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WEATHER LAB | How can it be dry outside when radar shows rain?

On occasion, radar will show precipitation falling, when in reality it is dry outside.

DES MOINES, Iowa — Has this ever happened to you? You check your We Are Iowa app and the radar shows rain or snow, but when you look out the window it is completely dry.

What is going on here?

Radar is a terrific tool to track precipitation, but occasionally it can be misleading.

One of the main culprits for these errors is something called virga, or precipitation that evaporates before reaching the ground due a layer of dry air in the lower atmosphere. 

Due to the curvature of the Earth, the farther away the radar beam gets from the radar, the higher up in the atmosphere it scans.

In fact, precipitation scanned 100 miles away from a radar is located about 8,000 to 10,000 feet above the ground.

Credit: WOI
On November 1, 2021, radar showed a wintry mix in parts of southern Iowa. However, most weather stations reported dry conditions.

When that precipitation is actually virga, the result is a radar map that doesn't paint a completely accurate picture.

This is most common in the winter months when the air is typically drier.

Credit: WOI

Another common radar error is ground clutter.

In most cases, temperatures get lower higher up in the atmosphere.

Sometimes, temperatures will warm with height just above the surface creating a temperature inversion.

When these develop, the air density is greater than normal.

This causes the radar beam to bend toward the Earth's surface and scan things like wind farms and even the ground itself.

Credit: WOI

As we mentioned in a previous weather lab, radar will scan any object even if it isn't precipitation.

The result is a radar map that shows fairly widespread precipitation.

There are several other possible radar errors, but they are far less common.

So the next time you see rain or snow on your app, it might be a good idea to take a peek outside or get the answers straight from the Local 5 Weather Team.

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