Doppler radar is like a normal radar in that it can detect areas of precipitation and measure the speed of falling precipitation. However, Doppler radar can do so much more; it can actually measure the speed at which precipitation is moving horizontally towards or away from the radar antenna. Because precipitation particles are carried by the wind, Doppler radar can peer into a severe storm and unveil its winds.

Doppler radar works on the principle that, as precipitation moves toward or away from the antenna, the returning radar pulse will change in frequency. A similar change occurs when the high-pitched sounds (high frequency) of an approaching noise source, such as a siren or train whistle, becomes lower in pitch (low frequency) after it passes by the person hearing it. This change in frequency is called the Doppler Shift and this, of course, is where the Doppler gets its name.

A single Doppler radar cannot detect winds that blow parallel to the antenna. Consequently, two or more units probing the same thunderstorm are able to give a three-dimensional picture of the winds within the storm. To help distinguish the storm's air motions, wind velocities can be displayed in color. Color contouring the wind field gives a good picture of the storm.

What appears on Digital Doppler XT is commonly referred to as base reflectivity imagery. This base reflectivity imagery displays the location and intensity of the radar echoes picked up by Digital Doppler XT. This display (in the various colors) is in units of DBZ or decibels. The higher the DBZ value, the larger the object. For example, hail and large raindrops would produce high DBZ values. In most cases, values greater than 15 are usually an indication that precipitation is reaching the ground. Likewise, values under 15 indicate precipitation evaporating before it reaches the ground.

Even a single Doppler radar can uncover many of the features of a severe thunderstorm. For example, studies conducted in the 1970's revealed, for the first time, the existence of the swirling winds of the mesocyclone inside tornado-producing thunderstorms. Mesocyclones have a distinct image (signature) on the radar display. About half of all the Mesocyclones identified have been associated with tornadoes. The time between mesocyclone identification and the tornado actually touching the ground is about 20 minutes.

Because the Doppler radar shows air motions within a storm, it can help to identify the magnitude of other severe weather phenomena, such as gust fronts, microbursts, and wind shears that are dangerous to aircraft. Certainly, as Doppler radar becomes part of the major radar network, our understanding of the processes that generate severe thunderstorms will be enhanced, and hopefully there will be an even better tornado and severe storm-warning system, resulting in fewer deaths and injuries.

The advantages of the Digital Doppler XT:

Substantially increase tornado warning lead time. Improve the detection and measurement of damaging winds, severe turbulence, wind shear and hail storms. Improve the forecast of the location and severity of thunderstorms. Increase the accuracy of identifying areas that are threatened. Substantially reduce the number of incorrect forecasts and false alarms.

Increase the accuracy of rainfall estimates for flash flood warnings. Improve water resource management and river flood forecasts.