Weather radar is a type of remote sensing instrument which estimates the intensity of
rainfall. By emitting brief pulses of energy and measuring the amount of power reflected
by weather echoes and returned to the radar, modern weather radars can detect even the
weakest of precipitation echoes.
NEXRAD is the commonly used acronym for the Next Generation of Weather Radar which began to be tested and implemented by the National Weather Service and the Federal Aviation Administration during the 1980s. These new Doppler radar systems, now more appropriately known as the WSR-88D (Weather Surveillance Radar - 1988 Doppler), have recently replaced the aging network of WSR-57 and WSR-74 radar systems which the National Weather Service and the Federal Aviation Administration had been using for the last several decades.
The WSR-88D provides several advantages over the its older predecessors, including:
The WSR-88D imagery which is available on INTELLICAST is referred to as base reflectivity imagery. Base reflectivity imagery displays the location and the intensity of the radar echoes detected by the WSR-88D. The echo intensity is displayed in units of DBZ.
The WSR-88D emits pulses of energy into the atmosphere at regular intervals. When this energy bumps into something (i.e. a raindrop, a snowflake, a mountain, etc.), some of the energy is scattered back to the radar dish. The amount of energy which is received at the radar dish is measured in units of DBZ (decibels). The higher the DBZ value the larger the object i.e. large raindrops and hail produce high DBZ values. In general, DBZ values greater than 15 indicate areas where precipitation is reaching the ground. On the other hand, DBZ values less than 15 usually are an indication of very light precipitation which in most cases is evaporating in the atmosphere before it reaches the ground.
Precipitation Intensity Scale
The WSR-88D operates in one of two modes, clear air, or precipitation. The legend on the right side of the image indicates the active operating mode, as shown below:
Generally speaking when there is no precipitation within the range of the radar site (230 Km, or about 140 miles), the radar system can be switched into "clear air" mode.
In clear air mode the sensitivity of the radar is increased dramatically. The WSR-88D can actually detect energy levels so small they are reported in terms of negative values ( i.e. -10 DBZ). This enables the radar to detect boundaries between air masses, temperature inversions in the atmosphere (warm air above cooler air), and non meteorological phenomena such as smoke plumes, and more.
So when you look out your window and see fair skies, yet the WSR-88D image is showing echoes over you, it generally means one of two things. First, check to see which mode the radar site is operating in. If it is in clear air mode the echoes on the image are simply indicating variations in the atmospheric conditions over your area, or some other non meteorological phenomena ( i.e. flock of birds or insects, smoke plumes, etc.) or ground clutter such as buildings, trees, hills, mountains, etc. If the radar is operating in precipitation mode, the echoes again may be caused by ground clutter. Ground clutter is most prevalent within 30 miles of the radar site.
The range of coverage of the WSR-88D system in base reflectivity mode is 230 km or about 140 miles. As a function of the WSR-88D no echoes are displayed beyond this distance. Often times this will result in a radar pattern ending abruptly in a circular pattern.