BAROMETRIC PRESSURE Weather data – Derived Variables
What is it?
The weight of the air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure.
Generally, the more air above an area, the higher the atmospheric pressure, this, in turn, means that atmospheric pressure changes with altitude.
For example, atmospheric pressure is greater at sea-level than on a mountaintop. 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 Vantage Pro2, Vantage VUE and Vantage Pro measure atmospheric pressure. When entering the location’s altitude in Setup Mode, these systems calculate the necessary correction factor to consistently translate atmospheric pressure into barometric pressure.
Barometric pressure also changes with local weather conditions, making 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 weather. 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.
The following section applies to Vantage Pro2, Vantage VUE and Vantage Pro systems only. The method described here is called the “NOAA” bar reduction selection on the Vantage VUE.
Parameters Used: Outside Air Temperature, Outside Humidity, Elevation, Atmospheric Pressure
PSL = PS * (R),
where PSL is sea level pressure, PS is the unadjusted reading sensed by the Davis barometer, and R is the reduction ratio, which is determined as follows:
First, Tv (virtual temperature in the “fictitious column of air” extending down to sea-level) can be determined as follows. The result is in degrees Rankin, which is similar to Kelvin except it uses Fahrenheit scale divisions rather than Celsius scale divisions:
Tv = T + 460 + L + C
where T is the average between the current outdoor temperature and the temperature 12 hours ago (in Fahrenheit) in whole degrees. L is the typical lapse rate, or decrease in temperature with height (of the “fictitious column of air”), as calculated by:
L = 11 Z/8000,
where L is a constant value with units in °F.
Z is elevation, which must be entered in feet.
The current dewpoint value and the station elevation are necessary to compute C. C is the correction for the humidity in the “fictitious column of air”. It is determined from a lookup table. The table consists of dewpoints in °F every 4°F and elevations in feet every 1500 feet. Linear interpolation is performed to obtain the correct reduced pressure value. For dewpoints below –76°F, C = 0; for dewpoints above 92°F, a dewpoint of 92°F is assumed.
Now, Tv can be determined. From this, the following can be computed:
Exponent = [Z/(122.8943111*Tv)]
Once this exponent is computed, R can be computed from the following:
R = 10^[Exponent].
Thus, PSL = PS * (R) can be calculated. Pressure can be in any units (R is dimensionless) and still yield the correct value.
This procedure is designed to produce the correct reduced sea-level pressure as displayed. This requires the user to know their elevation to at least ±10 feet to be accurate to every .01” Hg or ±3 feet to be accurate to every 0.1 mb/hPa.
This is a simplified version of the official U.S. version in place now. The accepted method is to use lookup tables of ratio reduction values keyed to station temperature. These are based on station climatology. These values are unavailable for every possible location where a Davis user may have a
station, thus this approach is not suitable.
It should be noted that if a sensor’s pressure readings require adjustment, the user can adjust either the uncorrected or the final reading to match the user’s reference, as appropriate. If the user chooses to measure uncorrected atmospheric pressure or use another reduction method, they should set their elevation to zero, or on a Vantage VUE, they can also use the “NONE” selection for bar reduction. Subsequently, output data using the WeatherLink can be read by or exported to another application and converted as desired.
The calibration of the sensor is a separate one-time function performed on the unit during the manufacturing process. It is a completely independent operation from the calculation the Vantage Pro2, Vantage VUE and Vantage Pro console makes to display a reading corrected to sea-level. The calibration is done to ensure the sensor reads uncorrected or raw atmospheric pressure (not barometric pressure) properly. Any properly functioning unit will read the uncorrected atmospheric pressure within specifications. However, limits in the displayable range of the bar value may prevent the user from setting an incorrect elevation for their location. That is, a user at sea-level, may see a dashed reading if they set their unit to 5000′ elevation or vice-versa. So, the best way to tell if a Vantage Pro(2) unit is functioning properly, is:
- use a reference that has been adjusted to indicate sea-level pressure and set the console to the proper elevation or
- use a reference that is reading the raw, uncorrected atmospheric pressure and set the console elevation to zero
and verify that these readings are comparable. The Vantage VUE console will display both corrected and uncorrected readings in the Weather Center, so it is not necessary to change the settings to view different values. Additionally, the Vantage VUE shows Altimeter Setting in the same location, which can also be used for verification if a nearby Altimeter value is available (see below).
ALTIMETER SETTING and CWOP
The CWOP program in NOAA prefers to receive altimeter setting data rather than barometric pressure.
This feature in WeatherLink 5.7 automatically calculates the correct altimeter setting using the userspecified elevation. Monitor II and Perception II users should set their barometer reading to match the altimeter setting of the nearest National Weather Service (NWS) weather station. Simply enter your zip code on the NWS home page to get the nearest observation. This is usually found at the “3 Day History” (detailed observation section) link under Current Conditions section. http://www.nws.noaa.gov/ . For users outside the United States, contact your country’s national meteorological service.
Altimeter Formula, A:
A = (P^N + K*Z)^(1/N), where P is the raw station pressure (in. Hg),
N = 0.1903, K = 1.313E -5, Z is elevation (feet).
“Smithsonian Meteorological Tables”. Smithsonian Institution Press, Washington, DC, 4th Ed. 1968.
“Federal Standard Algorithms for Automated Weather Observing Systems used for Aviation Purposes“. Office of the Federal Coordinator for Meteorological Services and Supporting Research, Washington, DC, 1988
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