Their results show that freezing rain and drizzle occur at all times of the year, including June, July, and August, when freezing drizzle can occur in the far northern regions.
Throughout the rest of the year, freezing drizzle occurs more frequently than freezing rain, and there is a general decrease in the frequency of freezing rain and drizzle toward the north. Eastern Newfoundland receives the most freezing rain and drizzle, exceeding h annually.
Baldwin included separate maps of the annual distribution of freezing precipitation and ice pellets in the United States. A comparison of his freezing rain map with Bennett's reveals a general agreement, even though the data periods were different —48 for Bennett and —69 for Baldwin and it appears that, based upon later climatologies of freezing rain and drizzle e. Baldwin's results for freezing rain also agree generally with Changnon and Karl , who showed national maximums in a portion of New York and Pennsylvania, an east—west zone across the Midwest, along the eastern Appalachians, and in the Pacific Northwest.
Additional support for this assertion comes from studies e. New England experiences the greatest number of days with ice pellets, where it occurs on an average of 12 days annually. The horizontal distributions of freezing rain, freezing drizzle, and ice pellets in the United States have been studied by Gay and Davis , Branick , Strapp et al. All of these studies generally agree with earlier freezing precipitation studies, but each has studied different aspects of the climatology, such as average duration, frequency of large-scale events, proportion of freezing to frozen precipitation, and average meteorological conditions during these precipitation types.
In the United States, freezing precipitation and ice pellets occur most frequently from November through March Robbins and Cortinas and are usually short lived Cortinas et al. The greatest frequency of freezing precipitation and ice pellets combined is found in the Northeast, although the greatest frequency of freezing drizzle occurs through the western portion of the central plains Bernstein and Brown ; Bernstein The temporal and spatial scales associated with these events indicate that they are typically mesoscale events Branick ; Gay and Davis ; Cortinas While these studies provide some information on the distribution of freezing precipitation, with the exception of Strapp et al.
Strapp et al. Ice pellets were also not included in their results. The purpose of our study is to show the characteristics of freezing rain, freezing drizzle, and ice pellets that occurred in the United States and Canada between and using surface observations from both countries. In this paper, we do not attempt to provide thorough explanations for the distributions.
Instead, this paper is meant to serve as a reference for weather forecasters and as a starting point for additional climatological research as it relates to freezing precipitation and ice pellets. Section 2 describes the data used in this study, section 3 describes the analysis, and section 4 lists the conclusions from this study.
This time period was selected because of the data availability at NCDC. Data prior to were eliminated because some station data were only digitized for 1 out of every 3 h, and we wanted to document the duration and evolution of these events using the highest observation frequency available, namely every hour. Data after were also eliminated to assure that present weather observations were taken by human observers and not by the Automated Surface Observing System ASOS in the United States.
The quality control procedures for each format are documented in reports published by the United States Air Force and the United States Department of Commerce Most of these observations were associated with ice pellet observations.
We speculate that a portion of these were actually hail observations erroneously reported as ice pellets, especially at very warm temperatures. Ice pellets can survive falls through layers of above-freezing air, but their survival becomes increasingly unlikely with increasing surface temperature.
A temperature threshold was chosen to isolate the ice pellet observations from the hail observations, since the focus of this study is not hail. Surface temperatures associated with each observation of freezing precipitation were examined by plotting a cumulative frequency distribution of the temperature data not shown.
No additional quality control procedures were performed on the data. Using these criteria, stations in North America were available for analysis Fig. Moreover, robust statistical descriptions e. All spatial plots were drawn by hand in order to preserve as much of the detail in the distributions as possible. Freezing precipitation and ice pellets have been observed throughout most of the United States and Canada Figs. Moreover, this distribution shows synoptic-scale and some mesoscale variability this variability will be discussed in the following subsections.
Of the three types of precipitation we examined in this study, freezing drizzle is the most widespread and occurs most frequently across the central part of each country and the eastern part of Canada and the western and northern coasts of Alaska. Ice pellets and freezing rain occur most frequently in the eastern part of each country.
Numerous investigators have shown that this thermal stratification can occur primarily as a result of airflow associated with extratropical cyclones and secondarily by the proximity of water bodies and topographical effects Stewart and King ; Stewart ; Bernstein ; Rauber et al. These conditions combine to produce several dominant areas of freezing rain. Most of the United States and Canada receive less than 10 h of freezing rain most years, with the highest frequencies near the St.
Lawrence River valley and Newfoundland, where usually 30 or more hours of freezing rain are observed annually Fig. These results are similar to those of Stuart and Isaac , who found in excess of 25 h of freezing rain annually in several regions across eastern Canada.
Regional maxima also occurred over the Columbia basin, across much of the Midwest, as well as along the east slope of and within the Appalachians. Freezing rain also occurs less frequently 5—10 h annually across the central plains of the United States and Canada, portions of central and western Canada, and a small part of Alaska. A distinct minimum in freezing rain frequencies was found along the western slope of the Appalachians, which was attributed to a rain shadow effect by Bernstein The variability associated with the annual distribution can be large at many locations, particularly in areas of frequency maxima Figs.
The freezing rain observations at some locations in eastern Canada and the United States varied from near 0 to 70 h annually Fig. Since these events often occur due to warm air advection above the cold surface on the cold side of warm and stationary fronts, annual variations in their frequency is likely associated with changes in the storm track and location of the surface freezing line.
A good example site is Greensboro, North Carolina GSO , where the track of low pressure centers relative to the southern end of the Appalachians and the location of the strength of high pressure over New England is important to creating a classical setup of midlevel warm air advection over surface cold air dammed along the mountains. The variability can also be greatly influenced by single events, such as the ice storm of described in the introduction , which produced a major fraction of the typical freezing rain hours per year in one storm.
That event is not included in the dataset used here. An analysis of freezing rain days Fig. For most of central and northwestern United States, the median number of freezing rain days is between 1 and 4. While in the northeast United States and extreme Canada, the median exceeds 7 days annually.
Although Changnon and Karl show more mesoscale variability in their maps of freezing rain days, the general spatial pattern agrees with their results, particularly during their latter data period — Based on this distribution and on results of other freezing rain studies Strapp et al. For example, the Atlantic Ocean and the Gulf of Mexico provide rich sources of warm moist air that can be advected over subfreezing surface layers, such as those that form in valley locations e.
Lawrence River valley or cold continental air masses that are in place as an extratropical cyclone passes.
Although extratropical cyclones also impact the western coasts, subfreezing surface air is often missing because the dominant synoptic circulation usually does not advect subfreezing surface air into the region.
There are exceptions, however, such as the Columbia basin, where cold continental air becomes trapped Whiteman et al. Freezing rain and ice pellets usually require an elevated warm layer and a subfreezing surface layer, although Strapp et al. John's, Newfoundland, no warm layer was present aloft. The most likely mechanism for ice pellet formation is through partial or complete melting of snowflakes in an elevated warm layer and then refreezing as they descend through the subfreezing layer.
Both Hanesiak and Stewart and Zerr found that incomplete melting of snowflakes in the warm layer was the primary factor in the production of ice pellets. The strength and depth of the cold layer were considered to be secondary factors. The geographic distribution of ice pellets is similar to that of freezing rain. Ice pellets occur most often in the eastern Canadian provinces, where several areas usually experience more than 20 h Fig.
In the United States, ice pellets usually occur for more than 10 h annually in the Great Lakes, the Northeast, and the mid-Atlantic states.
Along the mid-Atlantic and northeast coasts, the gradient in the occurrence of ice pellets is much less than that of freezing rain. Bernstein noted that this was due to ice pellets surviving to reach the surface at above freezing temperatures along the coast, where freezing rain would have changed to rain. Ice pellets also extended westward across Canada, and reached the Pacific coasts of British Columbia and extreme southeastern Alaska.
In eastern Canada and the United States, annual ice pellet totals were highly variable from year to year, ranging from near 0 to 70 in some locations Fig. Since the mechanism for ice pellets is similar to that for freezing rain, variability in the location of the storm track and surface freezing line are again likely to explain much of the annual changes in ice pellet frequency.
This thermodynamic environment supports supercooled droplet production due to the very low concentrations of active ice nuclei. Previous studies suggest that this is the primary process for the formation of freezing drizzle Bernstein ; Rauber et al.
Freezing drizzle occurs over a large portion of Canada; the central United States; western and northern Alaska Fig. It is the primary type of freezing precipitation to extend to the east slope of the Rocky Mountains, where upslope clouds with warm tops produce an excellent situation for freezing drizzle development.
The number of freezing drizzle days Fig. At many locations, the variability associated with the annual freezing drizzle distribution is usually larger than other types of precipitation Fig. The widespread geographic distribution of freezing drizzle suggests that several factors may contribute to its formation, including water source proximity, topography, and synoptic forcing. Several studies have noted that a large portion of freezing drizzle events near bodies of water were associated with onshore flow Strapp et al.
Bernstein also found freezing drizzle was associated with upslope flow along the slopes of major mountain ranges and in valleys where stagnant cold and moist air pools were present. We speculate that these locations are favored because most freezing drizzle in these regions occurs as a result of collision and coalescence within warm or subfreezing clouds Strapp et al.
Gentle synoptic forcing associated with warm fronts and stationary fronts has also been shown to be associated with freezing drizzle production, and likely explains much of its occurrence across the middle of the continent Bernstein et al. Relatively clean air is often present within the formation zones in all of the situations described above. Rasmussen et al. Fluctuations in the amount of sea ice present, as well as the frequency of warm-topped clouds may explain the large variability in the annual frequency of freezing drizzle there.
A plot of the two types of freezing precipitation combined shows a very broad distribution that covers all of the central and eastern United States and Canada Fig. In general, a broad swath extends from the western high plains through the Great Lakes region, into most of eastern Canada, the Maritime Provinces, New England, and the eastern slope of the Appalachians. Newfoundland receives the most freezing precipitation annually in the United States and Canada, with an annual frequency that is almost a factor of 2 larger than the highest frequency observed elsewhere.
It is also of interest to examine what fraction of winter observations i. This creates a thermodynamic profile that is conducive only to freezing precipitation, as shown by Strapp et al.
In the United States and Canada, freezing precipitation and ice pellets occur most frequently during the winter months December, January, and February; Fig. When the data are normalized to a day month, the months of maximum occurrence for freezing rain, ice pellets, and freezing drizzle are January, February, and December, respectively.
In the fall, there is a rapid increase in the frequency of freezing precipitation between October and December and a similar decrease between March and April, although the decrease of ice pellet frequency is more gradual in the spring. The results are consistent with other national studies. Changnon and Karl showed that peak months of freezing rain in the United States were December and January, with January averages highest in the East and December highest in the West.
Across most of Canada, Stuart and Isaac found that freezing precipitation occurs primarily between October and May, with March being the month with the highest frequency. The data also show that a seasonal dependence of freezing precipitation and ice pellet occurrence varies with latitude and, to a lesser degree, longitude. Since the spatial dependence is similar for all types of freezing precipitation and ice pellets, we only show histograms for freezing drizzle Fig.
At central longitudes, freezing precipitation moves from as far south as near the Gulf coast in midwinter to the Arctic coast in midsummer. The migratory nature of freezing drizzle is less evident, but still present in the western and eastern parts of the continent, though freezing drizzle is much less frequent at central and southern latitudes.
Freezing drizzle in coastal stations in the Canadian arctic and Hudson Bay areas is often maximized just before the sea freezes in the fall or after it thaws in the spring Strapp et al. Although freezing precipitation and ice pellets are rare across the United States and southern Canada during the summer months, they do occur at far northern sites, such as Barrow, Alaska, and Resolute Bay, Northwest Territories not shown. It is during these months that temperatures are moderate enough to allow liquid- phase clouds to exist and perhaps become prevalent at far northern latitudes.
Winter clouds tend to be glaciated in this region Stuart and Isaac ; Bernstein et al. Our examination of the surface observations indicates that there is an association between the occurrence of freezing precipitation and ice pellets and the diurnal solar cycle.
The details of this calculation are discussed by Kelly et al. The purpose of this normalization is to identify any relationship between the diurnal solar cycle and the occurrence of freezing precipitation. Our NST analysis shows a relationship between the diurnal solar cycle and freezing rain and freezing drizzle, but not for ice pellets Fig.
Freezing rain and freezing drizzle occur most frequently before sunrise typically the coldest time of the day then drop off sharply during the morning, reaching a minimum during the late afternoon. A similar diurnal trend was reported by Strapp et al. There is no apparent diurnal trend for ice pellets, perhaps because their occurrence is less sensitive to the surface temperature.
The expected reason for the diurnal trend in freezing precipitation is that the decrease in freezing precipitation occurs because of surface heating by insolation. However, other factors responsible for the decrease after sunrise can be identified by examining the temporal frequency distribution of different types of precipitation or cessation of precipitation reported immediately following each freezing precipitation event one or more sequential hours of freezing precipitation.
We counted every event and noted the hour NST and type of precipitation or cessation of precipitation following each event. The relative frequency was computed by dividing each precipitation type at each hour by the total number of events that ended in that hour.
For example, if freezing rain was observed between and NST, then no precipitation was reported at NST, we would count one event that was followed by the cessation of precipitation at NST.
This analysis shows that freezing drizzle or freezing rain events most commonly end with a period of no precipitation Figs. Those that end with a change in precipitation type are strongly dominated by snow, likely due to a decrease in cloud-top temperature or seeding from above cf. Politovich and Bernstein Changes to nonfreezing drizzle are markedly less frequent, but are maximized during daylight hours, when freezing drizzle events ending with a cessation of precipitation are at a relative minimum.
At this time of day, slight increases in surface temperature are likely responsible for the change, since both drizzle and freezing drizzle are typically formed via the collision—coalescence process.
When a freezing rain event ends with a change in precipitation type, that type is most often rain during daylight hours, but evenly distributed among several precipitation types, including snow, at night Fig. During the day, freezing rain events ending in snow or a cessation in precipitation are at a relative minimum.
Though snow is the most common precipitation type to follow an ice pellet event at any time of day, this change is also at its relative minimum during daylight hours. The same pattern holds true for freezing rain following an ice pellet event Fig. Claudine Hellbrugge Pundit. How do you deal with freezing rain?
What to do during an ice storm. Ice from freezing rain can accumulate on branches, power lines and homes. Never touch power lines. Avoid driving. If it is safe to do so, clear your surrounding property of the ice and snow, and sand or salt the ground. Check on elderly or vulnerable neighbours. Nordi Josy Pundit. Does it have to be cold to hail? Saul Dangl Teacher. Can flights take off in freezing rain?
While planes can be de-iced if still at the airport, icing is an extremely dangerous weather condition for flying , landing and take - offs. If freezing rain is occurring, it is likely that flights will be delayed or canceled as ice can build up on the wings, windshields and runways.
Monaim Viron Teacher. Does freezing rain melt snow? Freezing rain is actually melted snowflakes. Where in the world does freezing rain occur? The area most frequently hit by freezing rain extends over a broad region from Texas northward to Minnesota and then eastward into the Middle Atlantic states and New England. Onisor Salamo Teacher.
What color is freezing rain on the radar? Image 2: Blue shows where snow is most likely. Pink is mix. Green is rain. Temperature profile along with ground reports and radar returns are used to estimate the most likely precipitation. Layachi Renshaw Reviewer. Why is freezing rain so dangerous? Freezing Rain Effects.
Vicent Clara Reviewer. Is it safe to drive in freezing rain? According to the National Weather Service, freezing rain , sleet and other types of icing are dangerous to motorists because they make the roadways slick, which can prevent drivers from remaining in control of their vehicles.
Veronel Kleinhart Reviewer. What should you do if you hit black ice? If you do hit black ice , your first reaction must be to remain calm and avoid overreacting. Usually, the glaze is light but given enough time, can accumulate to the point at which roads and sidewalks become completely slick.
Weather Weather Talk: The difference between freezing drizzle and freezing rain. Written By: John Wheeler am, Jan. Suggested Articles.
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