Lesson 3/Learning Event 1
Learning Event 1
CONDITIONS NECESSARY FOR FROST ACTION
INTRODUCTION
One year has colder temperatures than another year. When you design you must do it to withstand the
coldest years. Sometimes you will have good records of the temperatures for previous years, sometimes
the records will not be so good. You have to consider temperature and other conditions that make the
situation susceptible to frost.
Terms Related to Temperature.
Average daily temperature. The average of the maximum and minimum temperatures for one day, or
the average of several temperature readings taken at equal time intervals, generally hourly, during one
day.
Mean daily temperature. The mean of the average daily temperatures for a given day in each of
several years.
Degree-days. The Fahrenheit degree-days for any one day equal the difference between the average
daily air temperature and 32F. The degree-days are negative when the average daily temperature is
below 32F (freezing degree-days) and positive when above (thawing degree-days). Figure 17 shows
curves obtained by plotting cumulative degree-days against time.
Freezing index. The number of degree-days between the highest and lowest points on a curve of
cumulative degree-days versus time for one freezing season. It is used as a measure of the combined
duration and magnitude of below-freezing temperatures occurring during any given freezing season.
The index determined for air temperature approximately 4.5 feet above the ground is commonly
designated as the air-freezing index, while that determined for temperatures immediately below a
surface is known as the surface freezing index.
Design freezing index. The average air freezing index of the three coldest winters in the latest 30 years
of record. If 30 years of record are not available, the air freezing index for the coldest winter in the
latest 10-year period may be used. To avoid the necessity for adopting a new and only slightly different
freezing index each year, the design freezing index at a site with continuing construction need not be
changed more than once in 5 years unless the more recent temperature records indicate a significant
change in thickness design requirements for frost. The design freezing index is illustrated in Figure 17.
Mean freezing index. The freezing index is determined on the basis of mean temperatures. The period
of record over which temperatures are averaged is usually a minimum of 10 years, and preferably 30,
and should be the latest available. The mean freezing index is illustrated in Figure 17.
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