"INTRODUCTION For a number of years@ the American Society of Heating@ Refrigerating and Air-Conditioning Engineers (ASHRAE) has provided@ in its Handbook ?C Fundamentals@ tables of climatic design conditions suitable for the proper sizing of heating and cooling systems. These tables include values such as dry-bulb temperature@ wet-bulb temperature@ dew-point temperature@ enthalpy@ and wind speed at various frequencies of occurrence over a long-term period@ corresponding mean coincident values of some other parameters@ and averages of some extremes (ASHRAE@ 2005). These climatic design conditions are widely used by the Heating@ Ventilating and Air-Conditioning (HVAC) industry@ and are referenced in a number of building-performance standards@ issued either by ASHRAE (Standard 90.1; ASHRAE@ 2004)@ or by other professional organizations (Manual J and Manual N; ACCA@ 2006@ 2008). The latest version of the Handbook (ASHRAE@ 2005) includes climatic design conditions for over 4@400 stations worldwide@ including over 750 in the USA@ over 350 in Canada@ and over 3@300 outside North America. Heating and cooling degree-days (DD) have long been used by HVAC engineers to assess a climate's severity. Degree-days can also be used to provide a simple estimate of annual load of a building@ as long as its indoor temperature and internal gains are relatively constant (ASHRAE@ 2005@ ch. 32 ?C Energy Estimating and Modeling Methods). Degree-days are made available by ASHRAE through Standard 169 (ASHRAE@ 2006) and will also be included in the next edition (2009) of the Handbook of Fundamentals. Climatic design conditions and heating and cooling degree-days are also available from other sources@ such as the Engineering Weather Data Handbook (AFCCC@ 2006). The availability of large sets of data in electronic format (Del Greco et al.@ 2006) now enables the calculation of climatic design conditions and degree-days for an ever increasing number of stations@ with data drawn from an ever larger number of years. As an example@ for the 2001 Handbook@ 12 years of data (1982-1993) were used to calculate climatic design conditions for 1460 locations worldwide. For the 2005 Handbook@ 20 years of data (1982-2001) were used for 4@422 locations. For the 2009 Handbook under development@ up to 30 years of data could be used@ and over 5@400 locations are expected. The use of a longer period is generally viewed as beneficial@ as statistics generated from a larger pool of data are more accurate. Climate Normals@ for example@ are defined by the World Meteorological Organization (WMO) as ""period averages computed for a uniform and relatively long period comprising at least three consecutive 10-year periods"" (World Meteorological Organization@ 1984). On the other hand@ scientists are realizing that climate is changing on a global scale because of anthropogenic factors@ more rapidly than even experienced in the past (Trenberth et al.@ 2007). This has HVAC engineers starting to question the validity of using climatic design conditions and degree-days based on the last 30 years of data. Will these conditions be appropriate for the next 20 to 30 years@ which is typically the life span during which HVAC systems designed today will operate? Should one instead derive different design conditions from shorter and more recent periods of record@ which will be more indicative of the climate of the future rather than that of the past? Guttman (1989) discusses the use of 30 years of data to derive long-term averages@ and warns against attaching a predictive value to normals. Colliver and Gates (2000) determined as 12 the minimum number of years needed to provide reasonably accurate estimates of climatic design conditions. They did not find@ however@ a significant interdecade variability of the design conditions. They suggest to use up to 30 years of data when available@ arguing that effects related to the evolution of climate are smaller than the additional variability that would be introduced by using a shorter period-of-record. Hubbard et al. (2005) recommended the use of a minimum of 10 years of data to derive climatic design conditions. They also looked specifically at the possible effect of long-term trends in climate on design conditions@ and found increases consistent with known trends in urban warming for areas undergoing rapid urbanization; however within the limited set of stations (17) used in the study they found no evidence that global climate change leads to a variation in climatic design conditions. More recently Huang (2007) has noticed an increase in cooling degree-days@ and a decrease in heating degree-days@ for stations in China over the 1973-2006 period. This paper will show that long-term changes in climatic design conditions can indeed be observed. Theses changes are clearly visible both on dry bulb and dew point design conditions@ and on heating and cooling degree-days. The paper will provide an estimate of the magnitude of this effect and show that it is more likely correlated to urban island effect than global warming. The paper also reassesses the errors introduced by using periods of records shorter than 30 years and provides an estimate of their magnitude."