First Frost or Freeze Observed Across Much of the Area on September 29th

Many locations across central and northeastern South Dakota, and west-central Minnesota experienced their first frost or freeze of the season on September 29th. Below is a list of low temperatures from select locations across the NWS Aberdeen forecast area:

 

Location Temperature (F)
Pierre 29
Oacoma 29
Faulkton 29
Turton 30
Watertown 32
Timber Lake 32
Redfield 32
Aberdeen 33
Eureka 33
Browns Valley 34
Gettysburg 34
Sisseton 35
Murdo 36
Leola 36
Wheaton 36
Mobridge 37
Britton 37
Ortonville 37
Wilmot 37
Roscoe 37
Milbank 37

 

One question that frequently arises with regards to frost formation:

The official temperature in "My Town" was warmer than freezing (32F)...but I still saw frost.  How does frost form when the low temperature does not fall below freezing?

The answer to this question is most often related to the height you are measuring the temperature. In other words, what is important is the temperature where the frost actually formed (i.e. on or near the ground).  In cases like last night, the temperature at the thermometer (a few feet above ground) remains slightly above freezing, while temperature where the frost forms is in fact 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 last year (on October 2nd) on a night with clear calm conditions.  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 October 1st, through 8am on October 2nd.
 
 
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) does indeed fall 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.  Science in action!    
 
What about my car or roof – they are above the ground and there was frost on them as well this morning? 
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.  

 

Mike Fowle - SOO



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