Mercurial barometer resides on the west wall of the weather house. As atmospheric pressure increases, the mercury in the barometer rises. Mercury in the tube falls as pressure falls.

A dial on the barometer is turned to level a measuring device equal to the meniscus of the mercury inside the tube. The measuring device consists of lines, each representing increments of 1/100 of an inch. Lines on the barometer shaft are in increments of 1/10 of an inch, and a final set of lines in increments of 1 inch are located on the measuring device.

A white plate on the wood mounting board provides contrast so that the observer can better see when the measuring device and the mercury are aligned. Once aligned, the lines on the tube and measuring device can be read to determine atmospheric pressure at station elevation in inches of mercury.

This measurement must then be corrected to sea level before being recorded to the National Weather Service.

To adjust station pressure to sea level, the observer must consider the temperature of the mercury in the tube. A thermometer mounted to the tube, called the tube thermometer, indicates the tube temperature.

Temperatures at or below 74°F require the observer to add 0.87" to the station reading to equate to sea-level pressure.. At or above 75°F requires the observer add 0.79" to the station reading to equate to sea-level pressure. The expansion of mercury in the tube varies with temperature, and in order to compensate for this variable expansion, these values are used to compensate.

The weather observer must also report barometric pressure in millibar units. To do this, s/he uses a inches-of-mercury-to-millibars-conversion chart. The Y-axis is pressure in 1/10 inch increments, and the X-axis is pressure in 1/100 inch increments. The observer locates along the Y-axis the increment closest to his/her barometric reading. S/he then follows across the X-axis to find the exact reading to the 1/100 of an inch. In the intersecting box is the equivalent value in millibar units. The photo at right shows the chart used.

Determining relative humidity and dew point temperature is done in a similar manner.