Degree Days
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- Degree days give a measure of the effect of the seasons' temperatures
on crop growth and fuel requirements, especially in the case of a building
which is continually heated. For each day that the average temperature
is one degree above the base temperature, one degree day has accumulated.
- Probably the most widespread application of the degree day concept
is the management of industrial and domestic heating. Heating degree
days are indicators of household
energy consumption for space heating. The air temperature in a building is
on average 2° C to 3° C higher than that of the air outside. A temperature
of 18° C indoors corresponds to an outside temperature of about 15.5° C.
If the air temperature outside is below 15.5° C , then heating is required
to maintain a temperature of about 18° C. The sum of the degree days over
periods such as a month or an entire heating season is used in calculating
the amount of heating required for a building. Degree Days are also used to
estimate
air conditioning usage during the warm season.
- Because of the relationship between temperature and the rate at
which plants grow and develop, the concept of accumulated degree days
has
been found useful
in quantifying the accumulated energy of a region or season. Such heat units
have widespread use in agriculture, particularly in the vegetable processing
industry to predict the date of harvest and the timing of successive harvests.
- Monthly and daily Degree Day values are available from our Synoptic
and Climate
stations. The base temperature for monthly values is 15.5° C.
Base temperatures for daily values are 15.0, 15.5, 17.0, 18.0 and 18.5 ° C
and include:
- Daily maximum and minimum temperatures
- Daily degree days above and below the base temperatures
- Monthly degree day values and the mean maximum, mean maximum, and
mean air temperatures
The method used in calculating Degree Days is as follows, 4 cases
are considered:
Case I
The minimum air temperature (Tmin) above the base temperature (Tbase).
In this case there are no degree-days below the base temperature
and the degree-days
above are given by 1/2(Tmax + Tmin) - Tbase.
Case II
The maximum air temperature (Tmax) below the base temperature. In this
case there are no degree-days above the base temperature and the degree-days
below
are given
by Tbase - 1/2(Tmax + Tmin).
Case III
The maximum air temperature above and the daily minimum air temperature
below the base temperature but the mean temperature which equals
1/2(Tmax + Tmin),
greater than the base temperature. In this case degree-days above
are given by 1/2(Tmax - Tbase) - 1/4(Tbase - Tmin) and degree-days below
by 1/4(Tbase
- Tmin).
Case IV
The maximum air temperature above and the minimum air temperature
below the base temperature but the mean temperature which equals
1/2(Tmax
+ Tmin), less than
the base temperature. In this case degree-days above are given
by 1/4(Tmax - Tbase) and degree-days below are given by 1/2(Tbase -
Tmin) - 1/4(Tmax
- Tbase).
If the mean temperature is equal to the base temperature the same
results obtain using either the formula for Case 111 or Case IV |