The main sensors are located at the top of the unit. The IR Thermopile detects
the temperature of the sky by sensing infrared radiation in the 8 to 15 micron
wavelength range. It uses a thermopile to do this. This is compared to the
measured ambient temperature to calculate the “Sky minus Ambient” reading.
A large reading indicates clear skies, as the sensor is exposed directly to the
cold of space. If the temperature is nearly the same as ambient, that indicates
low clouds and a high probability of precipitation.
Please note that very high cirrus clouds made of ice crystals are by their nature
very cold and may in some cases not be detected by the cloud sensor.
Fortunately, cirrus clouds do not produce precipitation, so they are not a risk
for your equipment.
Weather conditions can exist where rain falls without clouds overhead. An
example is a situation where high winds aloft carry precipitation from clouds
over an area that is not under cloud cover. For this reason, the Boltwood Cloud
Sensor III includes a very sensitive raindrop detector.
The rain sensor can detect individual rain drops. It will not signal a rain
condition until a couple of drops are seen in a short period of time, in order to
avoid false-triggering because e.g. an insect landed on it.
The Anemometer measures wind speed. It has two sensor stalks that operate in
a differential fashion. It works by detecting the temperature drop on the heated
stalk caused by blowing wind. The second unheated stalk provides a
comparison reference. Sunlight if present will heat both stalks identically,
which removes any bias caused by solar heating.
Note that the Thermopile, Anemometer, and Rain sensor stalks are user-
replaceable if they should ever become damaged. Contact our technical support
for assistance.
The Anti-Bird wires are optional. They keep birds from landing on the sensor.
Aside from directly blocking the sensor’s view of the sky, birds tend to leave
droppings, which would also block clear access to the sky in a somewhat more
permanent fashion. The wires give the bird a gentle poke in the underside,
which encourages them to fly off.
We recommend installing the Anti-Bird wires only if the sensor is installed out
of reach. Otherwise someone could accidentally poke themselves on them. The
sensor should be installed high up for a clear, unobstructed view of the sky and
to minimize wind blockage, etc.
The Sun Shield protects the weather sensors underneath from direct sunlight,
while allowing air to flow through freely.
Underneath the Sun Shield are power and roof cable connections, plus several
additional sensors. Keep this area clear and open to the environment, do not
cover it.
