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Answer: On a fair night with light winds and no frontal passages, the earth's surface loses heat by way of outgoing infrared radiation, and in turn cools the air in the lower atmosphere. This cooling process continues until incoming radiation from the rising sun begins to exceed the outgoing radiation, and the temperature starts to climb again. While there can be exceptions, the lowest temperature often occurs a little after sunrise as opposed to actually happening before the dawn.
Nov. 22, 2009 | Tags: cold, general meteorology

Answer: If you take eight years literally and go back that far from the date this answer is being added to our database, then we have not fallen into the single digits at all during that span. However, if you reach back a little farther, the most recent occurrences of single-digit lows at the airport were in January 2000 in the wake of a major snowfall, when the temperature dipped to 1 degree Fahrenheit on the 28th and 7 degrees on the 29th.
Nov. 18, 2009 | Tags: cold, past weather, records/extremes

Answer: Since fog is composed of tiny droplets, or in the case of ice fog tiny frozen crystals, separated by significant amounts of air, it could not and does not form a solid mass in the air, even when the particles themselves are frozen. The point regarding the air being less than freezing, though, is that water needs an appropriate surface upon which to initiate freezing at 32 degrees F, and droplets suspended in air will not typically freeze at that temperature unless they encounter something called an ice nucleus. Even then, with small droplets the ice nuclei are usually most effective at causing the droplets to solidify at temperatures below about 14 degrees F.
Nov. 6, 2009 | Tags: clouds, cold, visibility/fog/dust

Answer: Climate statistics for the Raleigh-Durham airport indicate an average of about 73 days each year with a low temperature at or below freezing, while there are about 4 days per year with a high temperature at or below freezing. As a very simplistic first approximation, this would suggest about 69 freeze-thaw cycles. Of course, whether a specific location or material freezes solidly or thaws completely can be impacted by its location, exposure to open sky or lack thereof, its exposure to wind that might reduce the time required for the item to rise or fall to a changed ambient temperature, and the amount of time on a given day that the air temperature spends significantly above or below freezing.
Nov. 2, 2009 | Tags: cold, normals, records/extremes

Answer: From records stretching back to 1944, here are the lowest daily high temperatures that have occurred in each month, along with the years in which those unusually cool days were recorded. January (17 degrees F, 1972/1985), February (18, 1958), March (20, 1980), April (40, 1959), May (49, 1946/1992), June (59, 1967/1997), July (67, 1984/1997), August (64, 1981/1989), September (53, 1984), October (47, 1980), November (32, 1976) and December (19, 1983).
Oct. 28, 2009 | Tags: cold, records/extremes

Answer: For the Raleigh-Durham airport, the earliest first occurrence of a freezing air temperature was October 3rd in 1974, while the latest first freeze was November 17, 1989. The average date for a first freeze is October 28th, with a standard deviation of +/- 9 days. Under the right conditions, frost can form on surface objects despite air temperatures that remain above freezing, and most reporting stations do not have reliable records of days with frost but not freezing temperatures, so all we can add regarding first frost dates is that they can potentially happen anywhere from a few days to a couple of weeks before the first freeze.
Oct. 26, 2009 | Tags: cold, past weather, winter weather

Answer: Our bodies lose heat more rapidly when the wind is blowing, which makes it "feel" colder than it really is, and wind chill is an estimate of the air temperature that would be required to produce the same rate of cooling in the absence of significant wind. A good explanation, an easy to use chart, and the formula used for calculations can be found at www.nws.noaa.gov/om/windchill/index.shtml.
Sep. 25, 2009 | Tags: apparent temperature, cold, cool sites, winds

Answer: It's unclear whether the "hurricane" references on the show are to large synoptic winter low pressure systems driven by horizontal temperature gradients (similar in many ways to the nor'easters that affect the U.S. Atlantic coast), which on rare occasion may produce sustained surface winds reaching 74 mph or higher, or whether they are referring to a phenomenon called a "polar low," which some meteorologists have referred to as arctic hurricanes. These systems rarely if ever produce hurricane force winds, but are quite intense nonetheless, and in addition can have some thermal properties (warm core, no frontal boundary) and an appearance on satellite (a clear "eye") similar to tropical cyclones. Even though the northern waters are not warm by any means, the important issue with driving these cyclones, whether in the tropics or polar regions, is the difference in temperature from the sea surface to the middle/upper atmosphere. You can learn more about these systems and see some interesting satellite images at nsidc.org/arcticmet/patterns/polar_low.html, and www.eumetcal.org.uk/polarlow/cometplows/polarlows/print_index.htm.
Aug. 25, 2009 | Tags: cold, hurricanes

Answer: On cold, clear nights with light winds it is common for the ground or for upward facing surfaces like roofs and car tops to lose heat by way of radiation and cool down faster than the air a few feet above. For that reason, while the reported temperature from a sensor 4-5 feet above the ground might be a few degrees above freezing, the frosted surface has indeed fallen to 32 degrees or below in order for ice to form.
Aug. 18, 2009 | Tags: cold, general meteorology

Answer: That is not at all a dumb question, so please don't feel compelled to "quit!" The reason official observations are made in the "shade" of a ventilated instrument shelter is that the objective is to measure the temperature of the air itself, by ensuring that the thermometer takes on that same temperature. A thermometer that is exposed to direct sunlight may be heated by the solar radiation so that it reports a temperature well above that of the ambient air. There is an opposite effect at night, as a thermometer exposed to open sky can cool by radiative heat loss to a temperature well below the air around it. So, thermometers need to be in the "shade" at night as well!
Jul. 15, 2009 | Tags: cold, heat