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The trick is understanding what it is we are looking at when we look at radar imagery. Just as it is hard for most of us to make heads or tails of the video produced by en ultrasound, looking at a radar return isn't always as simple as looking at it and saying, "Yep, there's the thunderstorm! There are the clouds" The experienced meteorologist will certainly be able to glance at a radar image and quickly be able to make out the important features, but most of us have to study the images quite a bit closer.

The experienced meteorologist should always be your first source of information. These pages regarding radar interpretation are meant to help you better understand what you're looking at--it's nor our intention to replace the meteorologist or provide you with sufficient information to skillfully interpret radar images. Meteorology and radar interpretation are fields and, like any field, require experience. It is good to understand the concepts but when it comes right down to it this is a very complicated topic and the ability of you or me to interpret images should always defer to the educated conclusions of those that have experience working with this technology and do this for a living.

BACKGROUND

The main difficulty most of us will have when looking at NEXRAD imagery--especially initially-- is differentiating between what is real and what isn't. Or, rather, what is important and what isn't.

NEXRAD is so sensitive that it is capable of picking up birds, insects, smoke... even dust, pollen, pollution and temperature inversions. This is especially true in "Clear Air Mode" (more on that soon) where NEXRAD's sensitivity is even further increased.

So there are two main things we need to be able to do when looking at NEXRAD images:

1. Recognize real meteorological objects and be able to identify them.
2. Identify known radar anomalies produced by birds, insects, smoke, dust, temperature inversions, radar failures, etc.

RADAR ANOMALIES

We won't talk much about radar anomalies on this page. They are the subject of many pages, the introduction to which can be found here or by clicking the appropriate link on the left sidebar. However, it is important that the reader visit the anomaly pages after reading this section on interpreting NEXRAD imagery so that he or she may recognize certain patterns that are clearly anomalies rather than real meteorological conditions.

OPERATING MODES

NEXRAD operates in one of two modes depending on prevailing meteorological conditions.

Clear Air Mode: NEXRAD operates in this mode when there isn't any significant nearby meteorological activity. For example, a clear and calm evening will usually result in the NEXRAD site operating in Clear Air Mode. In this mode the radar upates its image about every 10 minutes and its sensitivity is set to the highest setting. In this mode it is not unusual to pick up a significant amount of non-weather anomalies such as ground clutter, dust and generally insignificant weather phenomenon such as temperature inversions, fog, etc.

Precipitation Mode: NEXRAD operates in this mode when there is a significant amount of precipitation or weather activity nearby. In this mode the radar updates its image every 5-6 minutes (twice as often) and reduces its sensitivity so that radar returns are (hopefully) limited to significant meteorological conditions related to the precipitation.

NEXRAD PRODUCTS

NEXRAD produces two types of images which are known as "products." Each of these products has a distinct appearance which must be interpreted differently.

Reflectivity: This is the product most often seen on websites and which you will normally see on TV weather reports. It is a color-coded image such that certain colors reflect light rain, other colors reflect heavy rain, etc. This is useful for determining the location and severity of clouds and thundercells. If you wish to learn about interpreting a Reflectivity image, click here.

Velocity: This product is less popular on websites and seldom shown on TV, but provides very useful information about the internal makeup of a cloud or storm. The Velocity product shows the speed of objects (raindrops, dust, hail) in relation to the NEXRAD site. In short, this allows the interpreter to detect the motion of these objects which will often give you a good idea of the speed of the wind. It is also useful for detecting microbursts and other phenomenon which would otherwise be invisible to weather watchers. If you wish to learn about interpreting a Velocity image, click here.