The national radar weather difficulty.

The national radar weather offices employ is an optical instrument, which has problems similar to other optical tools. Issues such as unwanted reflections and refractions (bending). For instance, fibre-optics work by sending light down a duct of solid glass. None of the energy gets out due to the sharp changes in density (can you say "index of refraction") and the curvature at the boundary. We end up with a very efficient medium.

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What are the problems?

1. Nighttime radiation/Nocturnal inversions - temperature on clear nights INCREASES with height. Internal reflection can also cause national radar weather depictions and distant radio stations to come in more clearly, leaving a ghost-like figure on the picture. I remember as a kid, listening to Vancouver radio on summer nights in Medicine Hat, a thousand kilometres of mountains away. It worked late at night because of this effect.
2. Cool lakes and seas - the extra moisture near the surface gives the same result as above.
3. Sharp temperature and humidity changes such as those near a thunderstorm. Unfortunately this can distort the storm's image, just when the local weather radar is most needed.
4. Increasing temperature AND humidity with height - Sub-refraction. The opposite consequence to item #1 can happen and the national radar weather signals gets lost in space.




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5. Precipitation freezing or melting while falling - this forms a glossy boundary and looks very strong on the radar display. Meteorologists call this the bright band.
6. Radiation leaks from a radar - in several directions, all the time. This can make objects behind the radar, for example, look like they are in front. It picks up a reflected signal without distinguishing its direction.
7. Two or more distant objects - close to each other, will look like a single object.
8. Objects near the edge of the beam - a "still photograph" will be on the edge of the next photograph as well. When two are spliced together, the object will appear much larger than it really is.
9. The same storm would naturally appear more intense if closer to the radar. Radar systems compensate for this by installing circuitry which amplifies the signal from more distant objects.
10. The earth is not flat - its curvature requires the radar to deal with viewing storms at incorrect angles. The radar can miss part or all of a storm hiding in "the earth's shadow". Fortunately larger countries have national radar weather with multiple stations to help alleviate this problem.
11. If the shape of a storm is changed by wind, it looks to a radar like it covers more area than it actually does. Wind speeds and directions change at different heights, an effect known as "wind shear", and can stretch a storm significantly. Again, a network of national radar weather sites clarifies this somewhat.