Climatic Tables

by

Donald T. Jensen and Gaylen L. Ashcroft

Climate data from several dozen stations of the Cooperative Observer Network are presented in this book. The Cooperative Observer Network consists of volunteers who make observations of maximum and minimum temperatures and precipitation at the same time each day--usually 8:00 AM or 6:00 PM. Most volunteers also record snowfall and depth of snow on the ground.

Those perusing climate data tables for the first time frequently bump into unfamiliar terms. To enlighten the novice and refresh the memory of the occasional data user, we have defined some terms and described how mission data were handled in some of the analyses.

The data are summarized in three different types of tables--Monthly Climate Summary, Daily Climate Summary, and Freeze Dates and Freeze-free Season. Term definitions are grouped under the title of the table in which the term appears.

Monthly Climate Summary

In the Monthly Climate Summary, both normal and average values are given. Normals are 30-year averages and are re-computed each 10 years. The 1961-1990 monthly normals and their standard deviations are calculated and printed for each station that has data for 1961 through 1990.

The tables also give averages. Average monthly values and their standard deviations are calculated using all available data. The amount of data available varies from just a few years for some stations to more than 100 years for others.

An example shows how average values are obtained. For a station with 40 years of observations, 40 average monthly-maximum-temperature values can be calculated for January--one value for each year. These 40 values are then averaged to provide the January average monthly-maximum-temperature value printed in the table.

Standard deviation is a measure of the variability of the climate at a station. For example, if the 40 average monthly-maximum-temperature values are quite different from year to year, the standard deviation will be larger. If the average monthly-maximum-temperature values are quite similar for all years--i.e., the yearly values are only a few degrees above or below the mean--then the standard deviation will be smaller.

The tables also give monthly extremes. For temperatures, the monthly extremes are the highest and lowest monthly averages. For precipitation, the extremes are the highest and lowest monthly totals. Tables also contain daily extremes and the number of days with temperatures or precipitation amounts exceeding or lower than selected threshold values.

Heating degree days and cooling degree days are based on a threshold temperature of 65°F. Average heating-degree-day values are used to estimate the average amount of energy required to heat buildings. Average cooling-degree-day values are used to estimate the average amount of energy that is required to air condition buildings.

Growing degree days, base 40, are calculated with a lower threshold of 40°F, and an upper threshold of 77°F. Base-40 values are used to estimate the rate of growth for temperate-region crops such as small grains and alfalfa.

For growing degree days, base 50, the lower and upper thresholds are 50°F and 86°F, respectively. Base-50 values are used to model the growth of tropical region plants such as corn.

Daily Climate Summary

The data for each month are presented in two tables on a single page. One table contains temperature data for each day of the month. The other presents precipitation information for each day of the month. Temperature and precipitation values in the table are calculated using all available data.

Freeze Dates and Freeze-free Season

The last Spring freeze for a given year is considered to be the latest occurrence of a 32°F or lower temperature prior to July 24. The first Fall freeze is the earliest occurrence of 32°F subsequent to July 24. The length of a freeze-free season for a given year is the number of days between the last Spring and the first Fall freeze.

Missing data can distort values. However, deleting all years with one or more minimum-temperature value missing would leave little data for analysis. As a compromise, the following procedure is used when data are missing. The date of the last 32°F in the Spring is determined. If there are 10 or more days missing between the Spring date and July 24, the year is eliminated from the analysis. The same criterion is applied between July 24 and the date of the first Fall freeze. For length of the freeze-free season, a year is eliminated from analysis if either the Spring-date criterion or the Fall-date criterion is not met.