Chapter 5

Why does a cold, dry air mass exert a greater surface air pressure than an equally cold but more humid air mass?

A cold dry air mass has a greater density than a more humid air mass. This would mean that the humid air mass has lower air pressure. This is because the molecular weight of water is less than the molecular weight of dry air.

Air is a compressible mixture of gases. How does this property of air affect the rate at which air pressure decreases with increasing altitude?

Gravity causes the maximum air density to be at Earth's surface. As altitude increases, the air begins to thin rapidly, which is accompanied by a decline in air pressure.

How does a change in temperature affect air density? How does a change in humidity affect air density?

When air is heated, air density usually decreases. When the concentration of water vapor (humidity) increases, air density usually decreases as well.

At Minneapolis, MN, the air pressure typically drops more rapidly with altitude on a cold day in January than on a warm day in July. Explain the difference.

Air pressure drops more rapidly with elevation in cold, dense air than in warmer, less dense air.

What is an adiabatic process? Do adiabatic processes occur in the atmosphere? If so, provide an example.

An adiabatic process is a thermodynamic process in which no heat is exchanged between a mass and the environment. This does occur in the atmosphere. Examples of adiabatic processes occurring in the atmosphere are expansional cooling and compressional warming of air.

Compare the advantages and disadvantages of mercury barometer versus an aneroid barometer.

An aneroid barometer is more portable but less accurate than a mercury barometer.

Explain how air pressure tendency can be a useful indicator of future weather.

As a general rule, the weather becomes (or remains) fair as air pressure at Earth's surface rises and the weather turns stormy as the air pressure falls. Pressure tendency is an indication of an approaching low pressure (stormy weather) system or an approaching high pressure (fair weather) system.

A jet aircraft is cruising at the 600-mb level, that is, at the altitude where the air pressure is 600 mb. What fraction of the atmosphere's mass is below the aircraft?

Assume that the air pressure at sea level is 1000 mb. Then, 1000 mb − 600 mb = 400 mb. 400/1000 = 0.4. About four-tenths (or 40%) of the atmosphere's mass is situated below the 600- mb level.

On a particularly warm and humid evening, a sportscaster comments that baseballs hit to the outfield "will not carry far in this heavy air." How valid is the sportscaster's observation?

Baseballs hit to the outfield actually should travel further through a hot, humid atmosphere. Air becomes less dense with increasing temperature and increasing humidity and less dense air offers less resistance to a batted ball.

Why is the dry adiabatic lapse rate greater than the moist adiabatic lapse rate?

Because of the release of latent heat accompanying phase changes of water vapor, a saturated air parcel cools more slowly than an unsaturated air parcel. This causes the moist adiabatic lapse rate to be lower.

An unsaturated air parcel has an initial temperature of 15°C. If the parcel is lifted 1500 m and remains unsaturated, what is its new temperature?

Because the air parcel remains unsaturated, use the dry adiabatic lapse rate (9.8 C° per 1000 m). The temperature at 1500 m would be 0.3°C.

Distinguish between Charles' law and Boyle's law.

Charles' law defines the relationship between density and temperature in an ideal gas with constant pressure. Boyle's law defines the relationship between pressure and density when temperature is held constant. Together these two laws make the ideal gas law.

How does Dalton's law apply to the atmosphere?

Dalton's law states that the total pressure exerted by a mixture of gases equals the sum of the pressures produced by each constitute gas. The pressure balance is a condition in the atmosphere because at any point in a gas, or a mixture of gases, pressure is exerted equally in all directions.

Is there some altitude which clearly marks the top of the atmosphere? Explain your response.

No one altitude can be defined as the top of the atmosphere because it does not have a uniform density. The top of the atmosphere varies by location.

Why are air pressure readings adjusted to what they would be if the weather station were actually located at sea level?

Pressure readings are adjusted to sea level in order to observe changes in air pressure on a horizontal plane. It helps scientists identify air masses and weather changes and helps aid in weather forecasting.

The tropopause is higher over the tropics than over middle latitudes. Explain why.

The air is warmer over the tropics than over the middle latitudes. Colder air is denser than warmer air, causing the height of the tropopause to be higher over the tropics.

Provide a definition of air pressure that applies to Earth's surface and any altitude within the atmosphere.

The air pressure at a given location on Earth's surface can be thought of as the weight per unit area of the column of air above that location. The pressure at any point within the atmosphere is equal to the weight per unit area of the atmosphere above that point.

Why do temperature changes accompany the vertical (up and down) motions of air parcels?

The atmosphere compresses descending air, causing the air to warm. At the same time, when air rises, it expands and cools.

If water is used in place of mercury in a class-tube barometer, what is the required height of the tube? The density of mercury (Hg) is 13.6 g/cm3 and the density of water is 1.0 g/cm3 .

The glass tube for a mercury barometer is about 1 m in length. If water is used instead of mercury, the glass tube would have to be about 13.6 m in length because the density of mercury is 13.6 times the density of water.

On televised weathercasts, air pressure is often reported in units of length (inches) rather than units of pressure (millibars). Explain why.

Weathercasters typically use inches of mercury rather than millibars because "at home" barometers typically display in inches.