Welcome to You Ask Andy

Ruth Zinck, age 12, of Ottawa, Ont., for her questions:

Why does a barometer fall for storms weather?

Ruth's letter shows that she has thought deeply about her question. As we know, the barometer rises to indicate fair weather and falls to indicate stormy weather. The column of mercury in the tube rises and falls because of the weight of air pressing dorm on the mercury in the cup at its base. The more the air weighs, the higher the mercury is forced up in the glass tube.

So, it seems, that more air pressure indicates fair weather. And so it does. How then do you account for the weight of water vapor in the air ‑the extra vapor that must be there when stormy weather is ahead? This, says Andy, is a very reasonable question. There is also a reasonably answer to it.

'Mater vapor is a gas like the other gases of the air. When present it mixes, in various proportions, with the other gases of the air. It takes up space, or volume. The more water vapor present in the air, the more normal gases are forced out of place. And this water vapor happens to weigh less than normal air.

The volume of air ‑ the space it takes up ‑ remains the same. But moist air contains a large proportion of water vapor. This vapor reduces the overall weight of air because it has displaced heavier gases with a lighter gas.

Of course, water vapor is never more than a small percentage of the total air. Yet there is always some vapor in the air, oven over the deserts. The percentage of vapor ranges from almost nothing over the deserts to five percent. And the maximum five percent occurs only aver hot, steamy tropical bungles.

Over the deserts the air pressure is high. With only a smidgeon of light vapor in the air, the air presses down with all its might. It bears down in the cup at the bottom of the glass tube on the barometer. The liquid mercury in the cup can move only up the glass tube. When this pool of mercury is pushed down by air pressure, the column of mercury in the tube risers. The rising barometer indicates an air mass of high pressure. Fair weather is ahead, for there is little or no vapor in the air ‑ no vapor to turn into stormy rain, snow or sleet.

An air mass of low pressure contains more than its share of vapor. However, in our latitudes, the proportion of vapor never reaches anywhere near five percent. The proportion varies and shows itself on the barometer. As the vapor increases, normal air is displaced and the air mass becomes lighter. The pressure on the pool of mercury in the barometer is reduced and down goes the column in the glass.

The atmosphere tries to keep its weight even all over, a ray of light weight low pressure forms a sort of depression. High pressure air ground it tends to flow towards the depression. Hence, it becomes a center of stormy weather.

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