Visibility Meter

The summit is often bathed in clouds, but manual weather observations at the summit typically record the estimated visibility and any precipitation only once each hour.

Some research projects, such as the measurement of cloud properties or precipitation types and amounts, require a minute-by-minute record of the cloud density (visibility) and precipitation during the times of interest. The Vaisala model FD-12P is intended to fulfill that purpose. It normally operates unattended day in and day out. The minute-by-minute visibility and precipitation type and amount are recorded automatically on a computerized data logger inside the observatory on the summit.

IMAGE: The Vaisala model FD-12P visibility and present weather detector, used for a continuous, 24-hour-a-day record of cloud and precipitation conditions at the summit of Mt. Washington.

“Visibility” is a measure of how far you can see in the atmosphere. In clear air you can see for long distances, maybe tens of miles. This is “good” or high visibility. In fog (cloud) you may only be able to see objects a short distance away. This is “poor” or low visibility. For scientific purposes, visibility is recorded more precisely in feet or meters. The model FD-12P is calibrated for visibilities from ten meters (33 feet) to 50 km (30 miles).

The visibility is measured by a light beam that is projected downward inside one of the slanted “arms” seen in the photograph. Whenever any cloud is present at the location of the instrument, the tiny cloud droplets deflect some of the projected light up into the other arm where a sensitive photodetector is housed. The photodetector generates an electrical voltage that is proportional to the amount of light deflected up the tube. This deflected light is, in turn, proportional to the density of the cloud (i.e., the number of cloud droplets per cubic centimeter). This density determines the visibility at the location of the instrument.

The photodetector is calibrated in units of visibility so that the scientist or analyst can tell at a glance what the visibility was or is at any time, day or night. The type and amount of precipitation (if any) is detected by a sensor in the small, open box on top of the instrument.

This visibility and precipitation information is recorded continuously at the summit observatory for the use of any interested scientists.