DAVIS WeatherLink for Windows 4.0 - Manual (Page 105)

T E C H N I C A L R E F E R E N C E
Weather Data Calculations
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determines which bin contains the most markers and writes the corresponding
wind direction to the archive memory as the dominant wind direction.
Note: During intervals with very low wind speed, it is possible for no wind direction to be recorded,
because wind speed was always 0 when direction was sampled. In this case, wind direction
for the interval appears as a pair of dashes (--).
Temperature/Humidity Index
Temperature/humidity index (THI) uses the temperature and the relative humid-
ity to determine how hot the air actually “feels.” When humidity is high (i.e., the
air is saturated with water vapor) the apparent temperature will be higher than
the air temperature because perspiration cannot readily evaporate into the sur-
rounding air. Technically, THI is calculable only when air temperature is above
68° F (20° C) because it is a measure of heat stress, which is not significant at lower
temperatures. If temperature is below 68° F (20° C), the software reports THI as
being equal to outside temperature. Conversely, 125° F (52° C) is the highest T-H
Index for which calculation factors are available.
In terms of storage to the WeatherLink and database, temperature/humidity
index (THI) is a very special case. It is not stored in archive memory or in the data-
base, rather it is calculated “on-the-fly” as necessary (for example, when plotting
or displaying database information). When THI data is needed, the software cal-
culates an average THI for each archive interval based on the temperature and
humidity readings for the archive period.
Wind Chill
Wind chill is not stored in archive memory, rather it is calculated as the data gets
written to the database file. The software calculates wind chill based on the tem-
perature and wind speed readings for the archive period (that is, the average tem-
perature and the average wind speed during the interval).
Dew Point
Dew point is not stored in archive memory, rather it is calculated as the data gets
written to the database file. The software calculates dew point based on the aver-
age temperature during the interval and an approximation of the average outside
humidity during the interval. To arrive at an “average” outside humidity reading,
the software averages the outside humidity for the current archive interval and
the previous archive interval (if available). If no earlier entry exists in the archive
memory, the Weatherlink calculates dew point based on the current outside
humidity and the average temperature.