DAVIS Energy EnviroMonitor: Console - Manual (Page 17)

Energy EnviroMonitor Overview
Page 10 Energy EnviroMonitor Manual
Barometric Pressure
The weight of the air which makes up our atmosphere exerts a pressure on the
surface of the earth. This pressure is known as atmospheric pressure. Gener-
ally, the more air above an area, the higher the atmospheric pressure, which
means that atmospheric pressure changes with altitude. To compensate for this
difference and facilitate comparison between locations with different altitudes,
atmospheric pressure is generally adjusted to the equivalent sea-level pressure.
This adjusted pressure is known as barometric pressure. In reality, the
Health EM measures atmospheric pressure. When you enter the barometric
pressure for your location (see “Entering Barometric Pressure” on page 37), the
Health EM stores the necessary offset value to consistently translate atmo-
spheric pressure into barometric pressure.
Barometric pressure also changes with local weather conditions, which makes
barometric pressure an extremely important and useful weather forecasting
tool. High pressure zones are generally associated with fair weather while low
pressure zones are generally associated with poor air. For forecasting purposes,
however, the absolute barometric pressure value is generally less important
than the change in barometric pressure. In general, rising pressure indicates
improving weather conditions while falling pressure indicates deteriorating
weather conditions.
• Current Barometric Pressure (Alarm: rate of change)
• Trend of Barometric Pressure (rising, falling, steady) over previous one hour
Humidity
Humidity itself simply refers to the amount of water vapor in the air. However,
the amount of water vapor which the air can contain varies with changes in air
temperature and pressure. Relative humidity takes into account these factors
and offers a humidity reading which reflects the amount of water vapor in the
air as a percentage of the amount the air is capable of holding. Relative humid-
ity, therefore, is not actually a measure of the amount of water vapor in the air,
but a ratio of the air’s water vapor content to its capacity.
It is important to realize that relative humidity changes with temperature, pres-
sure, and water vapor content. If you have a parcel of air with a capacity for
10 g of water vapor which contains 4 g of water vapor, the relative humidity
would be 40%. Adding 2 g more water vapor (for a total of 6 g) would change
the humidity to 60%. If that same parcel of air is them warmed so that it has a
capacity for 20 g of water vapor, the relative humidity drops to 30% even
though water vapor content does not change.
• Relative Humidity (Alarm: High/Low)
• “Daily” High and Low Relative Humidity with time of occurrence (AutoClear)