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Weather Data Calculations
97
Rainfall
In calculating both daily and yearly rainfall totals for the software, the Weather-
link only checks the total rain register on the station. The Weatherlink compares
the current total rain value to the previous total rain entry in the archive memory
to determine the amount of rainfall which occurred during the interval.
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Setting or Clearing Total Rain
You must set and clear total rain from the software, not the weather station!
When you set or clear the Total Rain register from the software, the Weather-
link makes an internal note of the change so errors will not occur when calcu-
lating daily rainfall for the interval. If, however, you set or clear total rain from
the station, the Weatherlink does not make this internal note and may calcu-
late an inaccurate daily rainfall amount for the interval (often a negative num-
ber).
Degree-Days
Because temperature plays an important part in the rate of development of plants
and many pests (especially insects), a measurement which takes into account the
accumulation of heat with passing time is necessary to predict maturation.
Degree-days provide a measure for calculating the effect of temperature on the
development of plants and/or pests. One degree-day is the amount of heat which
accumulates when the temperature remains one degree above the base develop-
mental threshold for 24 hours. One degree-day is also the amount of heat which
accumulates when the temperature remains 24˚ above the base threshold for 1
hour.
Unlike strict time predictions of plant/pest development, degree-day predictions
hold true regardless of location or temperature fluctuations. As long as you know
the number of degree-days necessary for plant/pest development, you may use
degree-days as an accurate predictor. For example, you may know that it takes, in
general, three weeks for a specific pest to develop. What you will find, however, is
that the pest may take 4 weeks to develop in cooler weather and only 2 weeks to
develop in warmer weather. The time prediction can be off by up to a week in this
example, while the degree-day prediction should result in far greater accuracy.
The WeatherLink software uses the outside temperature data in conjunction with
the base and upper thresholds entered for each crop/pest to calculate degree-
days. You may choose between three possible methods for calculating degree-
days: “cut off,” high/low, or integration. (For a description of these calculation
options, see “Degree-Day Calculation Method” on page 86.)
Chilling Requirement
Certain fruit trees bear best when temperatures drop below specific levels for spe-
cific amounts of time during the dormant season. Chilling requirements provide a
measure of this dormancy. For more information, contact your local agricultural
agent or university agricultural extension.
