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Chapter 11: Atmosphere

Ch. 11.1 Atmospheric Basics

Atmospheric Composition

Air is a combination of gases: nitrogen, oxygen, and particles of dust/water droplets/ice crystals.

Permanent Atmospheric Gases

About 99% is Nitrogen and oxygen. Remaining 1% consists of Argon, Carbon Dioxide, water vapor, and other trace gases. Over Earth's history, the composition of the atmosphere has changed greatly.

Variable Atmospheric Gases

Concentration of some gases not as constant over time. Water vapor and ozone can vary significantly from place to place.
Concentration of water vapor and carbon dioxide play a role in regulating the amount of energy the atmosphere absorbs and emits.

Water Vapor

Amount of water vapor can vary greatly over time and from one place to another. Varies with seasons, altitude of a air mass, and with properties of Earth's surface beneath the air (ie: desert).

Carbon Dioxide

Currently 0.038% of atmosphere; up from 0.028%. Is cycled between the atmosphere, the oceans, living organisms, and rocks.

Burning fossil fuels has caused recent rise in carbon dioxide.


Molecules of three oxygen atoms; located 20-50km above Earth's surface. Ozone concentration varies seasonally at high latitudes. Minimum is in spring. Chlorofluorocarbons (CFC's) react with ozone and break it down causing a decrease in recent years.

Atmospheric Particles

Atmosphere contains variable amounts of solids: dust and soil, salt particles, ice, microorganisms (bacteria and fungi).

Atmospheric Layers

Five layers that vary by composition and temperature.

  1. Troposphere- closest to Earth and contains most of the mass of the atmosphere. Layer in which weather occurs. Temperatures decrease as elevation increases until tropopause.
  2. Stratosphere- contains the ozone layer. Temperature increases with altitude due to ozone molecules absorbing ultraviolet radiation from the sun until stratopause. 99.9% of atmosphere mass is below the stratopause.
  3. Mesosphere- temperatures decrease with altitude until mesopause.
  4. Thermosphere- temperature rises due to low density of air. Contains ionosphere which is composed of electrically charged particles.
  5. Exosphere- outermost layer with no clear boundary. Molecules spaced far apart and can escape into space.

Energy Transfer in the Atmosphere

Particles such as atoms and molecules are always moving. Movement is in all directions and speeds; random motion. A moving object has kinetic energy. The total energy of particles in an object is called thermal energy.

Heat is the transfer of thermal energy from a region of high temperature to a region of lower temperature. In atmosphere, thermal energy transferred by radiation, conduction, and convection.


Radiation- the transfer of thermal energy by electromagnetic waves. Solar energy is absorbed and reflected by Earth's atmosphere and Earth's surface.

Absorption and Reflection

Most solar energy that reaches Earth is visible light waves and infrared waves. Almost all the visible light energy passes through the atmosphere. The atmosphere absorbs some infrared waves and emits some.

About 30% of solar radiation is reflected into space by Earths surface, clouds, or the atmosphere. Another 20% absorbed by atmosphere and clouds. About 50% is absorbed by Earth's surface.

Rate of Absorption

Different areas absorb energy and heat at different rates. Example: water vs. land, light vs. dark objects


Conduction- the transfer of thermal energy between objects when their atoms or molecules collide. Occurs more easily with solids and liquids due to closer particles.
Conduction occur between Earth's surface and the lowest part of the atmosphere.


Convection- the transfer of thermal energy by the movement of heated material from one place to another. Occurs mainly in liquids and gases.
Convection currents are the main mechanism for energy transfer in the atmosphere.

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Ch. 11.2 Properties of the Atmosphere

Atmospheric properties describe weather conditions.


Temperature is a measure of the average kinetic energy of the particles in a material. The higher the temperature of a material, the faster the particles are moving.

Measuring Temperature

Fahrenheit (ºF) or in the U.S. or Celsius (ºC). The SI unite is Kelvin (K). Fahrenheit and Celsius are based on the freezing and boiling point of water. Zero point of Kelvin is absolute zero; lowest temperature any substance can have.

Air Pressure

Air pressure is the pressure exerted on a surface by the weight of the atmosphere above the surface.
As you go higher in the atmosphere, air pressure decreases as the mass of the air above you decreases.

Density of Air

The density of air decreases with increasing altitude.

Pressure-Temperature-Density Relationship

In the atmosphere, the temperature, pressure, and density of air are related to each other.

In a sealed container-

  • Air pressure and Temperature: Warmer air has higher pressure than cooler air.
  • Air Pressure and Density: In constant temperature, air with a higher density exerts more pressure than air with a lower density.
  • Temperature and Density: At the same pressure, warmer air is less dense than cooler air.

Temperature Inversion

An increase in temperature with height in an atmospheric layer; warm air on top of colder air.

Causes of Temperature Inversion

The rapid cooling of land on a cold, clear, winter night. The land does not radiate heat so lower layers of air become cooler than the air above it.

Effects of Temperature Inversion

Can lead to fog or low-level clouds.
Can worsen air pollution by trapping it close to the ground.


The movement of air is known as wind.

Wind and Pressure Differences

In the lower atmosphere, air generally moves from regions of higher density to regions of lower density.
Density differences are due to unequal heating and cooling of Earth's surfaces.
In the atmosphere: regions with high density have high pressure, regions with low density have low pressure.
Air moves from region of high pressure to region of low pressure, resulting in wind.

Wind Speed and Altitude

Wind speed and direction change with height in the atmosphere due to the presence of friction on Earth's surface.


The amount of water vapor in the atmosphere at a given location on Earth's surface.

Relative Humidity: the amount of water vapor in a volume of air relative to the amount of water vapor in a volume of air to reach saturation.

Dew Point: the temperature to which air must be cooled at a constant pressure to reach saturation.

Latent Heat: the extra thermal energy contained in water vapor compared to liquid water.

When condensation occurs, latent heat is released and warms the air. The release of latent heat can provide energy to a weather system, such as a hurricane.

Condensation Level

An air mass heats up as it sinks and cools off as it rises.
A air mass cools because the air pressure around it decreases as it rises, causing the air mass to expand.
If the air mass continues to rise, eventually it will reach saturation and condensation will occur.

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Ch. 11.3 Clouds and Precipitation

Clouds vary in shape, size, height of formation, and type of precipitation.

Cloud Formation

Uneven heating and cooling of the surface causes air masses to warm and cool. As the warm air mass rises, it expands and cools; this can cause water vapor in the air mass to condense.
When a rising air mass reaches the lifted condensation level, water vapor condenses around condensation nuclei.
Condensation nuclei- a small particle in the atmosphere around which water droplets can form.
Droplets can be liquid or ice; when enough accumulates a cloud is visible.

Atmospheric Lifting

Air rises when it is heated becoming warmer than the surrounding air; this is convective lifting.
Clouds can also form when air is forced upward or lifted by mechanical process.

Orographic Lifting

Occurs when an air mass is forced to rise over a topographic barrier. The rising air mass expands and cools, with water droplets condensing when the temperature reaches the dew point.
Many of the rainiest locations on Earth are located on the windward sides of mountain slopes.


Occurs when air masses move into the same area from different directions. When a warm air mass and a cooler air mass collide, the warmer, less-dense air is forced upward over the denser, cooler air.
This method is common at middle latitudes where severe storm systems form as cold polar air collides with warmer air.

Types of Clouds

Differences in cloud shapes are due to differences in the process that cause clouds to form. Cloud formation can also take place at different altitudes.

Clouds are classified according to a system developed in 1803 based on the altitudes at which they form and by their shapes. Three altitude classes: low, middle, and high.

Low Clouds

Form when warm, moist air rises, expands, and cools. If a cloud is formed it will flatten out and winds will spread it horizontally into stratocumulus or layered cumulus clouds.
Cumulus- clouds are puffy, lumpy-looking usually below 2000m.
Stratus- layered sheet-like clouds that can cover much or all of the sky in an area.
Nimbostratus clouds produces precipitation.

Middle Clouds

Altocumulus and altostratus clouds form at altitudes of 2000m-6000m. Made of ice crystals and water droplets due to the colder temperatures generally present at these altitudes. Middle clouds are usually layered and sometimes produce mild precipitation.
Altocumulus: white or gray, form large, round masses or wavy rows.
Altostratus: gray and form thin sheets of clouds.

High Clouds

Made of ice crystals, form at heights above 6000m where temperatures are below freezing.
Cirrus- often a wispy, indistinct appearance.
Cirrostratus: forms a continuous layer in the sky, vary from almost transparent to dense enough to block out the Sun or Moon.
Cirrocumulus: has a rippled appearance.

Vertical Development Clouds

If the air of a cumulus cloud is unstable, the cloud will be warmer than the surrounding air and continue to grow upward. Water vapor condenses, air continues to increase in temperature due to the release of latent heat. Can form a cumulonimbus and might reach tropopause. Can produce torrential rains, strong winds, and hail characteristic of thunderstorms.


Precipitation- all forms of water that fall from clouds to the ground. To become heavy enough to fall, droplets must increase their size by 50-100 times.


Is primary process responsible for the formation of precipitation in a warm cloud.
Coalescence- when cloud droplets collide and join together to form a larger droplet. Occurs as large droplets fall and collide with others to increase in size and fall to Earth. Typically a diameter of 0.5mm to 5mm.

Snow, Sleet, and Hail

Type of precipitation depends on the vertical variation of temperature in the atmosphere.

  1. Snow: in clouds far below freezing, ice crystals can form.
  2. Sleet: when droplets move up and down through freezing and nonfreezing air by air currents; or droplets that pass through freezing air near the surface.
  3. Hail: when up-and-down motion in a cloud is especially strong over large stretches.

The Water Cycle

About 97% of Earth's water is in the oceans. The continuous movement of water between the atmosphere and Earth's surface is known as the water cycle.
Liquid water evaporates to form vapor that rises and cools to form precipitation which joins a body of water, which then evaporates to start over.

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Page last updated April 3, 2017.