Question

You and a friend are watching TV on a rainy day when the weather reporter states that, "The barometric pressure is 28.8 inches and rising." Hearing this, you say, "It looks like fair weather is on its way." Your friend responds with the following questions. "I thought air pressure had something to do with the weight of air. How does inches relate to weight? And, why do you think the weather is going to improve?" How would you respond to your friend's queries?

Short answer

Rising barometric pressure indicates improving weather because high pressure often brings clear skies, and inches relate to mercury height in the barometer.

Step-by-step solution

Understanding Barometric Pressure

Barometric pressure, also known as atmospheric pressure, is the force exerted by the weight of the air above a surface. It is usually measured in inches of mercury (inHg) using a barometer. The number in inches indicates the height to which air pressure can push a column of mercury in the barometer.

Relating Barometric Pressure to Weather

Typically, a rising barometric pressure signifies that air is becoming denser, pushing down on the Earth's surface. This increase usually indicates that a high-pressure system is approaching, which is associated with calm, clear weather as opposed to low-pressure systems that often bring storms and rain.

Explaining 'Inches' in Barometric Measurements

The term 'inches' in the context of pressure refers to the height of a column of mercury that the atmospheric pressure can support in a mercury barometer. One inch of mercury is equivalent to 33.86 millibars (mb) of pressure. Thus, 28.8 inches of mercury signifies a pressure of approximately 975.3 mb.

Predicting Weather Based on Barometric Pressure

A rising measurement such as "28.8 inches and rising" suggests that a high-pressure front is arriving, which typically means improving weather conditions, such as clear skies and pleasant temperatures. Hence, your prediction that fair weather is likely on its way is based on this understanding of pressure changes.

Key concepts

Atmospheric Pressure

Atmospheric pressure, often called barometric pressure, is essentially the force exerted by the weight of the atmosphere above us. It is an invisible pressure that affects everyone and everything on Earth. This pressure is crucial in understanding weather patterns. Since the air is a mixture of gases, it has weight, and this weight generates a force over a specific area—which we measure as atmospheric pressure. You can think of it as the weight of a blanket of air that covers the planet. Atmospheric pressure is typically measured in units like inches of mercury (inHg) or millibars (mb). This measurement helps meteorologists to predict weather changes by analyzing the pressure values. Higher atmospheric pressure usually suggests stable weather conditions.

• Exerted by the atmosphere's weight.
• Measured in inches of mercury or millibars.
• Key indicator for weather predictions.

Mercury Barometer

A mercury barometer is a tool used for measuring atmospheric pressure. It was invented in the mid-1600s and has become a fundamental instrument for meteorologists. This device uses mercury because it is both dense and has a consistent volume change with temperature variations. In a mercury barometer, there’s a glass tube standing vertically in a container full of mercury. When the air pressure changes, it pushes on the mercury in the container, causing the level in the glass tube to adjust. The height of mercury in the tube, measured in inches, indicates the atmospheric pressure.

• Invented in the 1600s to measure atmospheric pressure.
• Uses mercury because of its density and stability with temperature.
• Changes in mercury height reflect changes in air pressure.

Weather Prediction

Predicting the weather involves understanding the changes in atmospheric pressure. Meteorologists use barometric readings to determine whether a weather system is a high-pressure system or a low-pressure system. A rising barometric pressure generally indicates that a high-pressure system is moving in, which is associated with clear skies and calm weather. Conversely, a falling pressure suggests the approach of a low-pressure system, often resulting in cloudy skies and rainy conditions. Rising pressure means the air is denser, suggesting less cloud formation and weaker winds. This knowledge allows people to prepare for changes in weather, helping in activities like agriculture, travel, and event planning.

• Involves tracking changes in atmospheric pressure.
• High-pressure systems bring clear skies.
• Low-pressure systems bring clouds and rain.

High-Pressure System

A high-pressure system involves air that is descending towards the Earth's surface. As it descends, it warms up causing clouds, which need cooler, buoyant air to form, to dissipate. This system is mainly associated with calm and clear weather conditions. When a high-pressure system is present, the atmospheric pressure is greater than in surrounding areas. This leads to lower humidity and often clearer skies, thus making activities outdoors more pleasant and predictable. It’s why increasing barometric pressure is associated with improving weather conditions.

• Air descends and warms, reducing cloudiness.
• Leads to clear and calm weather.
• Results in lower humidity and clearer skies.