Introduction

Air is a tangible material substance and as a result has mass. Any object with mass is influenced by the universal force known as gravity.

Newton's Law of Universal Gravitation states: any two objects separated in space are attracted to each other by a force proportional to the product of their masses and inversely proportional to the square of the distance between them. On the Earth, gravity can also be expressed as a force of acceleration of about 9.8 meters per second per second.

As a result of this force, the speed of any object falling towards the surface of the Earth accelerates (1st second – 9.8 meters per second, 2nd second – 19.6 meters per second, 3rd second – 29.4 meters per second, and so on.) until terminal velocity is attained.

Gravity shapes and influences all atmospheric processes. It causes the density and pressure of air to decrease exponentially as one moves away from the surface of the Earth.

Figure 7d-1 below models the average change in air pressure with height above the Earth's surface.

In this graph, air pressure at the surface is illustrated as being approximately 1013 millibars (mb) or 1 kilogram per square centimeter of surface area.

• Figure 7d-1: Change in average atmospheric pressure with altitude.
• Measuring Atmospheric Pressure

Any instrument that measures air pressure is called a barometer. The first measurement of atmospheric pressure began with a simple experiment performed by Evangelista Torricelli in 1643. In his experiment, Torricelli immersed a tube, sealed at one end, into a container of mercury (see Figure 7d-2 below).

Atmospheric pressure then forced the mercury up into the tube to a level that was considerably higher than the mercury in the container. Torricelli determined from this experiment that the pressure of the atmosphere is approximately 30 inches or 76 centimeters (one centimeter of mercury is equal to 13.3 millibars).

He also noticed that height of the mercury varied with changes in outside weather conditions.

## Torricelli's Barometer

Figure 7d-2: Diagram showing the construction of Torricelli's barometer.

The most common type barometer used in homes is the aneroid barometer (Figure 7d-3). Inside this instrument is a small, flexible metal capsule called an aneroid cell.

In the construction of the device, a vacuum is created inside the capsule so that small changes in outside air pressure cause the capsule to expand or contract.

The size of the aneroid cell is then calibrated and any change in its volume is transmitted by springs and levers to an indicating arm that points to the corresponding atmospheric pressure.

Figure 7d-3: Aneroid barometer.

For climatological and meteorological purposes, standard sea-level pressure is said to be 76.0 cm or 29.92 inches or 1013.2 millibars.

Scientists often use the kilopascal (kPa) as their preferred unit for measuring pressure. 1 kilopascal is equal to 10 millibars.

Another unit of force sometimes used by scientists to measure atmospheric pressure is the newton. One millibar equals 100 newtons per square meter (N/m2).

Atmospheric Pressure at the Earth's Surface

Figure 7d-4 describes monthly average sea-level pressure for the Earth's surface. This animation indicates that surface air pressure varies both spatially and temporally.

During the winter months (December to February), areas of high pressure develop over central Asia (Siberian High), off the coast California (Hawaiian High), central North America (Canadian High), over Spain and northwest Africa extending into the subtropical North Atlantic (Azores High), and over the oceans in the Southern Hemisphere at the subtropics. Areas of low pressure occur just south of the Aleutian Islands (Aleutian Low), at the southern tip of Greenland (Iceland Low), and latitudes 50 to 80° South.

During the summer months (June to August), a number of dominant winter pressure systems disappear. Gone are the Siberian High over central Asia and the dominant low pressure systems near the Aleutian Islands and at the southern tip of Greenland. The Hawaiian and Azores High intensify and expand northward into their relative ocean basins.

High pressure systems over the subtropical oceans in Southern Hemisphere also intensity and expand to the north. New areas of dominant high pressure develop over Australia and Antarctica (South Polar High). Regions of low pressure form over central Asia and southwest Asia (Asiatic Low).

These pressure systems are responsible for the summer monsoon rains of Asia.

We will examine this graphic again in topic 7p when global circulation is discussed.

 Figure 7d-4: Monthly average sea-level pressure and prevailing winds for the Earth's surface, 1959-1997. Atmosphere pressure values are adjusted for elevation and are described relative to sea-level. The slider at the bottom of the image allows you change the time of month. 05/07/2009 10:08color shading. Blue shades indicate pressure lower than the global average, while yellow to orange shades are higher than average measurements. (Source: Climate Lab Section of the Environmental Change Research Group, Department of Geography, University of Oregon – Global Climate Animations). (To view this animation your browser must have Apple's QuickTime plug-in. The QuickTime plug-in is available for Macintosh and Windows operating system computers and can be downloaded FREE from the World Wide Web site www.apple.com/quicktime).

## The Highs and Lows of Air Pressure | UCAR Center for Science Education

Standing on the ground and looking up, you are looking through the atmosphere. It might not look like anything is there, especially if there are no clouds in the sky. But what you don’t see is air – lots of it.

We live at the bottom of the atmosphere, and the weight of all the air above us is called air pressure. Above every square inch on the surface of the Earth is 14.7 pounds of air. That means air exerts 14.7 pounds per square inch (psi) of pressure at Earth’s surface.

High in the atmosphere, air pressure decreases. With fewer air molecules above, there is less pressure from the weight of the air above.

Pressure varies from day to day at the Earth’s surface – the bottom of the atmosphere. This is, in part, because the Earth is not equally heated by the Sun. Areas where the air is warmed often have lower pressure because the warm air rises. These areas are called low pressure systems. Places where the air pressure is high, are called high pressure systems.

A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet.

As the air rises, the water vapor within it condenses, forming clouds and often precipitation. Because of Earth’s spin and the Coriolis Effect, winds of a low pressure system swirl counterclockwise north of the equator and clockwise south of the equator.

This is called cyclonic flow. On weather maps, a low pressure system is labeled with red L.

A high pressure system has higher pressure at its center than the areas around it. Winds blow away from high pressure.

Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow.

Air from higher in the atmosphere sinks down to fill the space left as air is blown outward. On a weather map, you may notice a blue H, denoting the location of a high pressure system.

How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors in weather stations measure air pressure. These sensors are able to make continuous measurements of pressure over time.

In the past, barometers were used and measured how much air pushed on a fluid, such as mercury. Historically, measurements of air pressure were described as “inches of mercury.

” Today, meteorologists use millibars (mb) to describe air pressure.

### Air pressure depends on temperature and density.

When you inflate a balloon, the air molecules inside the balloon get packed more closely together than air molecules outside the balloon. This means the density of air is high inside the balloon.

When the density of air is high, the air pressure is high. The pressure of the air pushes on the balloon from the inside, causing it to inflate. If you heat the balloon, the air pressure gets even higher.

Air pressure depends on the temperature of the air and the density of the air molecules.

Atmospheric scientists use math equations to describe how pressure, temperature, density, and volume are related to each other. They call these equations the Ideal Gas Law. In these equations, temperature is measured in Kelvin.

This equation helps us explain how weather works, such as what happens in the atmosphere to create warm and cold fronts and storms, such as thunderstorms. For example, if air pressure increases, the temperature must increase. If air pressure decreases, the temperature decreases. It also explains why air gets colder at higher altitudes, where pressure is lower.

## The Basics of Air Pressure

Air pressure, atmospheric pressure, or barometric pressure, is the pressure exerted over a surface by the weight of an air mass (and its molecules) above it.

Air pressure is a difficult concept. How can something invisible have mass and weight? Air has mass because it is made up of a mixture of gases that have mass. Add up the weight of all these gases that compose dry air (oxygen, nitrogen, carbon dioxide, hydrogen, and others) and you get the weight of dry air.

The molecular weight, or molar mass, of dry air is 28.97 grams per mole. While that isn't very much, a typical air mass is made up of an incredibly large number of air molecules. As such, you can begin to see how air can have considerable weight when the masses of all the molecules are added together.

So what's the connection between molecules and air pressure? If the number of air molecules above an area increases, there are more molecules to exert pressure on that area and its total atmospheric pressure increases. This is what we call high pressure

## Air Pressure

The number of molecules in the atmosphere decreases with height.

The atoms and molecules that make up the various layers in the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface they exert a force on that surface in what we observe as pressure.

Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable.

Air pressure can be increased (or decreased) one of two ways. First, simply adding molecules to any particular container will increase the pressure. A larger number of molecules in any particular container will increase the number of collisions with the container's boundary which is observed as an increase in pressure.

A good example of this is adding (or subtracting) air in an automobile tire. By adding air, the number of molecules increase as well the total number of the collisions with the tire's inner boundary. The increased number of collisions forces the tire's pressure increase to expand in size.

The second way of increasing (or decreasing) is by the addition (or subtraction) of heat. Adding heat to any particular container can transfer energy to air molecules. The molecules therefore move with increased velocity striking the container's boundary with greater force and is observed as an increase in pressure.

Learning Lesson: Air: A weighty subject

Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.

In the International Space Station, the density of the air is maintained so that it is similar to the density at the earth's surface. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch).

Learning Lesson: A Pressing Engagement

Learning Lesson: Going with the Flow

Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, meaning a decrease in air pressure. In fact, while the atmosphere extends more than 15 miles (24 km) up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).

Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at ground level from one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we convert the station air pressures reading to a value with a common denominator.

The common denominator we use is the sea-level elevation. At observation stations around the world the air pressure reading, regardless of the observation station elevation, is converted to a value that would be observed if that instrument were located at sea level.

The two most common units in the United States to measure the pressure are “Inches of Mercury” and “Millibars”. Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. At sea level, standard air pressure is 29.92 inches of mercury.

Millibars comes from the original term for pressure “bar”. Bar is from the Greek “báros” meaning weight.

A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared).

Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.

How temperature effects the height of pressure.

Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.

Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.

Learning Lesson: Crunch Time

The H's represent the location of the area of highest pressure. The L's represent the position of the lowest pressure.

The most basic change in pressure is the twice daily rise and fall in due to the heating from the sun. Each day, around 4 a.m./p.m. the pressure is at its lowest and near its peak around 10 a.m./p.m. The magnitude of the daily cycle is greatest near the equator decreasing toward the poles.

On top of the daily fluctuations are the larger pressure changes as a result of the migrating weather systems. These weather systems are identified by the blue H's and red L's seen on weather maps.

Learning Lesson: Measure the Pressure: The “Wet” Barometer

The decrease in air pressure as height increases.

How are changes in weather related to changes in pressure? From his vantage point in England in 1848, Rev. Dr. Brewer wrote in his A Guide to the Scientific Knowledge of Things Familiar the following about the relation of pressure to weather:

The FALL of the barometer (decreasing pressure)

• In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind.
• In frosty weather, the fall of the barometer denotes thaw.
• If wet weather happens soon after the fall of the barometer, expect but little of it.
• In wet weather if the barometer falls expect much wet.
• In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.
• The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind).

The RISE of the barometer (increasing pressure)

• In winter, the rise of the barometer presages frost.
• In frosty weather, the rise of the barometer presages snow.
• If fair weather happens soon after the rise of the barometer, expect but little of it.
• In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.
• In wet weather, if the mercury rises suddenly very high, fine weather will not last long.
• The barometer rises highest of all for north and east winds; for all other winds it sinks.

• If the motion of the mercury be unsettled, expect unsettled weather.
• If it stands at “MUCH RAIN” and rises to “CHANGEABLE” expect fair weather of short continuance.
• If it stands at “FAIR” and falls to “CHANGEABLE”, expect foul weather.
• Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather.

These pressure observations hold true for many other locations as well but not all of them. Storms that occur in England, located near the end of the Gulf Stream, bring large pressure changes.

In the United States, the largest pressure changes associated with storms will generally occur in Alaska and northern half of the continental U.S.

In the tropics, except for tropical cyclones, there is very little day-to-day pressure change and none of the rules apply.

Learning Lesson: Measure the Pressure II: The “Dry” Barometer

The scientific unit of pressure is the Pascal (Pa) named after Blaise Pascal (1623-1662). One pascal equals 0.01 millibar or 0.00001 bar. Meteorology has used the millibar for air pressure since 1929.

When the change to scientific unit occurred in the 1960's many meteorologists preferred to keep using the magnitude they are used to and use a prefix “hecto” (h), meaning 100.

Therefore, 1 hectopascal (hPa) equals 100 Pa which equals 1 millibar. 100,000 Pa equals 1000 hPa which equals 1000 millibars.

The end result is although the units we refer to in meteorology may be different, their numerical value remains the same. For example the standard pressure at sea-level is 1013.25 millibars and 1013.25 hPa.

## How To Teach Kids About Air Pressure | Air Pressure Experiments

Although we rarely think about it, air surrounds us at all times and exerts a force on every inch of our bodies. This force, known as air pressure, is one of the most important topics in science, as it explains weather patterns, how airplanes fly and a variety of other wonders. In case you’re planning on teaching kids about air pressure, we’ve provided you with an explanation of the basics and some simple, fun and engaging experiments to demonstrate the power of this natural phenomenon.

### What Is Air Pressure?

The term “air pressure” is used in reference to the weight of air molecules pressing down on the earth. At sea level, air pressure is generally 14.7 psi (pounds per square inch), which means that 14.

7 pounds are pressing down on every square inch of our bodies. The reason we can still move our bodies freely is because the air is exerting pressure on us in all directions, and the reason we aren’t crushed is because the air pressure inside our bodies is the same.

Air pressure is determined by the following three factors:

• Temperature: As air gets warmer, it expands. This expansion causes the density of the air to decrease, which results in lower pressure. When air gets colder, on the other hand, it shrinks. This shrinking causes the air to become denser, which leads to higher pressure. This phenomenon is why areas near the equator, which are hot, generally have low air pressure, and areas near the North and South Poles, which are cold, have high air pressure.
• Altitude: The higher you are above sea level, the less dense the air is. As less dense air weighs less, it produces lower air pressure, which is why it can be difficult to breathe on top of a tall mountain. It also explains why your ears will pop when you’re going up or down a mountain in a car — your inner ear has air trapped in it, and as the air pressure outside decreases, the air trapped in your ear will cause the eardrums to push outward. This expansion is what causes the “pop.”
• Moisture: The amount of moisture in the air also affects the density of the air and, therefore, the air pressure. Water vapor is a light gas compared to the gases that make up the atmosphere, which is primarily oxygen and nitrogen. So when the moisture in the atmosphere increases, the amount of nitrogen and oxygen decreases per unit of volume, which causes the density of the air to decrease.

One of the most interesting aspects of air pressure is that when a pocket of air pressure is changed, things begin to move. This pressure difference that creates movement is what causes wind, tornadoes and many other weather phenomena.

When you’re discussing the movement of air, keep in mind that scientists speak in terms of the higher pressure “pushing” things, not lower pressure “pulling” things.

### How to Measure Air Pressure

Air pressure is commonly measured using a mercury barometer. A mercury barometer contains a column filled with mercury, and the higher the air pressure is, the higher the column of mercury will be. By measuring the height of the column, you can determine the air pressure.

These days, it’s more common to use a digital barometer, which is portable and more accurate than the traditional type. This device uses an electrical capacitor to measure air pressure.

### Air Pressure and Weather

Areas with low pressure are generally associated with bad weather. If an area has low air pressure, air from neighboring areas, which have higher air pressure, will move in. This change, in turn, will cause the air to move upward, as it has nowhere else to go. When the air moves up, water vapor will condense, which will lead to the formation of clouds and rain.

Areas with high pressure, on the other hand, are typically associated with good weather. In high-pressure areas, low-level air will spread outward, allowing air above to come down. This downward motion warms the air up, causing evaporation and leading to nice, dry weather.

### Air Pressure and Science Experiments

Here are 10 simple air pressure experiments for kids that can help them better understand its effects.

This experiment will allow you to create a tornado in a bottle. You will need:

• Water
• A transparent mayonnaise jar
• Liquid dish soap
• Food coloring
• Vinegar

To do the experiment, complete the steps below:

1. Pour water into your jar until it’s roughly two-thirds full. Then, add several drops of food coloring to the water. Any color is fine.
2. Add in one teaspoon of your liquid dish soap and one teaspoon of vinegar.
3. Put the lid on the jar. Make sure it’s on as tight as possible to avoid leaks and serious messes.
4. Shake the jar, then give it a twist so that the liquid inside will start spinning.

What you’ll observe is a small vortex that resembles a tornado.

### 2. Unspillable Water Experiment

In some situations, air pressure is stronger than gravity. This experiment demonstrates the strength of air pressure as it keeps the water in a glass in place — even when the glass is turned upside down.

This experiment requires:

• A juice glass
• Water
• An index card (4 x 6 inches)

The steps are as follows:

1. Fill your glass with water right up to the top. Allow the water to run over so that the lip of the glass is wet.
2. Put the index card over the full glass. Use your hand to press the card down firmly, making a good seal around the glass’s wet lip.
3. While working over a sink or tub, hold your card in place with one hand and turn the glass over. Then, let go of the card carefully. It will not move, and the water will remain inside the glass.

This experiment demonstrates that the force the air pressure exerts against the index card is even stronger than the force gravity exerts on the water in the glass. The air pressure keeps the card from moving.

### 3. Book Blowing Experiment

The book blowing experiment demonstrates how powerful compressed air can be.

For this project, you’ll need:

• Three books
• A large plastic bag that’s airtight

To perform this experiment, follow these four steps:

1. Stack three books on top of one another.
2. Ask the student to move the books by blowing in their direction. Of course, they won’t be able to.
3. Place the plastic bag on your table, then place the three books on top of it. The bag’s open end should hang out over the table’s edge.
4. Show that if you blow with enough force, the books will start to rise off the table. It’s the compressed air in the bag that’s causing the movement.

### 4. Caved-In Can Experiment

This experiment involves using the power of air pressure to crush a can. You’ll need:

• Water
• A large container
• Ice cubes
• A measuring cup
• An empty soda can
• A stove
• Potholders or tongs

Once you’ve acquired the materials, follow the steps below:

1. Fill the container with ice cubes and water. Set this container to the side so that you can use it later on.
2. Pour 1/2 cup of water into your empty soda can.
3. Put the can on a stove burner. If your student does this step, be sure to supervise them.