12 Severe weather
The term severe weather is a collective term for all unusual, unseasonal or extreme weather events. These weather phenomena often have serious or even life-threatening consequences and present a number of safety and security problems to museums, archives and libraries.
The term severe weather encompasses a variety of weather phenomena of differing intensities and impact, for example:
- Storms/hurricanes
- Localized strong wind events (tornadoes)
- Heavy precipitation (combined with thunderstorms, hail or sleet) or continuous rain
- Blizzards
- Rapid thaw
- Freezing rain/ice
Extreme heat and cold are not generally considered severe weather events, but they can also cause extensive damage.
Storms and hurricanes
Strong winds are classified according to the associated wind speeds as storms (Beaufort 9 to 12 with wind speeds of 75 to 118 km/h) or hurricanes (Beaufort > 12 with wind speeds > 118 km/h). Brief wind speeds reaching at least 118 km/h are referred to as hurricane-force gusts.
Wind strengths are described using the Beaufort scale, named after the English Admiral Sir Francis Beaufort (1774-1857, devised in 1806). This scale permits an estimation of wind strength based on the perceived effects of the wind.
Table 1: Beaufort scale (Wikipedia)
Tornado
A tornado, also known as a twister, is a column of air that rapidly rotates around an almost vertical axis. In principle, a tornado could happen anywhere at any time.
The Fujita scale, named after Japanese meteorologist Tetsuya Theodore Fujita (1920-1998, devised in 1971), is used to rate the intensity of tornado winds which are categorized based on wind speed. The scale has a total of 13 levels (F0 to F12), although to date the values above F6 have not been observed and are consequently only theoretical.
Table 2: Fujita scale (adapted from Wikipedia)
Precipitation (rain and snow)
In meteorology, precipitation is any form of water that falls under gravity. It can take liquid (rain) or solid form (hail, ice pellets, snow).
A severe precipitation event is classified with reference to the amount of precipitation (in the case of rainfall) or the height it reaches (in the case of snowfall) within a given time period. A distinction is drawn between heavy rain, very heavy rain, and very heavy continuous rain, as well as snowfall and drifting snow.
The following table gives the basic guideline values for classifying a precipitation event.
Table 3: Severe weather (based on classifications used by Deutscher Wetterdienst)
Thunderstorm
Thunderstorms, usually accompanied by strong winds and heavy precipitation in the form of rain or hail, are electrical discharges between different electrical fields in the atmosphere or between the atmosphere and the surface of the earth, and are experienced as thunder (auditory) and lightning (visual).
Approaching thunderstorms can be recognized by the typical appearance of dark blue storm clouds (cumulonimbus). A positive charge builds at the top end of a thundercloud while a negative charge builds at the base of the cloud. The temperatures within the cloud are analogous: around -30 °C in the positively charged region, and approximately -15 °C in the negatively charged region.
This temperature gradient results in strong updrafts and downdrafts moving at high speeds. The water droplets and ice particles then collide. In conjunction with the water changing state, this drives the electrical charging and discharging process.
The intensity of a thunderstorm is described on the basis of the number of electrical discharges. A slight or moderate thunderstorm is characterized by few or regular lightning flashes. In the case of a heavy thunderstorm with a large number of flashes, the flashes can no longer be associated with a particular thunderclap.
Rapid thaw
Snow thaws once the air temperature near the ground rises above freezing. The greater this temperature difference, the more quickly this happens. At the same time, heavy rainfall accelerates the thawing of an existing covering of snow.
If the ground cannot absorb the water, because it is saturated or still frozen, the thawed snow and rainwater run off very rapidly, often causing streams and rivers to flood.
A thaw is considered to be a severe weather event if there is continuous (heavy) rain and the depth of existing snow is over 15 cm.
Freezing rain/ice
Deposits of frozen water on the ground or on objects are referred to as ice. A distinction is drawn between ice and freezing rain.
Ice is formed by accumulations of water freezing over (e.g. snowmelt). The hazardous nature of this form of ice lies in the fact that it is usually not expected.
Freezing rain is the immediate freezing of very cold droplets of rain when they hit the ground or objects. It also occurs (briefly) when the raindrops themselves are not very cold, but they fall on extremely cold objects or frozen ground.
“Extreme heat” and “extreme cold” are addressed in the questionnaire for the Climate section.
Prevention – mitigating the impacts
General preventive measures
Early warning or notification of severe weather events is essential in order to have enough time to take or initiate protective measures. Severe weather is usually announced in good time via the media (TV/radio, press, internet). Various internet portals, e.g. the severe weather service of the Deutsches Wetterdienst (www.dwd.de), offer the option of receiving weather alerts with detailed information and timings on a regional basis via an email distribution service.
After that the severe weather warnings should be evaluated on the basis of the circumstances of the institution with regard to the specific objects in their care and appropriate protective measures should be taken. For this it is necessary to perform a risk assessment covering the whole of the property in advance. This assessment should identify the potential damage that may be caused on the basis of the various severe weather scenarios and define protective measures to minimize or avoid damage. At the same time, the institution’s response to any damage suffered should be planned. A clear allocation of duties to the staff concerned (based on position and/or function), taking account of the technical and organizational requirements, will ensure that the preventive measures and responses are carried out in practice. When describing the measures, also consider the safety of the personnel concerned.
In addition to the risk assessment, a resister of systems and building elements should be drawn up. This should contain a wide range of information about the structural design, the permitted loads, materials used, and any necessary (periodic) inspections. A list of local building firms and other service providers available at short notice, as well as any existing emergency materials, is also useful.
Preventive measures for storm damage
Storms or hurricanes are not events that can be avoided. By virtue of their mode of construction or location, the following structures can be considered particularly at risk:
- All free-standing buildings or structures that stand out from a connected group of buildings
- Structures with large roofs or facade surfaces
- Structures with irregularly shaped outer walls or roof surfaces
- Structures that have facade areas (e.g. archways) that remain open as a consequence of operational or management processes
Preventive measures can minimize or even prevent storm damage over the medium to long term. In particular, regular inspecting roofs and facades and maintaining the various structures and attached elements can minimize the risk of damage. The following is a list of the most important checks to carry out (based on VdS Guideline 2089):
- Loose lightning rods
- Damaged, loose or detached elements; bubbles or tears in the roof covering
- Missing or damaged roof shingles/boards and their anchorings
- Cracked or spalled chimney heads, covers and enclosures
- Loose antenna mountings
- Cracks in exterior masonry
- Damaged canopies, pest or rot infestation in wood
- Loose snow guards
- Ill-fitting skylights, rooflights and domes
- Detached or bent gutters and downpipes
- Irregular gravel coverage
- Missing or damaged weatherproofing
- Loose roof structures
- Loosely attached facade decoration in the form of architectural sculpture
- Advertising fixtures inadequately attached to the structure (e.g. flagpoles, information signs and advertising boards)
Another preventive measure to avoid storm damage is to put away light and movable outdoor objects (tables, chairs, container plants, promotional displays, etc.).
The threat posed by trees being uprooted trees or branches being broken off should also not be underestimated. Routine patrols and inspections coupled with appropriate maintenance of park areas can reduce this risk.
In case damage does occur, it is expedient to keep a stock of suitable building materials (e.g. boards, films, metal sheets) and tools to hand in order to limit the damage and prevent further damage.
Preventive measures for damage caused by precipitation
Extreme precipitation in the form of heavy or continuous rain, and also rapid thaws, can cause local flooding of watercourses and may also result in consequential damage such as wastewater backing up as a result of rising groundwater levels and overloaded drains. These not only cause damage to building and technical structures, they also represent a danger to life and limb. As the topic of flooding is already addressed in detail in the Flooding section of these guidelines it will not be discussed further here.
Precipitation is another major threat: heavy snowfall and drifting snow can cause considerable damage to building structures (roofs, skylights, etc.). Experience in recent years has shown that the collapse of roofs due to the weight of snow is a major risk. A recent example is the exhibition hall of the Point Alpha observation post in Geisa (Röhn/Thüringen) in December 2010.
To determine the maximum depth of snow on roofs, or define the point at which the snow must be removed, it is essential to know the load-bearing capacity of the roof construction. Based on these values and the form of the snow, it is possible to determine from Table 4 the height of snow which must not be exceeded. A specialist engineer or, in the case of public buildings, the relevant building and property management department of the local authority should be consulted to determine these figures.
Table 4: Weight as a function of snow type (from Niederösterreichischer Zivilschutzverband, Safety Ratgeber Wetterbedingte Naturgefahren, S. 22)
If severe weather warnings for heavy snowfall are issued, it should be checked whether existing snow needs to be cleared off the roof or whether there is still enough load-bearing capacity in reserve. If clearing is contemplated, the following safety principles should be observed:
- Before going on the roof, check whether additional loads are still safe. In cases of doubt, consult an expert (structural engineer).
- Observe occupational safety procedures. There is a risk of falling through the roof on non-load bearing roof elements (e.g. light domes, arcade rooflights etc.) and at the edge of the roof. Moreover, these roof elements may be covered in snow and consequently not visible. Likewise ensure access routes are safe.
- When clearing the roof it is essential to pay attention to the structural loading of the roof and support structures. In some circumstances, clearing only one side of the roof could lead to problems if the load is unevenly distributed. It is also expedient to consult an expert here.
- Pay particular attention to securing personnel engaged in clearing the roof to prevent any falls.
- Make sure any areas on public paths and roads below the clearing points are closed off.
It is advisable to go through this list and take action in good time before the start of the winter season. If the institution does not have staff or equipment capable of clearing the snow, it can engage a suitable firm to do the work (maintenance contract).
Following a long winter with a lot of snow, it is advisable to have an appropriate expert inspect the roof and roof structure. The general condition of the structure should be checked for the following defects in particular:
- Damage to the roof covering (leaks)
- Pest infestation in wooden structures (e.g. fungus)
- Water stains as a result of condensation
- General damage to the supporting structure (roof trusses)
- Corrosion of metal support structures
Snowfall in combination with moderate or strong winds is also a further threat if the roof covering is not watertight. Further damage can be avoided by regularly inspecting these areas and dealing with any problems at an early stage.
Institutions located in regions where there is a risk of avalanches should consult experts to assess the risk and take appropriate protective measures.
Hail is another form of solid precipitation. Strong updrafts in thunderclouds prevent small raindrops falling down from the cloud. The droplets are propelled higher and higher, then they freeze and new ice is formed. Once these ice particles can no longer be held by the updrafts, they fall to the ground as cold raindrops, ice pellets (particles < 5 mm) or hail (particles > 5 mm). The stronger the updrafts in the thundercloud, the larger the hailstones can become. Extreme specimens with a diameter of up to 8 cm are possible as individual ice particles, or up to approximately 10 cm as accumulations.
The larger the diameter of the particle, the greater the hazard and potential for damage. In extreme cases, hailstones can break through roofs and glass surfaces, and can also wreak havoc in parkland. If hail falls in small particles, local flooding can also be triggered.
To protect vulnerable objects from damage, it is advisable to use hail protection nets and tarpaulins. It is also possible to move mobile objects to safety.
To ensure they are protected against hail, when erecting new buildings or renovating old ones, hail-resistant materials should be used for elements of the building envelope such as windows, rooflights, skylights etc.
Preventive measures for damage caused by thunderstorms
In addition to storms and heavy precipitation, thunderstorms can also result in electrical discharges (lightning strikes).
Lightning strikes can cause fires, started by short circuits in electrical networks, as well as explosions and injuries to people. For instance, if a tree is hit by lightning, the water present in the tree vaporizes as a result of the intense heat, and this can have explosive consequences.
Lightning rods can be fitted to buildings to protect them from damage caused by lightning strikes. It is essential that these are installed correctly and are routinely inspected in accordance with the building regulations.
To avoid personal injury, observe the following precautions during a thunderstorm:
- Keep away from water faucets or showers
- Do not carry out any wet work during a thunderstorm
- Do not use a corded landline telephone
- Close windows and doors
- Do not touch gas, water or heating pipes
Keep at least 1 meter away from electrical equipment.