getting in from the east at that point.
Go ahead - click on it...then use your "Back" button to come back here.
Now, if you click on the Base (right) image, you get a different impression. The intense returns are still a fair distance away from the airport and there is even a clear band immediately adjacent to the field. In fact, there was no precipitation when I touched down. I had a pretty good margin. Have a look.
The NEXRAD weather radar system obtains a layered array of reflections in increments of 1/2 degree of elevation. The Base Reflectivity image is just the totality of reflections in the lowest 1/2 degree (i.e. pretty much horizontal). If you want to know what is going on near the ground you look at the Base Reflectivity image.
The Composite Reflectivity image is a synthesis that displays the most intense return obtained at any elevation above a given point on the map. So, if moderate precipitation exists 3,000 feet overhead, the system will paint the pixel corresponding to where you are standing yellow even if none of the precip is reaching the ground.
The NEXRAD image displayed on my Garmin GPSMap 396 (via XM Weather) shows Composite Reflectivity. (Note that the XM image is processed and synthesized a little differently than the National Weather Service (NWS) images shown above, but the principle is the same.) For cruise flight, you want a Composite Reflectivity image because that will give you "worst case" information and allow conservative decisions. If you were to rely on a Base Reflectivity image in planning your progress you could find a very unpleasant surprise waiting for you at 8,000 MSL.
Lots more interesting information on weather radar is available from NWS at this site. It's fascinating stuff and real-world useful. Highly recommended.