Modern Doppler weather radar sends out pulses of electromagnetic energy. When this energy encounters particles like raindrops or hailstones, a portion scatters back to the receiver. The strength of this returned signal is called reflectivity, which provides information about the presence and intensity of precipitation. Reflectivity is measured on a logarithmic scale called decibels of Z (dBZ). Different dBZ values are assigned specific colors, visually representing how strongly the radar beam is being reflected by particles in the air.
Understanding the Standard Reflectivity Scale
The reflectivity scale uses a progressive sequence of colors to indicate precipitation intensity. Low dBZ values, typically ranging from 5 to 30, are represented by shades of blue and green. These colors signify light precipitation, such as mist, drizzle, or light rain.
As dBZ values increase, the colors transition through the spectrum, reflecting a growing amount of water or ice content. Yellows and oranges, corresponding to values between 30 and 50 dBZ, indicate moderate to heavy rainfall.
The scale continues to progress into the reds, which represent very heavy rainfall, usually above 50 dBZ. These red areas are commonly associated with robust thunderstorms where the concentration of large raindrops is high. This progression establishes the visual language for interpreting the most extreme colors.
The Meaning of Extreme Intensity Colors
The appearance of pink and purple on a reflectivity map signifies the highest end of the dBZ scale, often indicating values of 60 dBZ and above. This extreme intensity means the radar is detecting a massive amount of reflected energy. These colors are characteristic of the most severe weather phenomena and signal powerful, potentially dangerous atmospheric conditions.
In most cases, the presence of pink or purple is a strong indicator of large hail within the storm structure. Hail is a much more efficient reflector of the radar’s energy than water droplets, and the larger the hailstone, the higher the dBZ value it produces. A reflectivity value between 60 and 65 dBZ suggests the presence of hailstones approximately one inch in diameter, though the exact size can vary.
Within a severe thunderstorm, these colors often highlight the storm’s core, where the strongest updrafts are located. These powerful upward currents suspend large hailstones high in the atmosphere, allowing them to grow before they fall. Furthermore, in a strong, tornadic thunderstorm, these extreme colors can sometimes represent a debris ball—a concentration of debris lifted by a tornado and detected by the radar.
Context Matters: When Pink or Purple is Not Severe Weather
While pink and purple are generally associated with extreme precipitation intensity, their meaning changes entirely when viewing a different type of radar product, such as a velocity map. Velocity products measure the speed and direction of wind relative to the radar, using colors like green for motion toward the radar and red for motion away. On these maps, a pink or purple color often indicates a technical error or artifact known as range folding.
Range folding occurs when the radar receives an echo from an earlier pulse during the listening period for the current pulse. This technical limitation prevents the radar from determining the wind’s true speed or distance. The resulting pink or purple coloration signifies that the data is unusable or ambiguous in that specific location, meaning the radar measurement failed.
Non-meteorological echoes can also cause extreme reflectivity values, resulting in pink or purple coloration on the map. Ground clutter, which is the radar beam reflecting off stationary objects like buildings, hills, or towers close to the radar site, can produce very high dBZ values. These returns are not rain, but they are strong reflections, and they typically appear as stationary, circular blobs near the radar’s center.
Large groups of biological targets, such as migrating birds or swarms of insects, can also produce high reflectivity that shows up as pink or purple. These biological echoes are often transient and move slowly or erratically compared to a storm. Meteorologists use specialized dual-polarization radar data to distinguish these non-weather targets from actual severe precipitation.