Characteristics of low-level temperature inversions, Whitehorse Yukon Territory: 1956--2003

Near-surface air temperature inversions are a common element of northern environments. Seasonal and inter-annual variations of low-level inversion characteristics were examined using 47 years of twice-daily radiosonde data from Whitehorse airport (1956-2003). Nocturnal low-level inversion events occurred throughout the year in 68% of the 0400h PST radiosonde ascents. Afternoon (1600h PST) inversion events, in contrast, were primarily a winter phenomena, limited to 24% of the readings between October and March, and were deepest and strongest during the winter months. Shorter, weaker, lower magnitude inversions appeared progressively more frequently during the 1974-2003 warming at Whitehorse. The impact (1956-2003) of regular low-level inversions on near surface temperatures in the Yukon River Valley is estimated to be an average annual reduction of -3.4°C. That is to say, in the absence of surface-based temperature inversion effects, the mean annual near-surface-temperature for Whitehorse (-0.7°C) would exceed 1°C, a temperature threshold at which permafrost could not be maintained. Overall inversion impact declined by about 1.5°C between 1956-1973 and 1974-2003