Why does frost form when it's above freezing?

How does frost form when the low temperature does not fall below freezing (32 degrees F)? 

The answer to this question is most often related to the thermometer height. In other words, what is important is not what the temperature is at the thermometer, but rather what was the temperature where the frost actually formed (i.e. near the ground). In these cases, the temperature at the thermometer height remains slightly above freezing, while temperature where the frost forms is at or below freezing!    
 

The thermometer indicates the temperature at the height of the thermometer, which at most weather stations is taken 2 meters (or about 6 feet) above ground level (AGL).  Because cold air sinks relative to warm air, and because the ground radiates heat very efficiently during calm, clear nights, the temperature at or near ground level can often be several degrees cooler than the temperature at the typical 6 foot thermometer height.   To illustrate this example, the NWS conducted a simple experiment during a clear calm night. One thermometer recorded temperatures at the standard 2 meter level, while a second thermometer was placed near ground level (directly below). The temperature trace of both thermometers is shown from during the period 3pm on day 1 through 8am on day 2. Click the image for a larger version.

Temperature Plot - Click to Enlarge

 

Note that the temperature 2 meters AGL (red trace) reaches a low temperature of 34 degrees F at 6am, while the sensor near ground level (blue trace) falls to 32 degree F from 6 to 7 am. So, while temperatures remained slightly above freezing a few feet above the ground, temperatures near the ground did in fact fall to freezing – allowing areas of frost to form.    
 

What about my car or roof – they are above the ground and there was frost on them as well? 

Certain materials like metal and glass radiate heat very efficiently, and therefore readily cool to the frost point.  In addition, cars and rooftops are often better exposed (from multiple planes) and thus radiate heat more quickly than other objects.  It should be noted that other meteorological factors such as wind speed and moisture also come into play when assessing the potential for frost.  Subtle changes in these values can also be factors why one area sees frost and another nearby location does not.  For instance, even a light wind speed of 3 or 4 mph can keep the atmosphere "mixed," thus preventing widespread frost formation.  

 
This article was written by and provided by Mike Fowle, Science Officer at the NWS Office at Aberdeen, SD. 


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