What Does a Barometer Measure — Reading Comprehension

A barometer is an essential scientific instrument used to measure atmospheric pressure. Atmospheric pressure is the weight of the air pushing down on a specific point on Earth's surface. Think of it as a vast, invisible ocean of air that exerts force on everything below it. Changes in this pressure are a key indicator of short-term weather changes. By understanding how a barometer works, scientists and meteorologists can forecast weather patterns and even determine altitude.

There are two main types of barometers. The traditional mercury barometer consists of a glass tube filled with mercury, sealed at one end and inverted into a reservoir of mercury. The weight of the atmosphere presses down on the mercury in the reservoir, forcing some of the liquid up into the glass tube. The height of the mercury column is a direct measure of the air pressure. A higher column indicates greater pressure, while a lower column indicates less pressure. The second, more common type is the aneroid barometer, which does not use liquid. It contains a small, flexible, sealed metal box called an aneroid capsule that has had most of the air removed. As atmospheric pressure increases, the capsule is squeezed inward. As pressure decreases, the capsule expands. These tiny movements are magnified by levers and move a needle on a dial, providing a pressure reading.

Weather forecasting relies heavily on these pressure readings. Generally, a high-pressure system is associated with fair, clear weather because the dense, sinking air prevents clouds from forming. Conversely, a low-pressure system is associated with cloudy, stormy weather. In a low-pressure system, the air is less dense and rises, allowing water vapor to condense and form clouds and precipitation. A rapid drop in pressure can signal that a storm is approaching, while a rapid rise often indicates improving weather conditions.

Barometers are also crucial for determining altitude. As you ascend to higher elevations, the air becomes thinner, and there is less air above you. This results in lower atmospheric pressure. For this reason, barometers are used in airplanes and by hikers to measure their height above sea level. This connection between pressure and altitude makes the barometer a versatile tool, linking the weight of our atmosphere to both the weather outside and our position in the sky.

Fun Fact: The first mercury barometer was invented by Italian physicist Evangelista Torricelli in 1643.

What does a barometer primarily measure?

How does a mercury barometer indicate high atmospheric pressure?

What is the key difference between a mercury barometer and an aneroid barometer?

A rapid drop in atmospheric pressure most likely indicates what change in weather?

Which of the following is an example of an aneroid barometer being used?

The passage states that high-pressure systems are associated with what type of weather?

Which of the following best describes the main idea of the passage?

A barometer's reading drops slowly over several days. What can you infer about the weather?