How can snow be described
Severe weather climatology: snowfall and drifting snow
1 Meteorological requirements and description
Snowfall is the term used to describe solid falling precipitation reaching the ground in the form of individual or connected or branched ice crystals (snow stars). Snowflakes are formed by the agglomeration and concatenation of snow stars in precipitation clouds.
Since there is less moisture in the air at low temperatures, heavy snowfalls with large snowflakes can usually be observed at temperatures around 0 ° C.
The decisive factors for the snowfall intensity and the size of the flakes are not the temperatures near the ground, but in the cloud in which the precipitation is formed. Heavy snowfalls can also occur at air temperatures below -10 ° C if a warmer air mass with temperatures close to 0 ° C slides onto the cold air close to the ground.
Light, occasionally moderate snowfall can also fall from flat inversion clouds in industrial conurbations and is known as industrial snow. In addition to inversion clouds close to the ground, industrial emissions (additional condensation nuclei) and slight vertical movements, which favor the failure of the fine-grained snow, are prerequisites for its formation.
Snow drifts occur when falling snow or snow lying on the ground is transported due to sufficient wind speed and is deposited behind obstacles or in depressions. The size of the snow drifts depends on the wind speed, the nature of the snow, the surface and the geometry of the obstacle. High wind speeds, dry and therefore loose snow, a slippery surface (e.g. a frozen lake or an icy blanket of old snow) lead to snow drifts in the lee of an obstacle, which quickly reach the height of the obstacle.
Dry snow can be transported over smooth ground from a wind speed of approx. 5 m / s. From an average wind speed of around 10 m / s or wind peaks of around 17 m / s, heavy snow drifts occur if there is enough fresh snow or loose old snow.
2 regional differences
The table below shows the mean annual frequency of days with a depth of fresh snow> = 1 cm and> = 10 cm for selected stations.
To estimate the number of days on which snow drifts can occur, the table also lists the number of days on which a total snow depth> = 10 cm was measured and at the same time wind peaks of at least 8 Bft (> = 17.2 m / s ) were observed. Under these weather conditions, strong snow drifts form if the snow is sufficiently loose. Since this evaluation does not take into account the nature of the snow, the number of days on which strong snow drifts actually form will be less than the frequencies given in the right-hand column of the table. However, the information is suitable to recognize regional differences and to be able to estimate the maximum possible number of days with heavy snowdrifts.
Frequency of days with fresh snow and with the potential for the formation of snow drifts, snow cover days with wind peaks above 8 Bft (period: 1981 - 2010)
(Islands: italic; Mountain top: CAPITAL LETTERS)
|Stations||Height above sea level in m|
Number of days with
Fresh snow height
> = 1 cm
Number of days with
Fresh snow height
> = 10 cm
Number of days with
Snow depth> = 10 cm
and Wind gusts> = 8 Bft
It is noticeable that days with fresh snow are observed around ten times as often in the summits of the German mountains as in the lowlands. The difference is even greater if only the days with heavy snowfalls (snow cover increase> = 10 cm) are considered.
In the lowlands (at the same altitude), snowfalls in eastern Germany are about twice as frequent as in the west.
The slightly higher frequency of snowfall in southern Germany can be explained by the higher elevation above sea level compared to the northern German lowlands.
Snowfall in winter in the coastal region of the Baltic Sea in north-eastern weather conditions is a regional peculiarity: the cold air absorbs heat and moisture over the mostly ice-free Baltic Sea. In the increasingly unstable stratified air, "train routes" of snow showers form, which locally lead to prolonged and abundant snowfalls in the Baltic Sea coast area. If the north-easterly wind blows strongly, significant snow drifts can also be observed on the Baltic Sea islands and near the coast.
In general, it can be stated that the meteorological conditions for heavy snowdrifts in the western lowlands occur on average less than once every 10 years. In the eastern inland lowlands and on the Baltic Sea coast, heavy snowdrifts are somewhat more frequent, but also not observed every winter.
In the exposed mountain areas, snow drifts occur on average several times in winter and in the low mountain range peaks in the sum of all days over several weeks in the winter half-year. On the Zugspitze, due to the extremely wind-exposed location and the high snow cover in winter, snow drifts are more the rule than the exception.
3 year cycle
The mean frequency of new snowfalls with an increase in snow depth of at least 10 cm is less than 1 outside of the low mountain ranges (see table) in relation to the year, i.e. heavy snowfalls do not occur every year.
The figure on the right shows the average monthly number of days with a depth of new snow> = 10 cm on the basis of 8 weather stations distributed across Germany (without summit stations, see table). Heavy snowfalls occur in the lowlands in the months of November to March. During the meteorological winter (December to February), the frequency is almost the same and, based on a winter month, is relatively rare with around 0.11 to 0.14 cases. In December, for example, new snow depths> = 10 cm only occur every 9 years (frequency 0.11) early in the morning at 7 a.m.
With increasing altitude above sea level, the frequency of snowfalls generally increases, also in spring. The figure opposite shows the mean annual cycle for 3 summit stations in German low mountain ranges (see stations in the table).
Heavy snowfalls can occur on the low mountain peaks from October to May. The months from December to March have almost the same frequency, which averages around 2 to 3 days per month.
Basically, it should be noted that the above figures are only mean values that give an indication of the probability of the occurrence of new snowfalls. In the individual years the frequency of snowfalls is very different and also very differently distributed over the individual months. There is great variability in winter weather conditions in all regions and altitudes in Germany.
Incidentally, the greatest snow depths in the peaks of the German mountains are measured in March or April. The absolute greatest snow depth was recorded on the 1142 m high Brocken on April 14/15, 1970 with 3.80 m and on the 2962 m high Zugspitze on April 26, 1980 with 7.80 m.
4 Effects and Damage
Snowfalls and drifting snow lead to impairments in road, rail and air traffic.
In addition to the depth of snow, the severity of the frost also determines the extent of traffic disruption. Heavy frost with temperatures around or below minus 10 ° C reduces the effectiveness of de-icing agents and increases the likelihood of technical malfunctions on means of transport.
Wet snow leads to snow breakage on trees and extreme snow depths on roofs can damage them or cause them to collapse.
5 Preventive protective measures and instructions
Please note the severe weather warnings and instructions on how to behave that are publicly distributed.
- Roads and railways are partly impassable after snowfall.
- Avoid Driving!
- Do your clearing and gritting duties!
- Trees can collapse under the heavy snow load.
- For planned train and air travel, it is advisable to obtain current information about existing and expected traffic restrictions.
- If masses of snow are deposited on roofs, it may be necessary to clear the roofs. Because of the risk of roof collapsing and the necessary personal safety against falls, this work should only be carried out by specialists or emergency services for disaster control.
Please also note the information on the website of the Federal Office for Civil Protection and Disaster Assistance BBK, e.g. the guide for emergency preparedness and correct action in emergency situations with downloads and a checklist.
6 Supplementary information for specialist users
Information on the maximum snow load on roofs can be found in DIN standard 1055, part 5. The design snow load for roofs shown in the building standard is regionally differentiated and takes into account, in particular, an increase in the design snow load with height above sea level.
Therefore, the probability of snow load damage to buildings in the upper mountain region hardly differs from that in the lower mountain regions and in the lowlands if the construction was carried out in accordance with the standards.
The increasing warming in the course of climate change tends to decrease the number of days covered by snow cover; on the other hand, the precipitation processes intensify. Therefore, from the decrease in the number of days with a snow cover, it cannot be deduced that the frequency of extreme snowfall events will decrease significantly and whether they will weaken in their intensity.
In addition, the predicted increase in passenger and freight traffic will tend to increase rather than decrease the vulnerability of transport connections to winter weather phenomena. In this way, snowfalls and snow drifts will retain their significance as winter storms for the transport systems in Germany and possibly also for buildings (snow loads on roofs) in the coming decades, despite climate change.
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