Answer to Question 1
ANSWER: An aneroid barometer houses a small, flexible metal box called an aneroid cell. Before the cell is tightly sealed, air is partially removed, so that small changes in external air pressure cause the cell to expand or contract. A mercury barometer consists of a long glass tube open at one end and closed at the other. Removing air from the tube and covering the open end, the lower portion of the tube is immersed into a dish of mercury. The column of mercury in the tube balances the weight of the air above the dish, and hence the height of the column is a measure of atmospheric pressure.
Answer to Question 2
ANSWER: As long as the upper-level diverging air balances the converging surface air, the central pressure in the surface low does not change. However, the surface pressure will change if upper-level divergence and surface convergence are not in balance. For example, the surface pressure will change if the mass of air above the surface changes. Consequently, if upper-level divergence exceeds surface convergence (that is, more air is removed at the top than is taken in at the surface), the air pressure at the center of the surface low will decrease, and isobars around the low will become more tightly packed. This situation increases the pressure gradient (and, hence, the pressure gradient force), which, in turn, increases the surface winds.
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A Mercury Barometer