![]() ![]() Not only does the radar detect the presence of the vehicles, but also their velocities. When the radar is switched into velocity mode, the display shows red and green shades – green representing movement toward the radar and red representing movement away from the radar site: What sets doppler radar apart from first-generation traditional radar tools is that doppler radar can detect velocities that is, it can detect the speed at which droplets are moving toward or away from the radar site. Take, for instance, the case of the National Weather Service doppler radar located just outside of Chicago, Illinois. On Augthe local doppler radar was able to detect traffic flowing on Interstates 55 and 57. When a low-altitude layer of warm air developed over the region, the radar beam was refracted lower to the ground than normal. The result was the radar beam being bounced off of vehicles along the area interstates. (Click each image for a larger view). While doppler weather radar is traditionally used to detect water and ice droplets in the sky – otherwise known as clouds and precipitation – weather radar has also been shown to detect birds, buildings, vehicular traffic, airplanes, and even the smoke from the collapsing World Trade Center towers and the disintegration of the Space Shuttle following the 2003 Shuttle Columbia disaster. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.Doppler weather radar has become ubiquitous in today’s American culture as people seek to stay updated on weather conditions from home, from their cell phone, and while on the road. But on calm weather days, meteorologists have time to examine some of their tools and instruments with a different eye. Hail is a good reflector of energy and will return very high dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values. The higher the dBZ, the stronger the rainrate. Typically, light rain is occurring when the dBZ value reaches 20. The scale of dBZ values is also related to the intensity of rainfall. ![]() The value of the dBZ depends upon the mode the radar is in at the time the image was created. ![]() Notice the color on each scale remains the same in both operational modes, only the values change. The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). One scale (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). Each reflectivity image you see includes one of two color scales. The dBZ values increase as the strength of the signal returned to the radar increases. So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used. ![]() Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). "Reflectivity" is the amount of transmitted power returned to the radar receiver. The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. ![]()
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