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How to detect freezing rain and freezing drizzle

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roe-dl edited this page Jun 30, 2023 · 5 revisions
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Preface

Freezing precipitation is a very dangerous weather situation. It can cause major accidents, especially in traffic. So it is important to know about it. A disdrometer cannot detect freezing rain as it is still rain what passes the detecting area of the device.

Hardware

There are devices available in the market to detect freezing precipitation. They consist of an oscillating rod that is exposed to the weather. If the rain freezes on the surface of the rod, the frequency changes. That change is detected. When the measuring time has passed a heater removes the ice from the rod, and the measuring cycle starts all again.

Such devices are very expensive, and even the governmental weather services do not generally use them.

Software

The means to detect freezing precipitation in software is to put the readings of different observation types together to get an estimate if the rain or drizzle is freezing or not.

This extension implements a rule called "frost indicator", defined by the German Weather Service DWD. If it rains or drizzles and the frost indicator is ON, freezing precipitation is assumed.

Frost indicator

The following table is processed top-down. The first rule that is met wins.

air temp 2m air temp 5cm soil temp 5cm frost indicator
<0°C <0°C any ON
any any <-0,5°C ON
other other other OFF
n/a ≥0°C n/a n/a
≥0°C n/a n/a n/a
n/a n/a n/a n/a

References

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