What causes weather-based radar errors?

One of the most popular tools for forecasters and the public regarding weather is radar.

While radar imagery only continues to improve, it isn’t always perfect and sometimes displays inaccurate information.

Radar stands for Radio Detection And Ranging. Inside a weather radar is a rotating dish or antenna that emits pulses of electromagnetic waves.

When these waves encounter precipitation, such as rain, snow or ice, it reflects back to the radar. The duration that it takes for the pulses to return measures distance, while the strength of return indicates intensity. This data is then available for forecasters to analyze and is displayed on radar imagery.

A common error that can show up on radar is known as virga. Radars scan at an increasing angle and sometimes detect precipitation that evaporates in dry air before reaching the ground. This can happen year-round but is most common with dry air masses.

You can also have the opposite effect when low-level clouds producing precipitation are missed by the scan, which can indicate no precipitation is falling when it indeed is. This is most common during snow squall events when clouds don’t tower as high and at the end of the radar scan, which is a problem that consistently plagues areas like southwestern Nova Scotia.

There is also the issue of what’s known as ground clutter. Since the radar beam begins close to the surface, it can sometimes pick up things like bugs, birds and aircraft.

While programs can be used to remove most or all of this clutter, static or clusters of returns near the radar’s location can signal ground clutter.

Still, radar imagery is often very accurate and, with advancements in technology, only continues to improve.