When the temperature is near 0°C, it becomes especially important to know if the precipitation is in the form of rain, freezing rain, or wet snow. At some automatic weather stations, particularly those at airports, additional sensors can be installed to gather this information. Let's take a look at the 2 pictured below, the icing detector and the precipitation occurrence sensor system (POSS). These 2 are normally used together to detect freezing rain or to differentiate between wet snow and rain.
At the very top of this instrument, you'll see a metal probe pointing upward. This probe vibrates slightly with a known frequency. As ice builds up on the sensor, the vibration frequency decreases and a signal is sent to the central computer that monitors the automatic instruments. When 2 mm of ice has accumulated on the metal probe, the sensor will de-ice itself automatically by briefly warming up.
Information from the icing detector is used with information from the precipitation occurrence sensor below to provide a more complete picture of precipitation types at sites with this equipment.
Different types of precipitation fall at different speeds. Rain, for example, falls much faster than drizzle or snow. The POSS uses a sensitive Doppler radar to measure how fast the precipitation is falling. This information, together with data from the temperature and icing sensors, is used to identify the main type of precipitation.
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