Why the Regional Weather Matters

An examination of your local outdoor climate is the place to begin a true analysis of the environmental conditions in storage vaults and exhibition areas. Once you have uploaded the weather data, review the graphs for a full year of temperature, relative humidity and dew point. Are you dealing with long, hot summers and high moisture content? Are you primarily concerned about winter dryness? Or are wide fluctuations in temperature or humidity worrying you? A closer look at the local environment is worthwhile since all the air in your storage spaces comes from the outside and is modified by your existing mechanical system. Are you doing the best you can to achieve a preservation environment?

Outdoor data typically shows moderate daily fluctuations in temperature. These ups and downs are less important than long-term seasonal trends. Most places have a winter season with cold temperatures and a warm to hot summer, but this can vary by region. How extreme are the seasonal temperature differences in your area?

The following graph shows a full year of temperature data from five regions in the United States—Las Vegas NV in the Southwest, Key West FL in the Southeast, New York NY in the Northeast, Medford OR in the Northwest and Rock Springs WY, which is on the west end of the Midwest. The key above the graph identifies the colors that correspond to each city.

You can see the arc of the general seasonal trend in temperature, along with regional variation – particularly for the Key West, FL data which remains relatively warm throughout the winter.

Daily fluctuations in relative humidity (RH) can be more significant than temperature fluctuations. Relative humidity trends over the year are a function of temperature and dew point. In many temperate areas the shape of the temperature and dew point graphs are similar, peaking in the summer when the air is hot and holds lots of moisture. The opposite is true in winter when air is generally cool and dry. In tropical locations the dew points are high all year long, the temperatures are uniformly quite warm, and the RH line is flatter in shape over the year and consistently high – which is again illustrated by the Key West FL data. Seasonal trends are less obvious on the RH graph.

The next graph shows a full year of relative humidity data from the same five locations. It illustrates both the extreme differences in outdoor RH between regional climates and the large night and day variation of outdoor RH.


Finally, the dew point graph below shows a full year of data from the same five locations. The large variation in the moisture content of the air in most of these cities is easier to read in the dew point graph, rather than the RH graph. What does your local dew point graph show you?

The following table gives you an overview of the average temperature, relative humidity, and dew point temperature for each of the cities illustrated in the preceding graphs. As you can see the range of averages is significant:

City Start Date End Date Average
Dew Point
Key West,FL 1/1/2009 12/31/2009 78.1°F 74.1% 68.9 °F
Medford, OR 1/1/2009 12/31/2009 51°F 69.6% 38.5 °F
New York, NY 1/1/2009 12/31/2009 54.1°F 71.6% 44 °F
Rock Springs, WY 1/1/2009 12/31/2009 40.2°F 59.9% 24 °F
Las Vegas, NV 1/1/2009 12/31/2009 69.2°F 26.1% 27.6 °F


Climate Zones and Collection Care
The US can be divided into climate zones ranging from tropical to desert to marine. These zones vary in temperature, humidity, and dew point because of geographical differences such as elevation, latitude and average rainfall. These climate zones are illustrated below.

This map and others illustrating climate zones are available online at:

Regional climate differences can have an impact on the storage recommendations for collections preservation. The descriptions below provide an overview of the preservation challenges presented by various climate zones.

Continental Climate – Warm to cool summers and cold winters

Protect collections in storage from extremes in temperature and relative humidity year round. Use cooling and dehumidification in the summer months to reduce temperature and humidity levels in storage.  Use heat and humidification in the winter season to increase temperatures and add moisture to acceptable levels.

Dry (Desert and Plains) Climate – Very dry, often cold at night and hot during the day, little rain

Manage wide fluctuations in temperature. Good air filtration is required to protect collections from dust and soot. Protect collections from high levels of sunlight, which can increase the rate of deterioration in vulnerable materials. High humidity is rare. Add moisture during periods of low RH which can lead to brittleness.

Sub-Tropical Climate – Warm and humid summers with mild winters

The climate is conducive to mold and mildew growth and biological decay. Exterior walls may be susceptible to condensation problems when warm humid air meets cold surfaces. Adequate moisture barriers and good air circulation are important. Increase temperatures and dehumidify to reduce high RH.

Tropical, Warm and Humid Climate – Heavy rainfall, high humidity and high temperatures

This climate encourages biological decay and insect growth. High RH increases the rate of deterioration due to moisture absorption. Moisture barriers, good air circulation, and the ability to dehumidify are very important.

Historically, regional climate influences the development of regional architectural styles. Thick adobe walls offer shelter from the sun and keep interiors cooler in hot, dry climates. Verandas, courtyards, porches and high ceilings provide protection from the sun, particularly in the south. Homes in tropical areas often feature elevated floors, louvered grilles and shutters, and balconies to promote air circulation.

Once you know the average seasonal dew point in your location, you can use IPI's dew point calculator (www.dpcalc.org) to help you find the ideal temperature and RH levels for a given space.