The Doppler weather radar transmits bursts of energy into the sky and listens for the signals that bounce back. This technology measures the location and movement of rain, snow, or hail, allowing us to visualize precipitation. This data is then translated into the familiar colorful maps seen on television and online.
Understanding Radar Reflectivity
The colors on a radar map are a direct visual representation of an atmospheric measurement called reflectivity. Reflectivity quantifies how much of the radar’s transmitted energy is scattered back to the dish by particles like raindrops, snowflakes, or hailstones. The size and number of these particles determine the strength of the returning signal, with larger particles reflecting more energy.
This measured return signal is converted into a logarithmic scale, known as Decibels of Z, or dBZ. Using dBZ values allows meteorologists to manage the vast range of signal strengths. Every color displayed on the map corresponds to a precise dBZ value, which relates directly to the estimated intensity of the precipitation.
The Spectrum of Intensity
The weather radar uses a spectrum of colors, progressing from cooler to warmer shades, to indicate increasing precipitation intensity. The lightest colors, typically blue and light green, represent the lowest dBZ values (5 to 30 dBZ), signifying very light rain or drizzle. These values often indicate minimal atmospheric activity that may not reach the ground.
Moving up the scale, darker greens and yellows signal moderate rainfall. When the reflectivity reaches the orange range, the precipitation is considered heavy, suggesting a significant increase in the rate of falling water. These mid-range colors (30 to 50 dBZ) represent a noticeable downpour that can reduce visibility and make travel difficult.
Interpreting the Red
The appearance of red on the weather radar indicates very heavy precipitation and high-intensity storms. Red colors are associated with dBZ values ranging from 50 to 60. This level means the radar is encountering a large concentration of precipitation particles, often indicating a torrential downpour with high rainfall rates.
High dBZ values are generated by intense thunderstorms, which carry the potential for severe weather. The presence of red warns of conditions that can lead to rapid street flooding, reduced visibility for drivers, and a greater risk of lightning activity. When the signal transitions into magenta or purple, the intensity is considered extreme, often exceeding 60 dBZ. This highest intensity is caused by large hailstones within the storm’s core, as hail is an efficient reflector of radar energy. Seeing red or purple echoes is a signal to seek secure shelter and heed weather warnings.