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Bright Band problems relate to the comparative reflectivity of particular object
types (in this case, precipitation). The supporting graphic shows the different stages
of precipitation, leading to the bright band effect.
It is normal for precipitation to first form at high levels in the atmosphere. In
this region, the temperature is normally below the freezing point of water. The
precipitation forms initially as a succession of very small ice crystals which begin
to fall towards the earth's surface.
- As these high levels, ice crystals fall, they form larger ice crystals (or snowflakes).
These flakes continue to fall as snowflakes in sub-zero temperature.
- At a certain point during its descent, the snowflake must pass through a point
where the temperature rises above freezing point. At this stage it will start to melt.
The initial melting will be to the exterior of the snowflake which will develop a
water coating. However, water is approximately 9 or 10 times as reflective as ice
to microwave energy. Therefore these large wet snowflakes will show a high reflectivity.
- The highly reflective melting snow will appear to the ground weather radar
as more intense precipitation than it actually is. This is what is known as the
bright band effect (an effect of high intensity of rainfall as it passes through
the melting level).
- As the melting snow continues to fall, it further melts until it becomes pure
rain. Raindrops are small and fall faster so both the size of the drops and their
concentration is reduced. The radar reflectivity is then more realistic in terms
of intensity of the weather seen.
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