Flytec Windwatch Pro - Manual (Page 23)

Temperature and Humidity
23
Temperature and Humidity
Temperature and Humidity Sensors
The Windwatch PRO is equipped with rapid-reacting, high-sensitivity and digitally-synchronized
humidity and temperature sensors that are located in the small opening below the wind speed
impeller opening. Therefore the protective slider must be open to perform accurate temperature
and humidity measurements. Since the accuracy of the relative humidity reading depends on
the temperature of the sensor, and the temperature of the sensor is influenced by the
temperature of the instrument’s housing, the difference between housing temperature and
ambient temperature must be compensated electronically. To perform this compensation, the
current temperature of the sensor is taken into account. You will achieve the most accurate and
conclusive readings by allowing air to flow freely though the sensor openings. This is
accomplished by holding the Windwatch PRO in the same way as when taking wind
measurements (i.e., WWP held vertically, upwind, and with an outstretched arm).
See the section below on Temperature Measurement.
Dew Point
The dew point is the temperature at which 100 percent relative humidity is reached
and fog or clouds begin to form. The difference between the current temperature and the dew
point is an important indicator in determining the likelihood of fog developing. Based upon the
altitude-related cooling formula (0.65 °C per 100 m height), it is also possible to predict the
altitude at which fog or clouds will form. A rule of thumb with respect to fog is that is that if,
during evening hours, the air temperature and dew point value move towards each other, there
is a high likelihood of fog during the night.
Temperature Measurement
The measurement of air temperature is one of the most difficult challenges in meteorology. The
official measurement setup for meteorological temperature measurement is as follows: the
sensor is mounted inside a test tube 2 meters above a freshly mown grass field. For ten minutes
before the measurement is to be taken, air is aspirated by a ventilator through the test tube and
directed at the sensor; only then is the temperature value read.
The Windwatch PRO was designed to approximate this procedure as closely as possible using
a handheld instrument. The small opening with three protective ribs is used to channel the air
and to direct its flow at the sensor. The sensor is thermally decoupled from the housing as much
as is possible inside a narrow housing.
To obtain precise and rapid temperature measurements, hold the WWP with the slider open,
facing the prevailing breeze. If there is no wind, wave the WWP back and forth several times, or
swing it in circles by its lanyard. This step helps to equalize the temperature of the WWP
housing with the ambient air. The housing temperature will be in equilibrium with the ambient air
when moving the WWP through the air no longer changes the temperature reading. The
temperature reading at that point will be the actual air temperature.