Climate guidelines glossary

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The information in this document is based on the current understanding of the issues presented. It does not necessarily apply in all situations, nor do any represented activities ensure complete protection as described. Although reasonable efforts have been made to ensure that the information is accurate and up to date, the publisher, Canadian Conservation Institute (CCI), does not provide any guarantee with respect to this information, nor does it assume any liability for any loss, claim or demand arising directly or indirectly from any use of or reliance upon the information. CCI does not endorse or make any representations about any products, services or materials detailed in this document or on external websites referenced in this document; these products, services or materials are, therefore, used at your own risk.

absolute humidity

A measure of humidity in terms of the weight of water vapour per unit volume of air. At 20°C, the absolute humidity of 100% RH is 17.3 g/m³.

Note: This measure of humidity is useful for engineers designing mechanical systems, but it is not useful for predicting the moisture content of objects in a collection.

See also: relative humidity

climate

The temperature and relative humidity (RH) of the air. When applied in a museum or archive context, it can refer to the long-term and short-term behaviours of temperature and RH.

Note: The broader term usually refers to long-term trends in the weather outdoors.

See also: environment

climate control

The maintenance of a particular temperature and RH inside a building, room or enclosure by the use of machinery (active climate control) or humidity buffers and insulation (passive climate control).

deliquescence

The formation of a solution by certain salts that absorb moisture from the air above a critical RH value.

Note: For example, table salt (NaCl) deliquesces at 75% RH and above.

enclosure

A physical barrier that surrounds an object (or objects). It can be a bag, a box, a display case, a storage cabinet, a shipping crate, etc.

Note: For the purposes of this resource, an enclosure blocks some or all of the agents of deterioration (for example, physical forces, water, pollutants and incorrect RH).

environment

All the conditions that surround and influence an object.

Note: For the purposes of this resource, this includes the agents of deterioration (for example, physical forces, water, pollutants, light, ultraviolet, incorrect temperature and incorrect RH).

See also: climate

lifetime

The time (usually in years) for an object to deteriorate to a state at which it no longer has value or utility to the institution.

Note: For example, the time required for digital media to become unreadable or the time it takes an archive document to become too brittle to be handled easily. For further details, consult Explanation of the mould and lifetime calculators − The idea of object lifetime.

microclimate

The local climate in a small region of space. A microclimate can be created on purpose by the use of an enclosure, which usually controls RH but may occasionally control temperature.

Note: A microclimate can also arise naturally, without sharp boundaries. For example, the air within a few centimetres of a cold floor has a much higher RH than the majority of the room and is often the cause of local mould growth.

microclimate enclosure

An enclosure designed to maintain a particular microclimate. It is usually designed to stabilize the RH, but it may also be designed to reduce temperature fluctuations, as in the case of shipping crates.

microenvironment

The local environment in a small region of space surrounding an object. A microenvironment can be created on purpose by the use of an enclosure in order to control several agents of deterioration.

Note: A microenvironment can also arise naturally, without sharp boundaries. For example, the air within a few millimetres of any material that is emitting volatile organic compounds will pollute an adjacent object.

microenvironment enclosure

An enclosure designed to maintain a particular microenvironment. It may be designed to limit one or more of the agents of deterioration.

proofed fluctuation

The largest fluctuation in RH that an object or collection has experienced in the past. It is a key parameter in sustainable climate control decisions because future fluctuations that are less than the proofed fluctuation have a very low probability of causing significant new fractures.

Note: In the most common Canadian situation, where the annual average of RH is moderate and the biggest risk to collections from fluctuations occurs during the low RH of winter, it refers to the lowest RH of winters past.

Note: For further discussion of the refinements in its application to the risk management of collections and decisions about climate control, consult Climate guidelines overview − Appendix B: Sensitivity to fluctuations and the application of proofed fluctuations.

relative humidity (RH)

RH is the ratio of the partial water vapour pressure to the saturation water vapour pressure at the same temperature. It can also be expressed as the ratio of the concentration of water vapour to the saturation concentration of water vapour at the same temperature.

Note: Unlike absolute humidity, RH is the measure of humidity that correlates well with the moisture content of objects, as well as with the rate of mould growth and with our own perception of damp and dryness.

Note: RH is expressed as a percentage.

See also: absolute humidity

risk management

The process within an organization that has as its goal the minimization of the largest risks to its collections.

Note: Risk management does not attempt to address all risks.

Note: For more detail, consult The ABC Method: a risk management approach to the preservation of cultural heritage.

sensitivity

The response of a material or object to a dose of an external agent, measured as a ratio between the response and the dose.

Note: For example, the number and size of fractures caused in an object by a certain size and number of RH fluctuations or the amount of fading of a colour after exposure to a certain amount of light.

See also: vulnerability

vulnerability

Adopted in various fields of risk (and disaster) management to refer to the susceptibility of an institution or a community to a hazard. It is a complex assessment that considers not only the direct effects on objects and people but also the eventual loss of value caused by those effects, given the mitigating factors of the resilience of the institution and community.

See also: sensitivity

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