28 FEBRUARY 2003
DATA DESCRIPTION AND APPLICATIONS
INTRODUCTION. This chapter summarizes each page in a typical site data
set and provides guidance for using the data.
DATA SET PAGE 1: CLIMATE SUMMARY. Figure 2-1 is a sample of Data
Set Page 1, which summarizes the site's climate.
Location Information. This section of Data Set Page 1 contains a
summary table that includes site name, location, elevation (above mean sea level),
W MO number, period of record (POR), and average (atmospheric) pressure not
corrected to sea level (higher elevations result in lower pressures). The W MO number
is a unique number assigned to every location in the world that takes and transmits
regular weather observations. The POR is the time frame over which the data used to
compute the statistics in this document were compiled.
Explanation of Design Values. Design values are provided for dry bulb
temperature, wet bulb temperature, and humidity ratio at specific percentile frequencies
of occurrence. The old EW D summer design values of 1%, 2.5%, and 5% were based
on the warmest four months of the year. In the United States this was standardized as
June through September. The new design values of 0.4%, 1%, and 2% are based on
the entire year. The old winter design values of 99% and 97.5% were based on the
three coldest months of the year (December through February). The new winter design
values of 99.6%, 99%, and 97.5% are based on the entire year. In other words, the new
design values are annual values, not seasonal values. In general, for mid-latitude
locations with continental climates (hot summer, cold winter), there are some
generalizations that can be made about the differences between the old and new
values. The new 0.4% annual value is comparable to the old 1% seasonal value. The
new 1% annual value is usually about a degree cooler than the old 2.5% seasonal
value. The new 2% annual value is similar to the old 5% seasonal value. The new
99.6% and 99% annual values are generally cooler than the old 99% and 97.5%
seasonal values; however, there is more variability between stations. The new design
values were instituted for several reasons. At some locations, the warmest or coldest
months of the year do not fall into the months listed above. It is easier to compare
locations that are in tropical or marine climates where there is less seasonal variability.
It is also more straightforward to compare southern hemisphere locations.
Dry Bulb Temperature
2-22.214.171.124 Median of Extreme Highs (or Lows). The dry bulb temperature extreme
high (or low) is determined for each calendar year of the POR along with the coincident
values for wet bulb temperature, humidity ratio, wind speed, and prevailing wind
direction. Median values are determined from the distribution of extreme highs (or