I got this image one May 1st. May 1st, 2003, with Casablanca Radar, in Havana. It was an unusual late cold front. Some years later, my friend Daniel Ernesto commented that he got very wet being at Plaza de la Revolución during the May 1st demostration. Bueno, para mis lectores hispano parlantes: este post va en inglés, porque se trata de un fenómeno de radar raro en Cuba, más propio de latitudes angloparlantes. Si se imaginaron que era sobre una banda de rock que va a visitar Cuba, se embarcaron. Well, frequently, I use this singular image in my conferences about Radar Meteorology. My friend Rinehart liked it so much that he used it as back cover for his book Radar for Meteorologists in its Spanish version.
The radar “bright band” is a phenomenon that shows up as higher reflectivity near the elevation of the melting layer (just around the 0 Celsius Isotherm) between snow and rain. Sorry for those who was expecting me to talk about a rock band. By the way, if talking about that kind of bands…The Beatles are my preferred, second place for the Rolling Stones, and third place for…well, I said I was not supposed to talk about that kind of band. I’m talking about the bright band.
The dielectric constant for ice is 0.197 while it is 0.93 for liquid water. This means that liquid water is considerably more “reflective” than ice for microwave radiation. The reflectivity factor (usually called Z) is proportional to the 6th power of the particle size. Since snowflakes are usually larger than raindrops, their size compensates somewhat for the lack of ice reflectivity, and they still show up pretty well on radar images (yellow in my image).
The bright band occurs in the layer where snowflakes are melting into rain. Two effects are occurring. The first is that when the snowflakes first start to melt, they melt from the outside in, so at first they have nearly the diameter of a snowflake (large) but are coated with liquid (high index of refraction), both combining for a high reflectivity. The second effect is that as the snowflakes melt and shrink into liquid water drops, their terminal velocity increases, and they start to fall faster, leaving fewer of them in the radar-sampled volume just below the melting layer. Hence the bright band becomes a vertical maximum in the reflectivity (look at the red thin line in my image).