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Instruments and Exposure

 

Location:

The weather station is located on Mount Benson near the city of Nanaimo on Vancouver Island.

Instruments:

The station is a Davis Vantage Pro 2 (Cabled). The instruments include:

1. Tipping Bucket Rain Gauge. The gauge is heated with a Davis rain collector heater during cold weather. This melts any snow that enters the gauge funnel and provides a pretty accurate measurement of snow water equivalent. In this way I can maintain a good record of total precipitation. The weakness is that there is no sure way of determining whether the precipitation is liquid or frozen other than by human observation. MSC engineers have never approved of heated precipitation gauges since the heat will cause some evaporation. I have to agree but feel that the amount of evaporation is slight and it is small price to pay to get the snow water equivalent data. Currently I compare the Davis tipping bucket gauge with the manual Type B gauge for each liquid precipitation event. When required I make adjustments to the TBRG to ensure that it is providing the same value as the Type B. The Type B gauge is the standard manual gauge used in Canada and is considered accurate to 0.2mm.

2. Anemometer. The wind sensors are atop a 5.5 metre pole located in my back yard.

4. Temperature and Humidity Sensors. The temperature and humidity sensors are enclosed in a fan aspirated Davis radiation shield. The fan is on a timer and runs only during the day.

5. Barometer. The pressure transducer is located in the house. I monitor my mean sea level pressure output and find that it agrees with Nanaimo airport sea level pressure to within 0.5 hPa. I do regular comparisons of the pressure values using an MSC regional inspection barometer. You can find the numbers on my Quality Control page.

 

Exposure:

East facing slope of Mount Benson at 194 metres elevation. The mountain rises to a 1000 metre high ridge 2.3 km to the southwest. The immediate area is country residential carved out of a second growth fir forest. The steep upslope exposure compounds the arctic outflow snow events that occur along the east coast of Vancouver Island at times during the winter months.

 

Interpreting the Data:

Rain Today is the total precipitation (rain plus melted snow) since midnight.

Hourly is the precipitation (rain plus melted snow) that has occurred in the past 60 minutes.

Monthly is the current month's precipitation to date (rain plus melted snow).

Total is the amount accumulated since the unit was last reset. I recently modified the total manually so it now reflects the annual accumulation for the calendar year. My climate summaries provide a much better record of total precipitation, have a look at them.

As a rule of thumb the relationship between snow and melted snow (water equivalent) is 10 to 1, in other words 1 cm of snow = 1 mm of water. This is only an approximation however, very dry snow will not yield as much water, wet snow more.

The Daily Precipitation Graph should show 1 step for each 0.2 millimetres of precipitation, either rain or melted snow (the gauge is heated), for the past 24 hours. However this does not seem to be the case, the steps are random and you must check the value on the Y axis to determine the value of the steps. The X axis is the time, there is a vertical line every second hour and the actual time in 24 hour clock mode is shown below each of the vertical lines. This graph is useful for determining the intensity of the precipitation and to see what time of day it occurred. The Y axis is the daily amount in millimetres, this value resets to zero at midnight.