1) Method

The Current Icing Product (Bernstein et al. 2005) was developed as a multiple data source, hybrid approach to the diagnosis of icing. CIP became an official Federal Aviation Administration and National Weather Service icing product in 2002. It combines satellite, radar, surface, and lightning observations with numerical model output and PIREPs to create an hourly, 3D diagnosis of icing and SLD. CIP uses these datasets to estimate the locations of clouds and precipitation, and then combines those using physically based decision trees and fuzzy logic.

CIP-sonde similarly examines the vertical structure of the atmosphere using coincident observations from soundings and surface stations. Based on this information, each profile is described by one of several icing scenarios (e.g., classical freezing rain). Within each scenario, fuzzy logic membership functions are applied to the data and then the potential for icing and SLD is determined at each level. The icing and SLD potentials are essentially the confidence or likelihood that those conditions were present, on a scale of 0 to 1. At high potentials, the presence of icing or SLD is considered to be very likely.

In Part I, it was shown that CIP-sonde was efficient at capturing icing events while warning for icing over a relatively small volume of airspace. The threshold of 0.15 resulted in the detection of 87.3% (POD_y) of good-quality positive icing PIREPs matched to soundings over North America, while only indicating icing at 1.6% of all altitudes for all soundings at all sites combined. The 0.15 threshold represented a reasonable balance between detecting the majority of icing and minimizing overwarning. Regardless of threshold choice, the geographic patterns and vertical distributions of icing and SLD were similar.

2) Input datasets

Vertical profiles of temperature and moisture were derived from a National Climatic Data Center database of balloonborne soundings from 1980 to 1994. Soundings were only included in this study if they 1) had matching surface observations that included the presence or absence of cloud cover, precipitation, and its type, 2) reached the −35°C and 400-hPa levels, 3) had at least 10 data points at or below those levels, 4) did not contain any superadiabatic layers, and 5) did not have any data that were misplaced vertically (e.g., pressure increasing with height). This limited the database to profiles that appeared to be of good quality, had information about the presence or absence of clouds and precipitation, were deep enough to reach temperatures where ice-phase cloud tops could dominate, and had adequate resolution at levels where icing typically exists. The 15-yr dataset resulted in up to ∼10 500 soundings per site. Horizontal coverage was fairly uniform across Europe and most of Asia (Fig. 1).

Surface observations were derived from a National Oceanic and Atmospheric Administration (NOAA) Techniques Development Laboratory database. At each sounding site, all observations made within a 100-km radius were considered in the assessment of cloud cover and precipitation type. The number of surface stations available varied by site, with several sites having fewer than 5 surface stations, while others had 20 or more. The presence or absence of clouds was determined using the maximum cloud cover reported at the valid time of the sounding (0000 and 1200 UTC). Only soundings matched to surface observations of at least “broken” sky cover were considered to have any chance for icing, since nearly all icing occurs when such cloud cover is present (Bernstein et al. 1997). Ceiling height was set to the height of the lowest deck that met this criterion. Surface observations were also checked for the presence of FZDZ, FZRA, PL, rain (RA), drizzle (DZ), and snow (SN), all of which provide important clues as to the presence or absence of icing, including SLD (see Part I).

1) Geographic distributions of icing

Over Europe, the primary full-year icing maximum occurred in a swath from Iceland and Scandinavia southeastward into Russia and southward to the Alps and Balkans (Fig. 2a). Since much of the western portion of this feature was over the ocean, it was primarily identified by the SIGMA index, but this was consistent with CIP-sonde results for Iceland, Norway, and several North Atlantic islands. High values in this region are not surprising, given the predominance of synoptic-scale storms and cloudy skies here (Joly et al. 1999; Rossow and Dueñas 2004).

Icing frequencies decreased gradually toward the southeast, but still exceeded 35% across western Russia, and 25% as far south as Turkey. They dropped off more sharply across France and through the Alps, toward Italy, reaching their minima over relatively cloud-free southern Spain and Greece. They also dropped off quickly between Iceland and the coast of Greenland, to the north of the main storm track. A strong gradient was present from Norway southeast to the Baltic Sea and the Gulf of Bothnia (between Sweden and Finland). This was likely associated with flow across the elevated terrain of Norway and northwestern Sweden. The gradient was reversed toward the northeast, across Finland. Dobesch et al. (2005) documented similar features in their surface-based assessments of icing frequency at 100 m AGL over this region.

The primary SLD maximum found using CIP-sonde was somewhat more confined to central and northwestern Europe (Fig. 2b). The area with SLD potentials exceeding 0.15 at least 8% of the time was focused around Germany, western Scandinavia, the United Kingdom, and Iceland. Many of these areas are located downstream of the North Atlantic and the North Sea. Relatively clean, maritime air masses with low drop concentrations can move onshore in these areas, enhancing the opportunity for SLD to form there. SLD has also been observed on numerous occasions within relatively clean source air over North America (e.g., Cober and Isaac 2006; Curry et al. 2000; Ikeda et al. 2007; Bernstein et al. 2004). During an icing research program over southern Germany, severe icing from nonclassical SLD was observed on two of the eight total flight days over a 2-week period. Both events occurred in northwesterly flow off the North Sea. Icing was found on 5 days and nonclassical DZ was observed on another (Hauf and Schröder 2006).

As was found for icing, SLD frequencies decreased sharply toward Spain and the Mediterranean Sea. They also decreased toward the east from Germany, falling to <3% across most of Russia, and from the Ukraine to the Baltic States. Values also appeared to decrease toward the Arctic and Greenland, but there were relatively few sounding sites in these remote areas and the trend may be partially attributable to a relative lack of surface stations there (see below). Still, some maritime sites with relatively few surface observations (e.g., Stornoway, Scotland, and Keflavik, Iceland) had some of the highest SLD frequencies found in this study.

While maximum icing and SLD frequencies on the order of 40% and 10% may seem large, recall that they were calculated using low-to-moderate thresholds and that they indicated the frequency of occurrence of conditions that were conducive to icing and SLD at any altitude up to ∼10 km. In CIP-sonde, all locations within 100 km of a sounding site were considered, resulting in a 314 000 km³ volume of airspace above each station. In SIGMA index calculations, analyses from 1° × 1° grid boxes were checked, each of which represented on the order of 10⁶ km³ of airspace, depending on the longitude. Thus, the icing frequencies presented here should not be interpreted as being representative of point or instantaneous values, which are expected to be much lower. Later in this section, it will be demonstrated that frequencies are lower within specific altitude bands and months. Overall, icing frequencies for Europe were similar to those found over North America in Part I, while SLD frequencies for Europe may be slightly larger.

Icing results found by the two methods generally agreed, though there were some notable exceptions. For example, both methods indicated that the European maximum extended across Germany and that values decreased toward the west and south from there, but CIP-sonde values over Germany tended to be larger than those from the SIGMA index. One possible cause for the larger CIP-sonde values was the number of surface observations matched to German soundings, relative to those in neighboring countries. To examine the possible effects of this factor, stations within the box shown in Fig. 2a were tested. Within it there were 54 sounding sites with a wide range of 1) matched numbers of surface stations and 2) icing and SLD frequencies.

The correlation coefficient between the number of stations matched to each site and the annual icing frequencies found at those sites was 0.02, indicating no correlation whatsoever. For SLD, the correlation coefficient was somewhat higher (0.30), but still fairly weak. These results do not explain what appear to be elevated CIP-sonde icing frequencies over Germany, but they do hint that SLD frequencies over Germany and other countries where the surface network was relatively dense (see Fig. 3) may be somewhat elevated. In contrast, SLD frequencies in portions of the Arctic may be underestimated because of the relative lack of surface stations there.

Frequencies of surface observations of freezing precipitation and subfreezing fog were relatively large over Germany and bordering areas of France and Benelux compared to surrounding areas, especially to the west (Fig. 3; Carriere et al. 2000). As will be discussed later, frequencies from German mountain stations roughly matched CIP-sonde icing estimates at those elevations from nearby sounding sites, including seasonal trends. These results suggest that the elevated CIP-sonde frequencies around Germany may be reasonable.

2) Surface indications

As expected, locations with high icing frequencies also had a high frequency of significant cloud cover in historical surface observations from 0000 and 1200 UTC (Fig. 3). Strong gradients in the frequency of clouds were evident across topographic features such as the Alps and Pyrenees. SLD-prone portions of Europe were quite cloudy and had a high frequency of DZ and RA, but most experienced little or no FZDZ, FZRA, or PL. Approximately 97% of the SLD that CIP-sonde identified in Europe occurred when DZ and/or RA was coincident with warm cloud tops (>−12°C), a combination that is commonly associated with the presence of SLD aloft. This value is somewhat larger than the 92% value found for North America in Part I and suggests that the “classical” SLD mechanism is even less common over Europe.

Because surface FZDZ and FZRA were rarely observed across most of Europe, very little of the SLD that CIP-sonde identified appears to have reached the surface. With the exception of northeastern Europe, FZDZ and FZRA were only observed with significant frequency at stations located at elevations above ∼1 km MSL (as in Carriere et al. 2000). This matches the lack of SLD below the 1-km level across most of Europe, discussed in the next section.

Like freezing precipitation, freezing fog (FZFG) tended to occur at elevations >1 km (Carriere et al. 2000). However, Makkonen and Ahti (1995) showed that the relief of the local terrain is more critical than elevation in determining the frequency and severity of icing at surface sites. In either case, an observation of FZFG is a good indicator of icing at the ground. But close examination of surface reports indicated that FG at T < 0°C was often not indicated as FZFG. Though a portion of such “cold fog” (hereafter CFG) observations was actually “ice fog,” most was likely to have been at least partially composed of supercooled liquid water (SLW). Overall, 85% of the 116 763 CFG observations used to make Fig. 3d occurred at T > −10°C, where SLW is expected. Thus, it is reasonable to use frequencies of CFG as surrogates for icing frequencies at the ground, though they likely represent somewhat of an overestimate.

It is useful to compare the frequencies of CFG and freezing precipitation from surface stations in elevated terrain with CIP-sonde icing and SLD frequencies at nearby sites for the same times (0000 and 1200 UTC). To eliminate the possibility that the data from the surface stations were used by CIP-sonde, surface stations used were always more than 100 km from all sounding sites. This distance complicates matters somewhat, since regional- and local-scale phenomena can affect the occurrence of icing and SLD. One-to-one correlations were not expected, since surface stations that provide point observations were being compared with sounding estimates from a 100-km radius cylinder. Also, FZDZ and FZRA only represent a subset of the SLD drop size spectrum, so their frequency at the surface should underestimate the frequency of SLD at the same altitude.

Still, comparisons proved to be quite favorable in many parts of Europe, especially between CFG and icing. This was true both in terms of overall annual values and seasonal variations both in areas with relatively high (e.g., Germany, Bulgaria, Romania, and the United Kingdom) and low (e.g., Italy, Spain, and Portugal) icing frequencies. For SLD, reasonably good matches with freezing precipitation were found in several regions with high frequencies (Germany, Ukraine, Poland, Yugoslavia), as well as relatively SLD-free areas (Spain, southern Italy). For example, CFG and freezing precipitation frequencies from Kleiner Feldberg (elevation of 805 m) were compared to the average of CIP-sonde icing and SLD frequencies surrounding that altitude at Meiningen, Germany. Results showed that surface frequencies for icing and SLD were 64% and 34% of those from the soundings. The favorability of such comparisons varied by location.

3) Seasonal movement of icing and SLD aloft

Monthly maps show that the icing and SLD maxima and minima moved during the year (Fig. 4). In January, icing frequencies exceeding 35% were found across much of Europe, especially its northern and eastern portions, and extended southeast through the Balkans to northern Turkey. Some places with notably high frequencies included Iceland, the northern United Kingdom, and areas extending eastward from Norway, eastern France, and the Balkans. Icing was indicated >25% of the time across the southern United Kingdom, western France, and northern Spain. Northern Spain was the site of a research flight campaign that featured numerous cases of both stratiform and cumuliform icing, with nearly all of it occurring within the temperature ranges covered by the SIGMA index and CIP-sonde (Bain and Gayet 1982).

A small amount of icing was also indicated over Italy, southern France, and Spain, as well as portions of northern Africa. Baddour and Rasmussen (1989) documented icing events over the mountains of Morocco during this time of the year. Note that January results from the two methods matched reasonably well, including many details such as the strong gradients found near the Adriatic Sea as well as across France and the United Kingdom. As shown earlier for annual results, there were some disparities in the monthly results, especially around Germany, where CIP-sonde values generally exceeded those of the SIGMA index.

In the spring, the icing moved northward with the storm track and by July most of it had shifted to the Arctic, from northern Russia and Norway to Iceland. Still, both indices found that values exceeding 15% remained over the United Kingdom and from Germany eastward. By October, the icing maxima have begun to move south and west toward their cool season centers. Again, CIP-sonde values were relatively high over Germany and its vicinity, but frequencies matched quite well across most of the continent.

January SLD frequencies were maximized across western Europe, including the United Kingdom, southern Scandinavia, Benelux, and Germany, into Poland. SLD frequencies exceeding 5% extended down to the north coast of Spain, southern France, the Alps, and Balkans. Following the icing pattern, the SLD maximum moved northward during the summer, and returned southward in the fall. Maxima found in places like the United Kingdom and Germany remained somewhat intact throughout most of the year, though frequencies waxed and waned. Maximum values occurred during the peak of the winter, when particularly cloudy, drizzly conditions tended to exist in these areas. It is interesting to note that SLD was somewhat common along the west coast of France and north coast of Spain from October to April, and then became very uncommon from June to September. In contrast, Arctic stations had most of their SLD between late spring and early fall, with relatively little during the winter, when clouds were more likely to be glaciated. Frequencies over much of Scandinavia tended to peak in the autumn, except in its southern sections, where the peak tended to last into January. Vedin (1998) found similar patterns in a surface-based climatology of freezing precipitation compiled for Sweden.

4) Vertical distributions and examination of individual stations

The altitudes at which icing and SLD occurred most frequently also changed during the year, as demonstrated in plots of their frequencies versus height and time of year for all European stations combined (Figs. 5a,b). Both techniques indicated a similar time–height pattern, though CIP-sonde values were generally somewhat larger than those from the SIGMA index. Icing altitudes generally rose in the spring and dropped in the fall, following the freezing level. The peaks in icing (20%–40%) and SLD (4%–7%) frequency occurred during winter and late fall, when they were most often found between 1 and 2 km. This pattern matches the relative lack of freezing precipitation and CFG at surface stations below ∼1 km. Icing extended upward to slightly higher altitudes than SLD because CIP-sonde only diagnosed nonclassical SLD when CTTs ≥ −12°C were present and only attributed SLD to the lowest cloud layer when multiple layers were identified. Icing potentials exceeding 0.15 were possible with colder CTTs and thus, colder temperatures, which were found at higher altitudes. Seasonal transitions were quite evident, as values decreased and moved to higher altitudes during the spring, then moved downward in the autumn, and then values increased to reach their maximum at 1–2 km from November to March.

Time–height distributions for individual stations sometimes had patterns that were very similar to the European average, while others were noticeably different. A reasonably good match was present at Trappes, France (near Paris; Fig. 6a), near the western edge of the continental icing maximum. To the south, Rome, Italy, had less frequent icing, with most of it during the cool season (Fig. 6b). Toward the southeast, patterns at Sofia, Bulgaria, (Fig. 6c) were fairly similar to those found over Paris and icing was found to climb from 1–3 km in winter to above 4 km in the summer. Nikolov and Moraliiski (2000) found similar seasonal, time–height patterns in icing at Bulgarian mountain stations between 0.8- and 3.0-km elevation. Point frequencies at these sites typically peaked at 12%–22%, which are somewhat smaller than the 20%–30% values inferred by CIP-sonde and the SIGMA index at 1–3 km. This serves as a good example of the difference between point observations and the larger-scale estimations made using the two icing algorithms.

To the northwest of the continental maximum, icing frequencies were generally maximized in autumn, winter, and spring, with a general increase across the United Kingdom toward Iceland. Moderate and high values were fairly persistent from autumn through spring over Belfast, United Kingdom, and Keflavik, Iceland (near Reykjavik; Figs. 6d,e). Icing at Keflavik was typically maximized in the 1–2-km altitude range throughout most of the year, only rising into the 2–4-km range during summer. Similar patterns were found over northern Scotland, and the nearby Shetland and Faeroe Islands (not shown). Oslo, Norway, to the southeast, had a pattern that reflected aspects of both the northern sites and the continental maximum (Fig. 6f). Hirvonen et al. (2007) reported that a Rosemount ice detector at the Luosto Fell site (well to the north of Oslo; elevation of 0.52 km) was prone to heavy icing between October and April. This observation matches the low-altitude, seasonal pattern at Oslo and other Norwegian sites (not shown) quite well.

1) Geographic distribution

This portion of the study focuses on eastern Asia and neighboring portions of Russia. Unfortunately, very little data were available for large portions of southwestern Asia, so those areas will mostly be discussed in terms of global patterns (next section). The primary Asian icing maxima were found in three swaths. The northernmost of these is an eastward extension of the European maximum, continuing over much of northwestern Russia, with values decreasing gradually toward the east to 20%–35% over Siberia (Fig. 7). The central maximum runs eastward from the Himalayan Mountains, across central China and South Korea to portions of Japan, then continues northeast into the Pacific. Full-year frequencies within this maximum were mostly on the order of 25%–45%, with higher frequencies along the Himalayas. Only the SIGMA index had sufficient data to detect the terrain-driven maximum there, because of a lack of sounding data. It was in this area that several icing accidents occurred during World War II (Aviation Safety Network 2008). A third maximum was found to the south, over Indonesia and Malaysia, and was associated with persistent cloudiness and showers in the intertropical convergence zone (ITCZ). There was only one SLD maximum and it was collocated with the central Asian icing maximum, from southeastern China to western Japan, and had frequencies of 5%–8%. Most stations in Asia had SLD (exceeding 0.15) less than 3% of the time. Overall, peak annual icing and SLD frequencies over Asia were lower than those found over Europe and North America.

As noted earlier, icing was found most often in places where at least broken skies were common and the clouds have a combination of ideal temperatures within them and at their tops. It is not surprising to note that the locations of the three icing maxima roughly match the areas with peak frequencies of cloudy skies (Fig. 8a). Though not observed as frequently as at some European sites, CFG was commonly reported at mountain stations within the central icing maximum, including Japan, China, and Taiwan. Several stations had CFG > 5% of the time (Fig. 8d) and had seasonal patterns that closely matched those from nearby sounding sites.

SLD maxima tended to occur where this combination was collocated with clean source air and/or the classical freezing rain process. Across Asia, the primary maritime air sources are the western Pacific Ocean and the large seas on its western edge. FZDZ at the surface was rare across Asia, except for at a few stations in southeastern China, where it was found in 0.2%–0.5% of all observations. Note that FZDZ frequencies decreased toward the east in Russia and were near zero over most of eastern Russia, matching the results of Berzukova et al. (2006).

FZRA was also rare across Asia, except in parts of southeastern China, where it was present in 1%–5% of all 0000 and 1200 UTC observations and has been reported to cause heavy icing on power lines there (Fig. 8c; Li et al. 2008). Most of the stations with significant FZRA were at elevations exceeding 1 km, though a few were in the lower terrain to the east. It was in this lower terrain, toward the east side of the southeast China FZRA maximum, that PL occurred most often, but with much lower peak frequencies of 0.2%–0.4%. These precipitation types and the classical FZRA process appear to have been important contributors to the presence of a pronounced SLD maximum over southeastern China (Fig. 7b). Overall, the classical structure was present in 10%–25% of all SLD cases in this region, compared to 5% for all of the Asian sites combined. Comparisons between frequencies of freezing precipitation at surface sites and CIP-sonde SLD frequencies at the same heights were reasonably good throughout southeastern China.

As was found for Europe, monthly maps indicate that there are seasonal movements to Asia’s icing and SLD maxima (Fig. 9). In January, April, July, and October, all three of the primary icing maxima were present, but their locations and intensities varied by month. During January, the northern icing area had a strong gradient from western Russia to Siberia. Siberia appeared to have almost no icing at this time of the year, because of bitter cold temperatures (<−30°C at the surface; Hydrometeorological Center of Russia 2008). Note that SIGMA index values tended to be larger than CIP-sonde values over western Russia, but similar toward Siberia.

The central maximum was fairly intense from the Himalayas across southeastern China and Korea to western Japan. Regional features embedded within this included north–south gradients across China, a secondary gradient across Korea, and a strong west-to-east gradient across Japan that was probably associated with the mountains there. The southern icing maximum was also quite active, especially between 10°S and 5°N, while there was a pronounced minimum between 10° and 25°N, where relatively few clouds tend to exist in winter (Rossow and Dueñas 2004).

With the warming of spring and summer, the central icing band weakened (except over the Himalayas), while the northern band shifted toward the north and northeast, and reached the Arctic by July. Arctic values peaked at more than 50% in midsummer, then decreased rapidly as cold air settled in during late fall and the main icing maximum moved southward. Monsoonal clouds and their associated icing also became quite evident in a swath from India and Burma to southwestern China in July. This appears to have been associated with a northward movement of the equatorial band of high-altitude icing. Summer was the only season when an adequate number of soundings was available for CIP-sonde to estimate frequencies in this region and comparisons between results from the two systems were favorable.

Seasonal patterns in SLD frequencies were similar to those found for icing. During January, the largest SLD frequencies (>7%) were present over the same region as the eastern portion of the central icing maximum and the locations where FZRA and PL were most common at the surface. The SLD maximum also extended from southeast China across Taiwan to western Japan. As part of their climatology of surface observations, Matsushita and Nishio (2004) found FZRA and PL in the ravines of western Japan during December and January. However, the classical FZRA structure was present in <5% of all Japanese SLD soundings. Based on this, and the fact that both FZDZ and DZ are rarely observed in this region, it seems that most of the SLD identified over Japan was associated with warm cloud tops and light rain at the surface.

Another strong gradient in SLD frequency ran from south to north across China. Both SLD maxima moved to the north and northeast during the late spring to briefly impact southeastern Russia, continued north to the Arctic by July, and returned southward in October.

2) Vertical distributions

As noted for Europe and North America (Part I), the altitudes at which icing and SLD occurred most frequently changed during the year over Asia, and the patterns could be different from region to region and station to station. Plots of their frequencies with both height and time of year for all Asian stations combined did not demonstrate this well, so the combined plots have been broken down into latitudes to the north and south of 25°N latitude (Figs. 5c–f).

Typical seasonal transitions in the northern latitudes were fairly evident, though weaker than those found over Europe and North America. Maximum icing frequencies were mostly found below 4 km from October to April, and in the 3–7-km range from June to August, with transition periods in May and September. Values and patterns from the two techniques proved to be quite similar. The southern icing pattern was quite different, due to tropical temperatures and persistent clouds found within the ITCZ, as well as monsoonal moisture over southern mainland Asia. Icing was mostly evident above 5 km, with a persistent maximum in the 5–7-km altitude band. While patterns from the two techniques matched reasonably well, the SIGMA index primarily indicated the presence of icing below 7 km, while CIP-sonde had icing up to 9 km. This is mostly due to the difference in the temperature ranges for the thresholds used by the two techniques. Some of the icing extended down to slightly lower altitudes during winter.

SLD frequencies were found to be quite low (<3%) in both latitude bands, but some SLD was still indicated at relatively low altitudes during the cool season and higher altitudes during the warm season. It is important to note that confidence in high-altitude SLD assessments is relatively weak because it is difficult to relate surface precipitation to such high-altitude clouds. While there is certainly potential for SLD to exist within these clouds, the methodology used here is probably inadequate to address a good portion of it.

Time–height distributions for individual stations revealed a fairly typical seasonal pattern at Taipei, Taiwan, and the more inland site of Guiyang, China (Figs. 6g,h). The results from these sites represent a transition from a southern latitude pattern to a more central Asian pattern, where cold season icing was common at lower altitudes. Frequencies at Taipei City were highest from January to June, as they climbed from 3 to near 7 km. On 21 December 2002, an ATR-72 lost control and crashed just to the west of Taiwan after encountering severe icing that included SLD at 5–6 km (Aviation Safety Council 2005). These altitudes were within the normal range for icing at this site for late December, which appears to be near the typical start to the main icing season over Taiwan. Guiyang, located within the southeastern China icing and SLD maxima, had peaks in icing and SLD frequencies from December to February, especially in the CIP-sonde results. SLD frequencies (not shown) exceeded 10% at altitudes below 2 km in January and February, the same months in which FZRA, PL, and RA occurred most often in this region. The seasonal change in altitude of icing was quite large at Guiyang and this feature was evident at numerous sites in east-central Asia, including over Japan.

Over southwestern and western Japan, icing and SLD appeared to be relatively common. Along the Sea of Japan, Wajima had a distinctive maximum below 3 km from November to March, when icing and SLD frequencies exceeded 25% and 6% (Figs. 6i, 10a). To the east, Tateno (near Tokyo; Figs. 6j, 10b) had similar seasonal patterns, but the wintertime frequencies were roughly one-quarter of the magnitude of those found at Wajima and they were slightly more elevated. This was likely due to an upslope–downslope couplet across the mountains of central Japan, with more frequent cloudiness and precipitation on the upwind, west side. Murakami et al. (2007) noted the frequent presence of clouds dominated by supercooled liquid water on the upslope side of west-central Japan in November and December, and a decrease in SLW occurrences to the east. Stratocumulus clouds sometimes form over the Sea of Japan as cold air blows across it (e.g., Inoue et al. 2005). When formed at the right temperatures, such “sea effect” clouds are likely candidates for icing. Heavy icing has been documented in similarly formed lake effect clouds over North America (Bernstein 2000).