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lab2 - Lab AOS 330 Measurement of Atmospheric Pressure 1...

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Lab - AOS 330 Measurement of Atmospheric Pressure 1 Objectives Gain familiarity with the techniques for reading standard barometers. Compute the sea level pressure and altimeter setting from the observed station pressure. Use the hydrostatic law to estimate the height of the AO&SS building from pressure observations. Introduction to the concept of instrument precision and its influence on derived quantities. 2 Materials Aneroid barometer Aneroid barometer handout 3 Introduction Determination of the atmospheric pressure is perhaps the most fundamental measurement in meteorology. In measuring atmospheric pressure, we should keep in mind that we are simply “weighing” the column of air above the level of the barometer. 3.1 Fundamentals Atmospheric pressure may be measured with a number of different types of instruments, the two most common being (1) the mercury barometer, in which the pressure of the atmosphere forces a column of mercury a proportional distance up into an evacuated glass tube, and (2) the aneroid barometer, in which the atmospheric pressure is measured by way of the degree of compression of an evacuated bellows. An aneroid barometer is considerably simpler to use than a mercury barometer; one simply reads the position of a pointer against a calibrated circular scale. The simplicity of the aneroid barometer is counterbalanced by its lower accuracy, since the internal linkage mechanism is subject to friction, and the elastic tension in the evacuated bellows may weaken with time. Consequently, aneroid barometers are usually used for routine pressure observations, but they must be calibrated against the much more accurate mercury barometer. Usually, it is sufficient to determine an additive (or subtractive) constant correction that brings the aneroid barometer reading into agreement with the mercury barometer. Some barometers have a mechanism for easy on-the-spot recalibration, although these are also more susceptible to inadvertent adjustments and must be checked often.
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