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Analysis of the -16°F Record Low at Laramie, Wyoming (27-Oct-2011)

Tony Bergantino, Wyoming State Climate Office

Temperatures (Click Graph to Enlarge)

On 27 October 2011, the temperature at the Laramie Airport dropped to a low of -16°F. The temperature that morning was the lowest in the nation and also broke the record at Laramie for the date. It was only 2 degrees above the all-time October low of -18°F set on the 30th in 1993. This -16°F temperature was the result of a wind shift that started to take place at 0543 MDT. At that time the wind was coming from a bearing of 205°. By 0545 the bearing was from 268° where it remained briefly before shifting again to about 200° for less than five minutes. A second shift again brought the wind from a bearing of 260° to 265°

This shift and resulting wind direction brought the airflow inline with the axis of the Big Hollow. The Big Hollow is a long wind-scoured basin to the west of the airport. It was formed during the Late Pleistocene and, though less than 10 miles in length, it is the largest such basin in North America. The northern rim bears about 80°/260° and the southern rim roughly 60°/240° creating a funnel west to east.

West of this scour is Lake Hattie while a half dozen other smaller lakes (often dry) run along the bottom. KLAR, the Laramie Airport station (ASOS), sits at about 7280ft elevation on a bench about 0.5 mile east of the eastern rim of the hollow. The hollow is about 2 miles wide near the eastern end. The eastern shore of the first of the lakes in the hollow (from the east) is about 2.25 miles from the eastern rim or about 2.75 miles west of the ASOS. The surface elevation of this lake is about 7130ft or 150ft lower than the ASOS.

Setting (Click Map to Enlarge)

Cold air accumulated in this sink during the morning of the 27th and, for much of the morning, a 5- to 10-knot wind was blowing from the southeast and the temperature at KLAR was fluctuating around 0°F. During the early morning hours there were a few quick shifts in direction, one of which briefly brought the wind from the south followed by another shift that brought the wind from the east for about as long (about 10 minutes each time). See map.

Starting at about 0445, approximately an hour before it's rapid westerly shift, the wind began a gradual change in direction, coming from about 150° to about 200° over the course of about 50 minutes. The wind remained coming from this bearing for about five minutes with the air temperature at 0°F and then made a sharp shift in about two minutes so that it was coming right through the Big Hollow.

By 0557 the temperature at the airport had dropped to -13°F as a result of the wind pushing the cold air out of the hollow and across the ASOS temperature sensor. After about 10 minutes the wind shifted a bit to being more southwesterly where it remained for another 15 minutes. There was another brief shift to 280° followed by a continued bearing of about 250° during which the final drop to -16°F occurred. By 1530 the temperature had worked its way up to 33°F.