Seldom do we question where weather statistics come from. The figures come from a continent-wide network of observers who maintain instruments in official recording stations known as Stevenson screens. A Stevenson screen is a white box standing three feet off the ground surface. The sides of the box are covered with down-sloping slats that prevent the entry of solar radiation but allow the circulation of air. Instruments housed in these shelters are routinely calibrated to ensure accurate readings. Thus standardized, temperature and relative humidity readings provide the official records of the weather bureau that can be used to develop a picture of winter across North America.

To us, winter is cold; to the weatherman it is average and record low temperatures. Official North American weather bureau records provide a perspective describing winter (See Table 1 below). Record lows tend to be located in the high country of the western United States or in the northern regions of the continent.

Table 1. Official low temperatures for North America. Data compiled through 1976.

State or Province	Location	Elevation ft (m)	Temperature (F)	Temperature (C)
Yukon			-81	-63
Alaska			-80	-62
Montana	Rogers Pass	5,470 (1,667)	-70	-57
Wyoming	Moran	6,770 (2,063)	-63	-53
North Dakota	Parshall	1,929 (588)	-60	-51
Idaho	Island Park Dam	6,285 (1,916)	-60	-51
Colorado	Taylor Park	9,206 (2,806)	-60	-51
Colorado	Bennett	5,484 (1,672)	-60	-51

It is cold, however, in more areas than those where the record lows have occurred. The distribution of cold is perhaps better understood by a map indicating the mean January temperatures (See Figure 1 below). Notice the -1.1C (30F) isotherm (line indicating the boundary of all equal temperatures).

Figure 1. Map showing mean January temperatures (F). In January, about half of the United States Experiences temperatures below freezing.

Winter is also wind. Winds affect us in many ways, including mechanical destruction, desiccation, and temperature depression. On the average, the highest maximum winds in the continental United States blow in the region of Wyoming, with very strong winds also occurring just off the Great Lakes and the coadts (See Figure 2 below).

Figure 2. Distribution of maximum expected wind speeds (mph).

Winter is snow. Mean annual snowfall provides a useful index of the impact of winter. On the map (Figure 3 below), note the location of the 40-cm (15.7 in) line (called an isopleth) for snowfall.

Figure 3. Map showing mean annual snowfall (cm) (after Schemenaur, 1981).

Winter is also less sun. During the winter, the northern hemisphere is further from the sun than the southern hemisphere, and it receives less insolation (INcoming SOLar radiATION) (See Figure 4 below).

Figure 4. Insolation for the continental United States (cal/cm2).

The yearly date when the snowpack forms and disappears is of critical importance to organisms (Figures 5 and 6 below). When cold temperatures occur in the fall before a protective mantle of snow has formed, many of the residant small mammals and plants can freeze or die of exposure.

Figure 5. Map showing the average date of continuous snowcover formation.

Figure 6. Map showing the average date of snowcover disappearance.

Plants as well as animals have adapted to conditions of winter. Below is an example of the color phases of the North American Weasel.

Figure 7. Winter color phases exhibited by the long-tailed weasel in North America. In the northern region, weasels turn white in the winter. South of the shaded zones weasels stay brown, while within the shaded zone weasels of both color phases may be observed during the winter. The narrowness of the band in the west is a product of a small sample size and compressed vertical zonation due to the presence of high mountains (adapted from Hall, 1951).