Latest on the Freezing Temperatures Across the Region:
Freezing temperatures continue to spread across the upper Midwest. The image below from the Midwest Regional Climate Center (MRCC) indicates locations that have already experienced low temperatures at or below freezing (32F). However, a widespread killing freeze (temperatures at or below 28F for several hours) has not yet occurred in many locations.
Thus, the NWS in Aberdeen will continue issuing Freeze Watches/Warnings until October 15th - or until a hard freeze occurs. Note: Frost Advisories will no longer be issued.
From the Science Corner:
One question that commonly 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 (e.g. near the ground). On certain nights, the temperature at the thermometer level (a few feet above ground) remains slightly above freezing, while temperature where the frost forms is in fact at or below freezing!
In other words, the height of the thermometer on 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 2-meter thermometer height. To illustrate this example, the NWS in Aberdeen conducted a simple experiment back on October 2nd, 2008 - during 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 3-5 mph can keep the atmosphere "mixed," thus preventing widespread frost formation.
Mike Fowle - SOO