The Old Farmer's Almanac 2015 (8 page)

Wow! Does this really mean that our faces will freeze in minutes while walking the dog? What does it mean?

The Windchill Temperature Index (we’ll just call it “windchill”) is an important piece of information that can come in handy for your comfort, health, and even survival. When a low temperature and high wind speed indicate a windchill at or below -25°F, the National Weather Service (NWS) issues a windchill warning. The dangers are real: Your appendages can freeze and your chance for survival is reduced, if you are not properly prepared. People are advised to bring pets indoors, children are not allowed outside during school recess, and the homeless are advised to seek shelter. Dangerously low windchill readings, especially when the ambient air temperature is below 32°F, can result in permanent injury from frostbitten skin and, when the body’s core temperature falls below 95°F, death from hypothermia.

Coldly Calculating

Windchill is not measured with an instrument; it is a calculated value based on temperature and wind speed. According to the NWS, the measurements for calculating windchill were originally, and for years, based on the research of Antarctic explorers Paul Siple and Charles Passel in 1940. Their findings were published in 1945. They measured the cooling rate of water in a small plastic cylinder attached to the top of a 33-foot-high pole. Because Siple and Passel’s results underestimated the time necessary to freeze human flesh, a new index designed to more accurately reflect how cold air feels on human skin was developed in 2001.

This calculation, still in use today, is based on the results of trials conducted by the Defence and Civil Institute of Environmental Medicine in Toronto, Canada. Six men and six women, with thermal transducers attached to their faces to measure the heat flow from their cheeks, forehead, nose, and chin, were placed in a chilled wind tunnel. Skin temperature readings were obtained at an average height of 5 feet—the typical height of the adult human face.

Temperatures used to calculate windchill are now taken at or below 50°F. Wind speed measurements are taken above 3 miles per hour.

Getting Under Your Skin

Our bodies radiate heat and generate a thin layer of warm air a few millimeters thick on the surface of our skin. When left undisturbed in low temperatures and no wind, this thin layer of air protects and insulates us from the cold. If we disturb this layer of air—for example, by blowing on a small portion of our skin—this area quickly falls to the ambient air temperature. Conversely, in warm (or hot) temperatures, such as in a sauna, blowing on the skin will disturb the protective layer of air and we will become uncomfortably warm, even hot.

Since cold is what matters most here, if you were to stand outside in a bathing suit when the temperature is 30°F (as I did in research for this article, but do not recommend), it would take a certain amount of time to lower your body temperature. If you set up a fan that moved the ambient air across your skin at 20 mph, the wind would increase your body’s rate (or speed) of cooling as if you were standing in still air at 17°F.

Here’s why this phenomenon is called the windchill effect: As the wind speed increases in cold air, it effectively increases the rate at which you lose heat from exposed skin. The wind has the effect only of cooling the skin quickly; with no wind, the skin would cool to the ambient temperature more slowly. In both calm and windy conditions, the skin’s surface temperature would never fall lower than the ambient temperature—in this case, 30°F.

It is worth pointing out that although the skin temperatures would never fall below the ambient temperature, when this temperature is below freezing, your skin would become frostbitten and eventually you would die from hypothermia.

This brought to mind my years growing up in the Berkshire Hills of Massachusetts. For several days during February 1958, when I was a third grader, the temperature fell to −10°F. Yet I vividly remember skiing on our local rope tow after school every day (and all day during school vacation) and walking home in the dark carrying my skis on my shoulder. My mother dressed me in two pairs of socks, two sets of long underwear, a sweater, a jacket, ski pants, insulated mittens, a trooper hat, a face mask, and a wool neck gaiter. (When I peeled off the last layers, I found that I was drenched in sweat.)

Nowadays, these conditions would require a warning from the National Weather Service. While a warning should be taken seriously, it should not be an excuse to cower from winter. While I am not suggesting that you don what I wore as an 8-year-old, if you know how to dress accordingly, you can enjoy the outdoors. With careful preparation, we should all be able to get outside and embrace the cold.

William L. Farrell
,
a retired mathematics teacher, writes from Dublin, New Hampshire.

These weather maps correspond to the winter (November through March) and summer (June through August) predictions in the General Weather Forecast (opposite). Forecast terms here represent deviations from the normals;
learn more
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See regional forecasts
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What’s shaping the weather? Solar Cycle 24 officially began on January 4, 2008, but there was minimal activity until early 2010. While activity has picked up since then, this cycle is the smallest in more than 100 years. As solar activity continues to decline from a low peak, we expect temperatures in much of the nation to be below normal this winter and above normal next summer. While Winter 2014–15 will not be as snowy as last winter, much of the northeastern quarter of the country still will have above-normal snowfall.

Other key factors in coming weather patterns include a continued warm phase in the Atlantic Multidecadal Oscillation (AMO), a cold phase in the North Atlantic Oscillation (NAO) during most of the winter, and the long-term cool phase of the Pacific Decadal Oscillation (PDO), despite some warm-phase intervals. Oscillations are linked ocean-atmosphere patterns that influence the weather over periods of weeks to years.

One of the keys to the upcoming winter will be the El Niño Southern Oscillation (ENSO), which many are forecasting to be in its warm El Niño phase. If this occurs, then winter will likely be much rainier than we are forecasting in California and the Southwest. We believe it more likely that El Niño will be in a weak to neutral phase.

Winter
is expected to be another cold one in the eastern half to two-thirds of the nation, with above-normal temperatures, on average, in the West. Snowfall will be above normal in most of the Northeast, although below normal in much of New England. Florida will have above-normal rainfall, while most of the southeastern and central states will have below-normal precipitation. We expect above-normal snowfall from eastern Arizona into the Big Bend of Texas and above-normal rainfall from parts of inland Washington into the northwest corner of Montana and just north of California’s Bay region. Other areas in the western third of the country, including most of California, should have below-normal precipitation.

Spring
will bring above-normal temperatures to the eastern half of the nation from late March through early May; temperatures will be below normal, generally, in the West. Precipitation will be below normal in most states near the Atlantic and Pacific coasts, but above normal in most spots in the central two-thirds of the nation.

Summer
temperatures will be above normal in most places, the main exceptions being the mid-Atlantic and southwestern states. Rainfall will be below normal in most of the nation’s midsection, which may reduce yields of corn, wheat, soybeans, and other crops grown within this area. The drought in much of California will likely continue as well, putting additional stress on our food supply.

The hurricane season
will not be particularly active. Storms that do arrive will threaten the Gulf states much more than the Atlantic coast. A major hurricane strike is most likely to occur in late August in the area from Louisiana into eastern Texas.

Autumn
temperatures will be above normal in most of the eastern half of the nation and below in the West. Precipitation will be below normal in most of the southeastern and western states and above normal elsewhere.

Learn how we make our weather predictions and get a summary of the results of our forecast for last winter.
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There will be four eclipses in 2015, two of the Sun and two of the Moon. Solar eclipses are visible only in certain areas and require eye protection to be viewed safely. Lunar eclipses are technically visible from the entire night side of Earth, but during a penumbral eclipse, the dimming of the Moon’s illumination is slight.

March 20: Total eclipse of the Sun. This eclipse will not tie visible from North America but will be visible from Greenland, Iceland, Europe, North Africa, and north western Asia.

April 4: Total eclipse of the Moon. This eclipse will be visible from North America. The eclip se will be best observed from the western states and provinces. The Moon will set during the eclipse for observers in the eastern regions. It will enter the penumbra at 5:00
A.M.
EDT and the umbra at 6:15
A.M.
The Moon will leave the umbra at 9:45
A.M.
and the penumbra at 11:01
A.M.

September 13: Partial eclipse of the Sun. This partial eclipse will not be visible from North America but will be visible from parts of southern Africa, the southern half of Madagascar, the southern Indian Ocean, and the eastern part of Antarctica.

September 27–28: Total eclipse of the Moon. This eclipse will be visible from North America. The eclipse will be best viewed from the eastern half of North America. The Moon will be rising during the eclipse for western regions. The Moon will enter the penumbra at 8:10 P.M. EDT and the umbra at 9:07 P.M. on September 27; totality begins at 10:11 P.M. On September 28, the Moon will leave the umbra at 12:27
A.M.
and the penumbra at 1:24
A.M.