A Rise in Mid-Winter Thaws on the Summit

Thaw Events: Eve Cinquino – Bates College (graduated), B.S. Computer Science

Observer Guide: AJ Grimes

Mentor: Eric Kelsey, PhD Mount Washington Observatory/Plymouth State University

Commonly referred to as a “January Thaw” for its common occurrence in January, periods of unusually warm weather in the middle of the winter season can spell trouble for winter snowpacks. These episodes can create unpleasant and dangerous skiing conditions as well as increase avalanche likelihood, leading to a decline in skier turnout and an increased risk for those who still choose to ski. The mountain ecosystem on Mount Washington also relies on a thick, insulating layer of snowpack to protect vegetation from harsh conditions. The most important thaw events for snow melt occur when both temperature and dewpoint are above freezing, which leads to warm, wet weather that can rapidly melt the snowpack. Under these conditions, water vapor condenses onto the snowpack in the same way that water vapor condenses onto a cold soda can on a humid summer day. In addition to this added liquid water to the snowpack, heat is released during condensation, which effectively warms and melts the snowpack. This project examines how both temperature and dewpoint related thaw events are changing at the summit and their effect on snow depth during meteorological winter (December-February) for 1939-2020, building off of the work of Ethan Rogers and Laura Kee, previous summit interns.

Rates of event frequency (number of events/winter), length (hours/winter), and intensity (thawing degree hours/winter; TDH/winter) were found to be significantly increasing (p < 0.05) for both temperature and dewpoint over the Observatory’s history. These results indicate that mid-winter thaws are becoming stronger and more common on the summit. The most significant trends were in intensity, with dewpoint increasing by 2.59 TDH/year and temperature increasing by 3.16 TDH/year. Temperature thaw events where dewpoint remained below freezing were calculated as well, but were not found to be increasing significantly. Average snow loss during the three types of thaw events (all temperature events, dewpoint events, and temperature events with dewpoint below freezing) was calculated as well, finding that during dewpoint events, snow melted at a rate of 0.41 cm/hr. This rate is almost double that of temperature events (0.28 cm/hr) and quadruple the rate of melt during temperature events where dewpoint remained below freezing (0.10 cm/hr), confirming that dewpoint thaw events are indeed the most detrimental for snow melt on Mount Washington.

Figure 1: Trends in winter thaw occurrences based on summit temperature, where (a) is the total events that occurred per season (DJF), (b) is the number of hours when temperature was above freezing each season, and (c) is the total temperature thawing degree hours per season. The blue dashed lines are the Sen’s slopes, or linear trends in the datasets. Error bars account for missing observations where a thaw likely occurred.

Figure 2: Trends in winter thaw occurrences based on dewpoint, where (a) is the total events that occurred per season (DJF), (b) is the number of hours when dewpoint was above freezing each season, and (c) is the total dewpoint thawing degree hours per season. The blue dashed lines are the Sen’s slopes, or linear trends in the datasets.

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