Cold air in northwestern Canada
A very cold arctic air mass had been building over central Alaska and northwestern Canada during the latter half of November 2006 (surface air temperatures colder than -40 F/-40 C have been reported daily over that region since 21 November). A NOAA-15 AVHRR 10.8Âµm “IR window channel” image centered over the southern Yukon Territory (above) on 27 November (surface analysis) revealed that the coldest air (-40 to -50 C, darker blue enhancement) was settling into the lower elevations of the river valleys. Narrow lakes along and south of the Yukon Territory / British Columbia border exhibited significantly warmer IR brightness temperatures (-10 to -20 C, orange to yellow enhancement), due to heat radiating upward through the snow and ice covered lake surfaces.
A similar IR image centered a bit farther east over the Northwest Territories (below) showed warmer brightness temperatures over the higher terrain of the Mackenzie and Selwyn Mountains (-20 to -30 C, yellow to cyan enhancement) — those higher terrain features rose above the level of the strong temperature inversion which was trapping the coldest air near the surface at lower elevations. This IR image also revealed a comparatively warm signature (0 to -20 C, red to yellow enhancement) from the snow and ice covered surface of Great Bear Lake in the northern portion of the image. The 1-km resolution of the NOAA-15 AVHRR instrument showed the small-scale structure of these temperature features much better than the “4-km” resolution of GOES-11 (which had degraded to an effective resolution of about 12 km, due to the ~65 degree satellite viewing angle) — this is quite apparent looking at a NOAA-15 / GOES-11 IR image fader (Java applet).