Barometry  Evangelista Torricelli credited with invention of barometer.  Gasparo Berti may have been first.

Atmospheric Pressure  Pressure is a force applied to, or distributed over, a surface, measured as a force per unit area.  The pressure of air is due to the “hits” or impacts of the air molecules on the surface, or other molecules, on which the force is exerted.

 Pressure can also be thought of as energy per unit volume since the pressure exerted on a surface is a result of millions of these “hits” by molecules and the number of hits, (which is determined by density), and the force with which the molecules hit, (determined by their kinetic energy), determines the pressure exerted.

 We can then say that atmospheric pressure is dependent on: l Speed of the molecules of air l Mass of the molecules of air l Frequency of impact of the molecules of air l gravity

Sea Level Pressure  Using the Hydrostatic Equation and the General Gas Law Equation one can arrive at the following equation:  From which the pressure at sea level (P sfc ) can be determined. Note that it includes the height of the station and the average temperature.

ASOS Sea Level Pressure Calculations  NWS uses the following: l Sea Level Pressure (mb) = P s R l Where, Where P s is the pressure at ground in inches of mercury. R is a value dependent on the average 12 hour temperature, determined by taking the current temperature and the temperature 12 hours ago and averaging them. R values are calculated for each station by NWS

P s is determined from the equation: where –P is the pressure measured at the instrument in inches of mercury –H is the height of the instrument above the ground, usually 5 feet, –T is the 12 hour average ambient temperature in degrees Rankin. –Rankine scale is the Fahrenheit equivalent of the Kelvin scale.

General Errors of Pressure Measurements  Wind l Ideally the atmosphere would be static. However, wind produces dynamic pressure errors, a “pumping” action on the barometer.  Temperature l Changes in physical characteristics of the instrument due to temperature changes affects the pressure value obtained.  Instrument Error - l errors in construction or calibration

Direct and Indirect Pressure Measurements  Direct: l Measures pressure due to weight / force directly exerted by atmosphere.  Indirect: l Measures pressure by sensing a change (produced by pressure) to some characteristic of a material.

A. Direct Pressure Measurements  Piston Gauge: l Primary standard barometer. l Used to calibrate secondary barometers under laboratory conditions. l Not used to directly measure atmospheric pressure. l Total weight of piston, weight carrier and added weights determines the pressure against which a secondary barometer is calibrated.

1. Mercury Barometers  Work on the principle of balancing the weight of a column of the atmosphere against the weight of a column of mercury. l Cistern barometers Adjustable - Fortin Fixed - Kew l Without cistern Siphon Barometer

Characteristics of Mercury  High Specific Gravity (density), l g/cm 3 at 0 o C  Low vapor pressure, mm Hg at 0 o C  Small volume coefficient of expansion, l x 10-4 cm 3 / o C  Low specific heat, cal/g o C  Low freezing point, o C  High boiling point, o C

Corrections to Mercury Barometers  Instrument l Determined by calibration against a standard barometer. l Correction table provided by manufacturer.  Thermal l To account for changes in volume of mercury and scale length changes due to temperature.  Gravity l To account for changes in gravity from standard gravity resulting from elevation of the barometer and latitude of the barometer

Thermal Correction  General Equation l s = coefficient of linear expansion of scale l m = coefficient of volume expansion of mercury l T = temperature of thermometer l T hg = standard temperature of mercury l T s = standard temperature of scale l R s = observed height of mercury column:

 If standard temperature for both mercury and scale are both 0 o C, then the pressure corrected for temperature is:  An error of about 1 o F in temperature will result in about 0.1 mb error in pressure.

Gravity Correction  Value of gravity for latitude change  Value of gravity for altitude change  Pressure corrected for instrument, temperature and gravity is:

Common Mercurial Barometers  Adjustable Cistern l Normal Barometer Secondary Standard in England Used to calibrate other barometers

Diagram of the National Physics Laboratory Normal Barometer  The separation of the two mercury surfaces is determined using two micrometer microscopes and cross wires.

New Normal Barometer  A modern Normal Barometer using lasers to determine the separation of the mercury levels.

 Another Adjustable Cistern Barometer l Fortin Barometer A secondary Standard in the U.S.

Reading the Fortin Barometer

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