From the Science Corner:
• Low temperatures on Thursday morning dipped into the lower to middle 30s across portions of northern South Dakota.  A few notable low temperatures are listed below:
• • Herreid (RWIS) = 30F
• • Aberdeen (ASOS) = 31F
• • Frederck (RWIS) = 32F
• • Bowdle (SDSU) = 33F
• • Mansfield (APRS) = 33F
• • Mobridge (ASOS) = 36F
• One question that commonly arises with regards to frost formation:
• The official low 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?
• There can be several reasons for this.  First, low temperatures can vary significantly over short distances.  For example, lower elevations such as valleys or river bottoms are often signifcantly cooler than higher elevations such as hills or ridgetops.  So while the "official temperature" at a specific location might have been 36F - the temperature in your location could be several degrees cooler.
• Second,  the height you are measuring the temperature is also very important!  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!
• 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.

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