THE SATELLITE DETECTIVE
AN ACCIDENTAL DISCOVERY OF A BEAUTIFUL LAKE
by Phil Lutzak – February 2008
On a recent weekend, in my morning perusal of satellite image loops, I noticed that a small yellow spot became visible over southeastern Quebec in Canada for a few hours in the early morning (Figure 1 below). It appeared around 0845Z (3:45AM EST) and lasted until about 1245Z (7:45AM EST). Although I knew that the large area of blue color over the northern U.S. and eastern Canada was caused by very cold temperatures accompanying an arctic air mass, that yellow spot really captured my eye.
Determined to find out where this little "yellow spot" was, and what caused it, I looked up the local surface map for 12Z, or about 7AM local time. Figure 2a, below left, shows the corresponding surface weather map, with the area where the "yellow spot" appeared to be. Notice that, although there are a lot of "m", or "missing", sky conditions present, weather stations near the suspect area show clear to partly cloudy skies with light or calm winds. Coupled with the fact that high pressure was cresting overhead, I knew it was not an area of clouds, but most likely a local cold spot. I then looked up the terrain on Google Earth (Figure 2b) and easily found the culprit - a large lake right in the suspect area. This cold spot is beautiful Lac St. Jean, Quebec.
Now I knew where it was. But what caused it? Why is Lac St. Jean much colder than most surrounding areas? A large, frozen lake such as this will radiate heat energy away rapidly in all directions at night, and this would have contributed to its cooling more than the surrounding areas, but only if there was little snow cover on the frozen Lake and the surrounding land. However this 2008-02-22 snow cover chart shows that the lake and environs were all well snow covered, so that explanation does not seem realistic. Looking further for the solution, with help from the astute Professor Lee Grenci of Penn State University, I discovered that Lac St. Jean (average elevation ~ 400 feet) is located in a valley surrounded by 500 to 1100 foot mountains (see Figures 3a and 3b). This means that at night, as the air cools, the heavier, more dense cold air will drain from the surrounding mountains down into the "bowl" where the lake is. This is the true source of most of the very cold pool of air that sits over Lac St. Jean on cold, clear, and calm nights.
In order to document that the temperatures in the area corroborated the evidence of a local cold pool, I looked for cities with weather reporting stations at and near the lake area. The closest location I could find near the Lake was Normandin, which is a few miles from the northwest corner of the lake. In order to see if temperatures were colder both north and south of the Lake area, I located a reporting city northeast of Lac St. Jean called Chutes Des Passes, and one to the southwest called Parent. These cities are highlighted on the HPC surface map and Google Earth map in Figures 4a and 4b below. Unfortunately, the HPC surface map does not display data from Normandin or any other weather station close to Lac St. Jean.
Finally, look at the infrared satellite color key in Figure 6 below, which matches the temperatures the satellite detects with various colors. It is of interest that the point in the color key where blue changes to yellow is -30C, and that is exactly what happened in our satellite images from Figure 1. As the temperature fell below -30C at Lac St. Jean between about 9 and 13Z, the colors over that area on the image turned yellow. As the temperature rose above 30C shortly before 13Z, the image color returned to blue. It's very satisfying when the science works out so well, and I "discovered" a beautiful Lake in the process!