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Chapter 8 - Atmospheric Circulation

Our atmosphere is the gaseous mass or envelope surrounding the Earth, and retained by the Earth's gravitational field. Other planets and moons in the Solar System have atmospheres. The atmosphere plays many important roles in moving water in the world's ocean basins, and for supporting life on Earth!

Earth's atmosphere is:

Density stratified - air is compressed and most dense near the surface and grows increasingly rarefied skyward.
• About 100 kilometers thick between the ocean/land surface and the vacuum of space.
Composed mostly of gases, mostly nitrogen (as N2) and oxygen (as O2), and trace amounts of other gases (including CO2, argon, water vapor); and traces of liquids and solids in suspension or falling as precipitation: suspended water (clouds, water droplets and ice crystals), traces of organic compounds, and suspended particles of dust from a variety of sources.
Click on thumbnail images for a larger view.
Layers of the atmosphere as viewed from space.
Fig. 8-1.
Layers of the Earth's atmosphere as seen from space.

Structure of the Atmosphere

The Earth's atmosphere is subdivided into levels (Figure 8-2):
Layers of the atmosphere
Fig. 8-2. Structure of Earth's atmosphere.
* The troposphere is the lowest portion (up to about 6-8 miles [10-13 km]) where all weather takes place and contains about 80% of the air's mass and 99% of water vapor.
* The overlying stratosphere contains an abundance of ozone which absorbs ultraviolet radiation, protecting life on land and in the shallow ocean extends up to about 31 miles (50 km).
*The mesosphere is the part of the earth's upper atmosphere above the stratosphere in which temperature decreases with altitude to the atmosphere's absolute minimum.
* The thermosphere the region of the atmosphere above the mesosphere and below the height at which the atmosphere ceases to have the properties of a continuous medium (about 60 miles [100 km]). The thermosphere is characterized throughout by an increase in temperature with height, where the charged atomic particles of the solar wind begins to interact with atmospheric gases.

Energy Transfer Through the Atmosphere

The amount of energy coming into the Earth from the Sun is equal to the energy reflected and radiated back into space. The atmosphere, oceans, and land absorb and release energy. Living things also absorb and release energy. Some of the energy stored in organic matter is preserved when it is buried in sediments. Geothermal energy is also a trace of the energy radiated into space. The rate of energy transfer also varies due to cloud cover and ice and snow coverage.

Incoming solar radiation
involves all wavelengths of the electromagnetic spectrum. Figure 8-3 shows the wavelengths and intensity of solar energy striking the top of the atmosphere and the energy reaching the surface. The atmosphere is transparent to most wavelengths, but part of the solar spectrum are absorbed by certain greenhouse gases in the atmosphere including water vapor, carbon dioxide, ozone, methane, and other gases. Shorter wavelengths (UV and blue light) is diffused in the air—making the sky blue. Longer wavelengths are less diffused—making sunsets and sunrises red (Figure 8-4).

Energy that is not reflected back into space is radiated back into space in wavelengths longer than visible light (mostly in the thermal infrared portion of the electromagnetic spectrum).
Specturm of solar radiation absorbed by the atmosphere
Fig. 8-3.
Wavelengths of solar energy transmitted and absorbed by the atmosphere.Electromagnetic energy transfer in the atmosphere
Fig. 8-4.
Energy transfer through the atmosphere

Composition of the Atmosphere

Nitrogen (N2) - 78%
Oxygen (O2) - 21%
Argon - 0.9%
Carbon Dioxide (CO2) - 0.036%
Others < 1 % - Neon, Helium, Methane (CH4), Krypton, Hydrogen (H2), traces of other compounds

Other trace gases
in variable amounts include nitrogen oxides, ozone (O3), sulfur dioxide, hydrocarbons, and more. These gases are released by volcanic eruptions, lightning, wildfires, erosion, and pollutants of many kinds from human activity. Major sources of air pollutants included gases and smoke released by fossil-fuel energy consumption, industrial releases, agriculture, and leaks of refrigerator and air conditioner coolant compounds.

Composition of the atmosphereFig. 8-5. Chemical composition of the atmosphere (major and trace gases)

Water Moisture in the Air (Humidity)

The amount of water vapor in the air can range from trace amounts up to about 4% by volume. Humidity is a term used to describe the relative amount of water vapor dissolved in the air. In general terms, humid refers to moist air and arid refers to dry air conditions.

Warm air can hold more moisture than cold air. As warm, moist air cools, the relative humidity increases. When air has reached the maximum amount or water it can hold it is called saturated - this occurs when clouds form and moisture condenses to for water droplets. As the air continues to cool, microscopic water droplets grow in size and it may start raining! The atmospheric temperature below which water droplets begin to condense is called the dew point. The dew point is the temperature at which moist air reaches 100% saturation. Dew can form on objects and consists of tiny drops of water that form on cool surfaces at night when the atmospheric vapor condenses (such as on grass) (Figure 8-6).

Weather reports frequently report the relative humidity. Relative humidity of 100% indicates that the dew point is equal to the current temperature when the air is holding the maximum amount of water vapor (it is saturated), and water vapor will begin to condense into water droplets, forming fog (clouds). Moist air is less dense than dry air. This explains why moist are rises to form clouds.

The amount water moisture that air can hold depends on factors including air temperature, air pressure, and the amount and kinds of particulate matter dispersed in the air (see cloud condensing nuclei [CCNs] discussed below). For example, warm, humid air near sea level may be clear clear during warm daylight hours, but as the air temperature drops at night the relative humidity will increase until it reaches the dew point and fog begins to form. Fog is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface (restricting visibility) (Figure 8-7). As the air continues to cool, the condensing droplets water form mist. When the mist droplets grow large enough to be influenced by gravity, it will fall as precipitation.

The dew point is called the frost point when the temperature is below the temperature that water freezes. Below freezing, water moisture in the air will sublimate directly into ice, forming frost, snow, or hail (Figure 8-8).

A hygrometer is a device that can measure the humidity in the air. There are many kinds of hygrometers and different ways to measure humidity in the air. Modern hygrometers use sensors that can directly measure the electrical, optical, thermal, and other means to accurately measure water content in the air. Hygrometers are part of any meteorological station, and can be measured with regional radar and satellite sensor data. Hygrometers are important devices for measure moisture in soil.

National Weather Service (NOAA) - Relative Humidity Calculator (enter data for current temperature and dew point to determine relative humidity).
Dew forming on leaves.
Fig. 8-6. As the atmosphere cools, the moisture content dissolved in the air will condense to form water droplets, such as illustrated by dew on leaves. As the air continues to cool, more and more moisture will be released and water droplets
Fog layer beneath the Golden Gate Bridge, California.
Fig. 8-7.
A Fog Bank Fog under the Golden Gate Bridge, CA. where water moisture condenses in the air near the cool ocean surface. The warm moist air above the bridge remains clear because it is above the dew point.
Large frost crystals on frozen ground.
Fig. 8-8.
Frost forms when air moisture directly sublimates from the air onto cold surfaces. These spectacular frost ice crystals formed on frozen ground in South Dakota on a day when the high temperature only reached -11º Fahrenheit.


The Water Cycle

The water cycle involves all processes by which water circulates between the Earth's oceans, atmosphere, and land. It involving precipitation as rain, snow, hail, drainage in streams and rivers, and return to the atmosphere by evaporation (converting liquid water to water vapor), sublimation (converting ice into water vapor), and transpiration (water released by plants). The weight of the atmosphere provides the pressure needed to keep water liquid on the surface of the planet. Planets and moons with thin or no atmosphere may have water as ice, but there will be no permanent bodies of liquid water. The Water Cycle is also called the Hydrologic Cycle. On land, the water cycle involves movement of water down slope in the form of surface runoff and streamflow, groundwater flow, and glacial ice flow that moves water back to the oceans.
Water Cycle (NASA version)
Fig. 8-9. The Water Cycle

Atmospheric (Barometric) Pressure

A barometer is an instrument measuring atmospheric pressure, used especially in forecasting the weather and determining altitude (Figure 8-10).

Air pressure on the planet is directly related to the mass of the air column above at any location under the influence of gravity: Pressure = Force/Area.

Atmospheric air pressure is reported as average air pressure measured at standard sea level. Reported barometric air pressure at elevations above sea level are adjusted to be equivalent to air pressure measured at sea level at locations closest to where measurements are taken.

How Air Pressure Is Reported

Barometric units most used to describe atmospheric pressure includes atmospheres, millibars, and PSI (pound-per-square-inch).

The weight of ONE ATMOSPHERE (Earth's atmosphere above us) is equal to the weight of the Earth's average air pressure at standard sea level.

One atmosphere (on Earth, on average) is equivalent to:

14.7 pounds-per-square-inch (psi) - this might mean something when you add air to you car's tires.
29.92 inches of mercury (a historic measure of air pressure that is still widely used).
406.8 inches of water (33.9 feet) - how deep you'd need to dive in a freshwater lake to double the weight of the atmosphere.
seawater (33.4 feet) - seawater is slightly denser than freshwater.
1.01325 bars (one bar was supposed to be equivalent to the weight of one average Earth atmosphere; the slight number above 1.0000 bars is from adjustments from atmospheric-pressured data that was later compiled from locations measured around the world. It was determined that the average weight of the Earth's atmosphere was slightly higher than the standard one bar was originally established between the year's 1793 and 1795 by the European science community as an attempt to add an air pressure standard to metric system. When analytical devices are calibrated, they use the revised metric unit: millibars.

The average weight of one Earth atmosphere is now commonly reported as 1013.25 millibars (mb).

Fig. 8-10. Barometers

Atmospheric Pressure Drops With Increasing Altitude (Elevation)

Elevation and air pressure have an inverse relationship - air pressure decreases with increasing elevation (Figure 8-11). At an elevation of about 18,000 feet you would be above about half of the atmosphere. That, of course, depends on changing weather conditions! An common altimeter is a type of barometer that measures air pressure to report elevation, but altimeters must be adjusted to match local weather conditions.

What is the difference between altitude and elevation?

If you are flying an airplane, you need to know this! Technically, altitude is the vertical distance from the Earth surface (land or water) to an object (such as an airplane). Elevation is a the vertical distance between a location on the ground and global sea level.
Air pressure with altitude
Fig. 8-11.
Atmospheric pressure decreases with altitude on a curve.

Can you feel changes in Atmospheric Pressure?

The answer is a most definite yes! As you go up in elevation, air pressure trapped within your ears is not equalized with the air pressure outside, so your ears tend to occasionally pop as you climb in altitude, as your ear ducts release air when you swallow. Older people commonly complain about bone and joint pain when a storm is approaching and air pressure starts falling.

The opposite is true when you go down in elevation, such as on an airplane descending from high altitude. Anyone who has frequently flown can tell you about crying children complaining ear aches because their ears have not readjusted to air pressures at low elevations.

Density of Warm Air vs. Cool Air

As air is heated it expands (moving atoms apart). This reduces the density of air in unconfined space. As a result warm air rises. Conversely, as air cools, it condenses (moving atoms together) and increases it's density in unconfined space. As a result cold air sinks. Because the atmosphere is unconfined, dense cool air will sink and flow to displace warm air in another location (Figure 8-12).

Density of Moist Air vs. Dry Air

Air saturated with water vapor is less dense than dry air. As a result, moist air will rise relative to dry air if air temperatures and pressures are the same.
Hot air and cold air
Fig. 8-12.
Differences in air pressure at different levels in the atmosphere drive the movement of air.

Atmospheric Convection

Convection is the circulation of fluid due to density differences. Atmospheric convection works like a pot of boiling soup, warm fluid rising (in middle) and cool fluid falling (on sides). A rising storm thunderhead is an example of atmospheric convection. Warm moist air rises, expands, releases energy as clouds form. After releasing its heat and moisture, the cooled air sinks, displacing warm air below (Figure 8-13).

Air convection
Fig. 8-13.
Atmospheric convection.

Air Pressure Gradients and Air Pressure Systems

• Surface winds blow from high to low pressure - this is called a pressure gradient—displayed as lines of equal barometric pressure on a weather map (Figure 8-14).

An air mass is a body of air with a relative horizontally uniform temperature, humidity, and pressure:

High pressure systems have dry conditions with sinking air masses.
Low pressure systems have wetter conditions with rising air masses.
Air pressure gradient and air pressure systems
Fig. 8-14.
Air pressure gradients and air pressure systems.

Types of Air Masses and How They Form

An air mass is a large body of air with relatively uniform temperature, humidity, and pressure. Air masses move with the global atmospheric system and can change as the move over landmasses and oceans, picking up or loosing warmth and moisture as they move.

Types of air mass are classed by where they form:
Polar - source regions above 60° north and south:
Polar Maritime
(cold and moist)
Polar Continental
(cold and dry)
Temperate - between 25° and 60°N/S:
Temperate Maritime
(cool and wet)
Temperate Continental
(warm and dry)
Tropical - source regions within about 25° of the equator:
Tropical Maritime
(warm and wet)
Tropical Continental
(hot and dry)

As air masses move they change to match the attributes of the next region. For instance, if a polar (or Arctic) air mass moves south over the North American continent it will become warmer and dryer (becoming a temperate-continental air mass; see example in Figure 8-15). If it moves east over the Atlantic Ocean it may become warmer and pick up moisture and become a temperate-maritime air mass. When a maritime air mass moves over a large landmass it can loose its moisture, heat up, and become a continental air mass.

Air masses can move rapidly (if air pressure gradients are high). Air masses can control the weather for a relatively long periods ranging from days to months. They can also stagnate in one region causing long periods or rain or drought. Tropical storms and hurricanes can form in association with tropical-maritime air masses. Most weather occurs along around air masses at boundaries called fronts (discussed below).
Origin of air masses affecting North America
Fig. 8-15.
Origin of air masses affecting North America's weather. Air masses move as air pressure gradients change over time.
A Year in Weather (2013)
NASA YouTube animation
- a global mercator map showing storm systems around the world for a year starting in January, 2013. Note the tropical cyclones (typhoons) in the Eastern Pacific, the weather patterns in the Intertropical Convergence Zone, and the Antarctic circumpolar region.

Dust, Aerosols, and Cloud Condensation Nuclei (CCNs)

Cloud condensation nuclei (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water vapor condenses. CCNs are aerosols, an aerosol is a colloidal suspension of microscopic particles dispersed in air or gas. The aerosols can be a combination of solid particles and liquid compounds (liquid water or organic residues).

Examples of CCNs include:
- dust particles (clays) - most are from wind storms in desert regions (see NASA video with Figure 8-16)
- soot from fires
- volcanic ash
- salts from sea spray
- sulfate compounds released by phytoplankton in the oceans
- pollen and organic aerosol compounds released by land plants (Figure 8-17)
- pollution (smog) from urban areas and from agriculture activities. (Figure 8-18).

CCNS are abundant in the air. The adhesion properties of water, allows water droplets (or ice) to form and grow on CCNs, until gravity is strong enough for droplets to fall as rain or snow. However, too many CCNs in the air can prevent water droplets or ice crystals from growing large enough to fall as precipitation (rain or snow), contributing to often thick haze or smog).

Dust/aerosols in the atmosphere
Fig. 8-16.
Dust from desert regions is a major source of CCNs. CLICK HERE to see an animation of aerosol movement in around the world.

Natural aerosol haze from plants in the Appalachian Mountains
Fig. 8-17.
Natural CCN aerosols released by plants produce the haze of the Smoky Mountains region of the Appalachian Mountains.
Smog in NYC in the 1970s
Fig. 8-18. Smog in NYC in the 1970s. Air pollution from human activity is an increasing source of CCNs in the atmosphere. Smoke from manufacturing, vehicles (particularly diesel-burning trucks), coal-burning power plants, and construction dust are significant sources. Efforts to regulate CCNs have helped reduce smog in the US, but what about China?
Dust is emitted from dry soils rich in alluvium (silt- and clay-sized fractions) when surface winds are strong enough. Such conditions are exist in the arid and semi-arid regions of the world, and dust events can be observed from nearly all continents: Africa, Asia, Australia, North and South America, Aerosols form from sulfate compounds produced from oxidation of sulfur dioxide, which is emitted mostly from burning fossil fuels (petroleum and coal) such that major plumes of sulfate are observed in East and South-East Asia, and from Europe and the United States.

How does air pressure relate to weather?

Increasing high pressure (above 1000 millibars) corresponds with clear, sunny weather.
Decreasing pressure (below 1000 millibars) corresponds with cloudy, rainy weather.
Videos: Earth From Orbit - a website hosting NASA/NOAA weather satellite videos showing major storm activity as seen from space.

Highest barometric pressure (record):
1084 millibars (32.01 inches of mercury)
Agata, northern Siberia, on December 31, 1968.
The weather was clear and very cold at the time, with temperatures between -40° and -58°
Lowest barometric pressure (record):
870 millibars (25.69 inches of mercury)
West of Guam (Pacific Ocean) on October 12, 1979 In the eye of Super Typhoon Tip which involved wind speeds of 165 knots (305 km/h; 190 mph).

Why does San Diego have the best weather in the US?

Highest air pressure: 1033 millibars (February, 1883)
Lowest air pressure: 987 millibars (January, 2010)

This is the lowest range in the United States! (46 mb) !



Weather is the state of the atmosphere at any place and time in regards to conditions: sunshine, heat, dryness, cloud cover, wind, precipitation (rain, sleet, snow, hail), etc.


Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals.
The dew point is when the relative humidity reaches 100%. The base of a cloud marks the boundary where relative humidity has reached saturation. Cloud tops can rise until they encounter warmer air in the stratosphere. There they stop rising and spread out forming anvil-shaped thunderheads shapes (Figure 8-19).

4 general types of clouds (there are many sub-types)
Cirro-form: high level, wispy - fair weather clouds if ice crystals, typically above 20,000 ft (6000 m)
Cumulo-form: low to high level cotton-like puffy clouds with flat base at 100% humidity level, can rise to 60,000 feet.
Nimbo-form: rain clouds (low to mid level) - clouds typical thicken and lower as precipitation begins.
Strato-form. uniform flat cloud layer at any level, forms fog at the surface (coastal marine layer an example)

Names of clouds can include combinations of forms as they change. For instance, a small, puffy white cumulus cloud can build up and become an altocumulus cloud, before rising even further to become a cumulonimbus (thunderstorm) that can develop a high anvil-shaped top as the rising moist air at top the cloud encounters the stratosphere and can't rise any higher.

Figure 8-20 illustrates common forms of clouds. Also see Types of Clouds (examples from California).

Cloud base and top of thunderhead
Fig. 8-19.
Cloud base and tops of a thunderstorm (a cumulonimbus cloud).

Cloud types
Fig. 8-20. Common types of clouds.

Lightning and Thunder

Lightning is giant spark, or series of sparks (electrical discharges), that leap through the air.

Lighting occurs as mostly as intra-cloud lightning (leaping between different parts of a thunderstorm, Figure 8-21) or cloud-to-ground lightning (Figure 8-22). Lightning is caused by the buildup of between positive and negative electrostatic charges within the clouds or between the clouds and the ground. The air acts as an insulator between the buildup of charges until they become great enough to overpower the insulating capacity of the air.

The passage of lightning has jagged path. Lightning probably follows the interconnect paths created by ionizing radiation particles passing through the atmosphere from outer space. These particle create very short-lived plasma passages through the atmosphere that allow electricity to propagate through the insulating air, creating the rapid electrified flashes we see as lightning. The discharge of lightning temporarily equalizes the charged regions in the atmosphere or the ground, until opposite charges can build up again.

Lightning tends to strike high places (closest to the cloud), such as the tops of building, telephone poles, trees, antennas, but this is not always the case. Lightning will strike any place where the electrical charges build up and where a stepped leader (an initiating passage of electrical discharge) arrive first. Typically, a negatively stepped leader leaving a cloud will arrive at the ground followed by a more powerful, brighter return stroke or multiple strokes moving in the opposite direction (sometimes the inverse of this occurs). A lightning bolt can be a complicate mix of stringers and leader besides the passage of the main bolt of lightning. In many cases lightning may strike water or a low lying area even if nearby trees are present. Lightning will often appear as a rapid series of strokes as discharges from different parts of the cloud utilize the previously ionized air passage created by an initial discharge giving the appearance of multiple strokes following the same path. In sky-to-ground lighting, the bright return stroke to the sky is estimated to travel about 60,000 miles per hour. In contrast, when conditions are right spider lightning is an unusual slow spread of lightning though lower stratus clouds at the base of a thunderstorm that appears to propagate in all directions away from an initial sky-to-ground stroke.

What causes lightning is complicated, and various theories apply. Areas of positive and negative charges can build up within the same thunderstorm cloud (Figure 8-23). The formation of precipitation (starting with cloud-condensing nuclei [CCNs] to the growing droplets that become rain drops, snow, sleet, and hail) all have surface area that are growing or diminishing as they move upward or downward through a cloud. Changes in surface area of droplets and ice crystals, and the frictional interaction of particles create the negative or positive charges and as condensation or evaporation takes place. Some researchers suggest that heavier, growing precipitation particles carry negative charges to the lower part of clouds as they descend with down-drafts. Updrafts may transport positive charges from near the ground upward through the cloud.

Lightning is associated with thunderstorms, but they are also known to occur in association with the clouds associated with volcanic eruptions, hurricanes, tornadoes, forest fires, snow storms, and even discharges from the ground during earthquakes.

The power or intensity of lightning varies with the volume of atmosphere hosting electrostatic charges and the distance lightning travels. A typical lightning stroke only lasts about 0.2 seconds. However, a typical lightning bolt can generate up to one billion volts, and they average between 5,000 to 20,000 amps of electrical current (as much as 200,000 amps have been measured - enough to briefly power a small city!). Lighting can heat the air to temperatures around 15,000 to 60,000 degrees Fahrenheit (or much higher). This causes the air to rapidly expand, creating shock waves we hear as thunder. Up close, thunder form a nearby lightning strike sounds as a sharp clap-like high frequency crackle and initial boom, followed by an extended low-frequency rumble as sound wave arrive at different times from farther distant parts of the lightning’s path (higher frequency sounds are absorbed as they travel over longer distances through the air). Thunder can be heard for distances of 25 miles or more under the right conditions.

Estimates vary, but as many as 10,000 fires are started each year by lightning, and lightning kills between 6,000 and 24,000 people and lightning injures as many as 240,000 people each year worldwide.

Intra-cloud lightning
Figure 8-21. Intra-cloud lightning.
Lightning bolt
Figure 8-22. Sky-to-ground lightning with leader bands.
Lightning charges in a cloudFigure 8-23. Lightning charges (positive and negative) can build up in different parts of a cloud relating to updrafts and down-drafts, and the changing character of precipitation within a cloud.
How far away was that lightning strike? After you see a flash of lightning start counting seconds. For every 5 seconds the lightning bolt is one mile away. It is advised to take shelter immediately if the sound is less than 5 seconds!
Lightning strike map of the United States 2015-2019.Lightning strike map of the United States 2015-2019.

Weather Fronts

A weather front is a boundary separating two masses of air of different densities (Figure 8-24). Fronts are classified as to which type of air mass (cold or warm) is replacing the other.

A cold front forms along the leading edge of a cold air mass displacing a warmer (less dense) air mass. Cold fronts are typically narrow bands of showers and thunderstorm and are most commonly associated with severe weather condition.

A warm front is the leading edge of a warmer air mass replacing (riding up and over) a colder air mass. If the front is essentially not moving (i.e. the air masses are not moving) it is called a stationary front. Warm fronts typically have a gentle slope so the air rising along the frontal surface is gradual. This configurations results in widespread stratus (strato-form) cloud layers with precipitation near the rear of the frontal boundary. Warm fronts typically quite extensive, and can create typically gray skies and dismal weather—an all too common occurrence in parts of the Midwest and Northeastern United States as slow-moving warm fronts stall over the regions. This can happen any time of year. It reflects that warm, moist air is flowing above cooler air down below, creating the gray stratus cloud layer in-between.

Colliding air masses can have both warm fronts. For instance, when a warm, moist air mass (such a maritime-tropical air mass) encounters a cold, dry air mass (such a polar continental air mass), both warm fronts and cold fronts can form as air rotates around a center of low pressure (as illustrated in the lower graphic of Figure 8-24). This rotation is driven by the Coriolis effect (discussed below).
Weather Fronts
Fig. 8-24.
Weather fronts between air masses: cold fronts and warm fronts.

How do you say which way is the wind blowing?

We name wind direction based on which direction it is coming from (from high pressure to low pressure). For instance, if the wind is moving off the Pacific Ocean directly onto the land in California we call it a west wind, or to clarify, out of the west. The direction of wind is named for the direction it is coming from, not in the direction that it is moving towards.

Weather and Climates

Weather is localized atmospheric conditions in the short term (described in terms of minute, hours, or possibly weeks).
is the prevailing weather conditions in an area in general or over a long period (years, decades, etc.).

Weather is typically constantly changing as air masses move across a region. A most common question on a long-distance phone call is: "How is the weather out there?" The answer can typically be "it's sunny," "it's cold," "it's snowing," "or "it's raining cats and dogs," or any other description of the prevailing atmospheric at any time in a location.

Climates are controlled by both geographic factors and regional weather patterns. Different regions (climates) typically have seasonal cycles. For instance, the Eastern United States typically has 4 seasons and have frequent weather fronts between polar air masses from Canada and tropical air masses from the Gulf and Atlantic regions. In contrast, California typically has 2 seasons, summers are dry and winters have short rainy periods. Patterns in weather repeat each year and are typically consistent and predictable with seasons of the year. Examples include monsoons in India and the US Desert Southwest, Hurricane season in the tropics, etc. Figure 8-25 illustrates how California's weather and climate is influenced by regional geography and prevailing weather conditions.

History shows that climates change. The time spans for changes can range in cycles ranging from years and decades to centuries, or thousands of years. Droughts can start an last for years. Desertification (such as what is happening in Africa) has been progressing for centuries. Parts of the world experienced Mini-Ice Age conditions between the 13th and 19th centuries. Climate change has impacted civilizations throughout recorded history. A classic example is illustrated in the history of the Chaco Culture in the US Desert Southwest (Figure 8-26). See more climate cycle times and events in history on NOAA's Paleoclimatology website).
California's precipitation and climates are controlled by geography
Fig. 8-25.
California's weather and climate is influenced by regional geography and prevailing weather conditions.
Chaco Culture Fig. 8-26. Chaco Canyon in New Mexico was at the center of a regional 13th century society impacted by climate change.
See NOAA's Climate Change Impacts website - time lines with many links and animations.

Climate Variability

California's history of droughts illustrates climate variability (Figure 8-27). Climate not only vary on a seasonal and annual basis, but there are larger scale fluctuations that impact different regions of the world. One is the El Niño/Southern Oscillation (ENSO) (discussed in Chapter 9). Because the atmosphere is an open system, changes in one region can affect other surrounding regions. Regions that may experience dry conditions for decades may suddenly have a severe rain period, or regions that are typically wet can sustain drought. Likewise, regional temperature average can swing through cyclic periods. Some of these changes can be progressive and represent long term changes. For instance, during the last ice age, Southern California was very wet, and large lakes filled many of the basins between mountain ranges. By about 5,000 years ago, the lakes dried up as the climate changed, then they returned as wet conditions returned for periods of time. The last major drying period was about 500 years ago, as recorded by the evidence of Indian village sites associated with fishing on the shores of SoCal lakes that now are mostly barren desert. See a website on Climate Variability (NASA).
California's drought cycles
Fig. 8-27.
California's cycles of drought and wet periods is an example of climate variability.

Effects Of Uneven Heating Of Earth By the Sun

The amount of energy Earth receives from the Sun is not evenly distributed (Figure 8-28). More solar energy (per unit area) is delivered to the equator than near the poles.

• The equatorial regions are warmer than the poles because direct sunlight is concentrated and little is reflected.
• In polar regions, light strikes the earth at an angle; it is diffuse and much of it is reflected back into space.
• The seasonal variations (winter and summer) also affect the distribution of heating of the planet.

This imbalance between the solar heating in the tropics and at the polar regions is a major factor in atmospheric movement on Earth and other planets with atmospheres.

Solar energy by latitude
Fig. 8-28.
Solar energy budget by latitude

The Coriolis Effect on Atmospheric and Ocean Circulation Systems

Heat from insolation (short for INcoming SOLar radiATION) is the driving force behind the fluid motion of the atmosphere and the oceans. However, the patterns of motion are also influenced by the forces created by the rotation of the Earth on its axis. Any mass moving in a rotating system experiences a force (the Coriolis force) the acts perpendicular to the direction of motion and to the axis of rotation. Because air has mass, air currents maintain momentum when moving from a location of high pressure to low pressure. However, because the Earth is rotating, the rotation causes a right-turn deflection in the Northern Hemisphere and a left-turn deflection in the Southern Hemisphere (Figure 8-29).

The Coriolis effect influences all moving objects, especially ones moving over large distances (such as intercontinental ballistic missiles). The Coriolis effect causes objects or moving masses of air to:
• Change direction—not speed.
• Maximum Coriolis effect occurs at poles.
• No Coriolis effect occurs at equator.

Rotation of pressure systems due to the Coriolis effect:

Northern Hemisphere:

• High pressure turns clockwise
• Low pressure turns counter-clockwise

Southern Hemisphere: opposite of N.H.
• High pressure turns counter-clockwise
• Low pressure turns clockwisespacer_bar

So... which way does the water spin in a toilet in the northern hemisphere, southern hemisphere, and on the equator?
Coriolis EffectFig. 8-29. The Coriolis effect is cause by the rotation of the Earth on its axis. This rotation causes air masses moving from high to low pressure to deflect.

Earth's Atmospheric Circulation System

The global atmospheric circulation system influences the movement of air masses in general wind belts that move air in rotating masses within zones around the planet. These wind belts seem relatively stable when viewed in a long-term view (decades). However, fluctuations may occur on seasonal or annual basis. The wind belts are influenced by the Coriolis effect and large-scale convection patterns in the atmosphere (Figure 8-30).

These relatively stationary wind belts impact the surface of the oceans, creating currents that circulate waters in the oceans.

Studies of the atmosphere have show that their are 3 major atmospheric systems called circulation cells (Figures 8-30 and 8-31).
World wind zones
Fig. 8-30.
Global wind circulation patterns impact regional climates and drive the large current systems in the global ocean circulation system.

Circulation Cells in Earth's Atmosphere

Three major circulation cells move air, heat, and moisture through the atmosphere between the equatorial regions to the polar regions. These cells are constantly changing due to regional air pressure changes under the influence of the Coriolis effect.

Hadley cells
(0° to 30° N and S of equator)
• Responsible for the Trade Winds: They blow NE in N. Hemisphere and SE in S. Hemisphere.

Ferrel cells
(30° to 60° N and S of equator)
• Responsible for the Prevailing Westerlies in both hemispheres.

Polar cells
(60° to 90° N and S)
• Responsible for the Polar Easterlies in both hemispheres.
Circulation cells in the Atmosphere
Fig. 8-31.
Hadley, Ferrel, and Polar circulation cells in Earth's atmosphere redistribute convectional heat.

What is the Jet Stream?

A jet stream is a narrow, variable band of very strong winds in the upper troposphere. They are predominantly westerly air currents encircling the globe several miles above the Earth. There are typically two or three jet streams in each of the northern and southern hemispheres. These high-speed wind currents often move at speeds exceeding 250 miles (400 km) per hour at altitudes of 6 to 9 miles (10 to 15 km). Jet streams are influenced by moving air masses and the Coriolis effect causing them to meander and sometime split. See the location of the jet streams in Figures 8-31 and 8-32.
Polar and tropical jet streams
Fig. 8-32.
Polar and Subtropical jet streams

Equatorial Doldrums and Inter-Tropical Convergence Zone (ITCZ)

The equatorial doldrums are associated with the inter-tropical convergence zone (ITCZ) the region that circles the Earth near the equator, where the trade winds of the Northern and Southern Hemispheres converge (Figure 8-33).

The doldrums are:

• Area of low atmospheric pressure with lots of rain.
• Located on equator where there is least influence of the Coriolis effect.
• Low wind area with calms, sudden storms, and light unpredictable winds

Seasonal shifts in the location of the ITCZ affects rainfall in many equatorial regions, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. The ITCZ moves north during winter in the northern hemisphere and south in the summer.
Intertropical convergence zone
Fig. 8-33.
The doldrums are the belt of clouds along inter-tropical convergence zone. This belt of clouds (with lots of rain) migrates north and south across the equator with the seasons.

The Tropical Easterlies (Trade Winds)

During the age of sailing ships, ship captains learned take advantage of the prevailing wind belts to cross the oceans. Two belts of trade winds encircle the Earth, blowing from the tropical high-pressure belts (Hadley Cells) to the low-pressure zone of the equatorial inter-tropical convergence zone. The tropical easterly wind belts near the equatorial region are also called the Trade Winds. Trade winds blow steadily toward the equator from the northeast in the Northern Hemisphere, or the southeast in the Southern Hemisphere (see Figures 8-30 and 8-31).

Horse Latitudes

The horse latitudes are belts of calm air and sea occurring in both the northern and southern hemispheres between the trade winds and the westerlies (roughly 30-38 degrees north and south of the equator). Horse latitudes separate the Hadley and Ferrel Cells. It is a region also called the subtropical high—a belt of very dry because of high pressure, little rain. Horse latitudes roughly correspond with major desert regions of the world. The horse latitudes got it name from historic legends describe ships becoming becalmed when crossing the horse latitudes and running out of water and unable to re supply. Sailors would throw horses on the ships overboard.

Horse latitudes
Fig. 8-34.
Location of the horse latitudes (subtropical highs).

Weather data animations:

Global atmospheric circulation video - High Speed Weather—Satellite Infrared of the entire globe (NASA data)

See animations current data from NASA Global Geostationary Weather Satellite (GOES weather satellite).

Current San Diego Weather Radar
(accuweather.com) - this website provides current weather radar conditions for local, state, regional, and national scales with looping animations.

Global weather animation (infrared)
Fig. 8-35.
Weather satellite data animation.

The Coriolis Effect Influences Superstorms

Large rotating storms are called hurricanes (near North America), typhoons (near Southeast Asia) and cyclones (in the Indian Ocean). All are the same, caused by warm moist winds being drawn to the center of low pressure near the center of the storm (called the eye in well developed storms). North of the equator the Coriolis effect causes low-atmospheric pressure to rotate counterclockwise, but south of the equator they rotate in a clockwise direction. The lower the air pressure in the eye of the storm, the greater the wind speed and rotation. Note on the map in Figure 8-36 that there are no hurricanes along the equator or near the poles. These are regions where the Coriolis effect is not a significant force in deflecting storm winds to cause rotation.

Superstorms not only can cause major wind damage and flooding, but can erode and redeposit vast quantities of sediments, both offshore and onshore, heavily impacting impacting both communities and ecosystems.
Storm paths of Hurricanes, Typhoons, and Cyclones
World map showing historic paths of hurricanes, typhoons, and cyclones. The large storms are the same (different names for different regions); storm rotation is influenced by the Coriolis effect.

Tropical Cyclones, Hurricanes, and Typhoons

Tropical cyclones are large rotating air masses with low atmospheric pressure (Figures 8-37 and 8-38).

Tropical cyclones are called hurricanes in the Atlantic Ocean or in the Pacific near North or South America, and Hawaii). Tropical cyclones are called typhoons in the Western Pacific Ocean region. They are simply called cyclones in the Indian Ocean region.

Northern Hemisphere Example:

• Storms Intensify over warm water (>77 degrees F); warm water provides water vapor.
• Water vapor provides fuel for storm in the form of latent heat energy as water vapor condenses.
• Storms die over land and cool water.
• High winds, tornadoes occur near storm center and along feeder bands.
• Sea level can rise in front of storm called a storm surge.
• Classified by maximum sustained wind speed (see rating storms below).
• Hurricanes and other storms rotate counterclockwise in the Northern Hemisphere because of the Coriolis effect.

Atlantic hurricane
Fig. 8-37.
Hurricane Andrew (1992) was for a time a Category 5 hurricane with sustained winds of 175 mph (280 km/hr).

Ratings Storms (By Maximum Sustained Wind Speed)

Tropical depression (<38 mph)
Tropical storm (between 38 and 74 mph)
Tropical cyclone (>74 mph)

Saffir-Simpson Scale
: 5 categories of hurricane intensity based upon wind speed: (see a NOAA animation).
• Category 1 is from 74 to 96 mph.
• Category 2 is from 96 to 110 mph.
• Category 3 is from 111 to 130 mph - level considered a "superstorm" (Katrina, 2005).
• Category 4 is from 130 to 155 mph (examples: Andrew, 1992, Hugo, 1989).
• Category 5 is >155 mph (Camille, 1969).

Naming storms:
Alphabetical lists of names are assigned each year to storms that develop in each of the ocean basins. Names of notoriously damaging storms are retired to remind people of their impacts and legacy.

The term superstorm is used to describe any powerful and destructive storm that affects a large area or region. Tropical storms and cyclones can be superstorm, but other massive storms in temperate and polar regions can be become superstorms. Nor'easters are extra-tropical superstorms that typically impact the Northeastern United States in the fall and winter season, causing massive amounts of snowfall and coastal flooding.
Hurricane Katrina
Fig. 8-38.
The eye of a hurricane is the center of low pressure. Hurricane Katrina (2005) shown here, was the most costly and destructive hurricane disaster in US history, killing more than 1,800 people.

Severe Weather

Severe weather conditions can occur anywhere, but some area are more susceptible to severe weather than others due to regional geography and climate factors. Severe weather includes strong convective thunderstorms, winter storms (severe cold, blizzards, and ice storms), damaging wind storms and tornadoes, flooding, dust storms, extreme heat, and firestorms.

Atmospheric scientists are constantly monitoring weather conditions to make predictions of potential severe weather conditions (and potential disasters), using ground-based weather observations combined with remote sensing data (satellite, airplane, Doppler radar, etc.). These are combined with historic weather data in order to make weather predictions.

The Federal government has been recording statistics of deaths and property damage due to weather-related activity for many decades. Floods and droughts (with associated famines) have remained the most deadly disasters worldwide. In the United States, extreme heat events and floods remain the numbers 1 and 2 killers (Figure 8-39). Tornadoes are number 3, but they are perhaps the most terrifying because of their unpredictable occurrence and suddenly destructive behavior.


Tornadoes are mobile funnel-shaped rotating vortexes of wind that form and advance beneath large storm systems (Figure 8-40). Tornadoes vary considerably in their destructive power—how strong their winds are, how long they are in contact with the land surface, and the distance they travel. Storm systems can often produce multiple tornadoes (called a tornado outbreak). A single storm cell can sometimes produce multiple tornadoes simultaneously. The 2011 Super Outbreak (April 25-28) produced 392 "confirmed" tornado in 21 states (between Texas and New York) with four rated as F5 tornado on the Fugita tornado intensity scale (see table below).

Fugita Tornado Intensity Scale

F-Scale # Wind Speed Intensity Phrase
F0 40-72 mph light damage - branches off trees, minor damage to roofs
F1 73-112 mph moderate damage - roof and window damage, mobile homes overturned, cars pushed off roads
F2 113-157 mph significant damage - roofs off homes, mobile homes destroyed, large trees down/uprooted, cars push off roads
F3 158-206 mph severe damage - roofs and walls torn from well constructed homes, most trees uprooted, heavy cars lifted off ground and thrown
F4 207-260 mph devastating damage - well-constructed homes leveled, cars thrown and large object moved like battering missiles
F5 261-318 mph incredible damage - homes lifted off foundations and ripped apart, trees debarked, concrete structures damaged, skyscrapers topple

Figure 8-41 shows a tornado probability map of a typical day (June 23) in the United States based of historical tornado data; data for each day of the year shows that tornado activity in the country varies significantly from season to season and from one region to another. The central Great Plains region is commonly called Tornado Alley because it statistically experiences the greatest number of tornadoes in any given year, but weather and climate data show that trends are changing (as well as better recording of data). The Great Plains and Midwest typically experience clashing air masses—cool and dry air masses that move east from the Rocky Mountains and Canada collide with warm, moist air masses moving north from the Gulf of Mexico and Atlantic regions. When conditions are right, large thunderstorms that display intense convection and rotation can generate vortexes that descend as funnel clouds—these become tornadoes when the start to impact the surface.


Drought is a prolonged period of abnormally low rainfall. Drought conditions commonly lead to other disastrous weather conditions including dust storms, heat waves, and firestorms, all of which can be catastrophic. The worst droughts in US history occurred in the 1930s and 1950s, resulting in Dust Bowl conditions throughout the Great Plains and Midwest that lead to severe economic damage and social upheaval and migrations. Drought periods have alternated with serious flooding in intervening years. Figure 8-27 (above) shows the cyclic nature of droughts and flooding periods for the State of California. During recent droughts, devastating firestorms have ravaged communities throughout the regions around San Diego, Los Angeles, Santa Barbara, San Francisco, and throughout the Sierra Nevada region—as urban development has spread into areas where vegetation is naturally apt to burn on a frequent basis during drought conditions (Figure 8-42). Conversely, flood conditions during wet El Niño years can potentially be more catastrophic to California than drought. Recent investigations into the impact of a California mega-flood event that happened in California in the winter of 1871-1872 suggest that if were a similar event were to happen today it could potentially be the most destructive natural disaster to impact the United States—possibly causing nearly three times as much damage than a great earthquake in the region.

To see information about current and forecasts of regional storm activity see:
NOAA/National Weather Service Storm Prediction Center
Weather fatalites in the United States for 2016.
Fig. 8-39. NOAA severe weather statistics averages for the United States.

A tornado
Fig. 8-40. this F3 tornado occurred on May 3, 1993 in Oklahoma.

Tornado probabilities map of the United States
Fig. 8-41. Tornado probability map of the United States showing the likelihood of a tornado occur based on historic data for June 23 of any year.

Firestorm of the Zaca Fire in Santa Barbara County in 2007.
Fig. 8-42. A flame front of a firestorm near Santa Barbara in 2007.

Snow fall at Crater Lake Lodge, Oregon
Fig. 8-43. Massive snow accumulation can be anticipated in places like Crater Lake, OR. However, giant winter storms can have long lasting effects, not just the cold and ice, but the shutting down of regional infrastructure and economies.

What is the Greenhouse Effect?

The greenhouse effect is the trapping of the sun's warmth in a Earth’s lower atmosphere. This happens because lower atmosphere due to the greater transparency of the atmosphere to visible radiation from the Sun than to thermal infrared radiation emitted from the surface (Figure 8-44). A glass green house will let sunlight in, but captures some of the thermal energy within the enclosed interior. A greenhouse gas is any gas that absorbs and emits energy in the Thermal infrared range. Primary greenhouse gases in earth's atmosphere include: water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3).
The greenhouse effect.
Fig. 8-44. The greenhouse effect
is enhanced by the presence of greenhouse gazes in the atmosphere.

Global Warming and Earth's Greenhouse

Earth is currently growing warmer at an alarming rate! The weather records compiled from around the world indicated that there has been a significant rise in global temperatures over the past century. This rise in temperature is linked to the increasing amount of carbon dioxide and other greenhouse gases accumulating in the atmosphere (Figure 8-45). The rise in carbon dioxide and other greenhouse gases is a result of consumption of fossil fuels, deforestation, and other human impacts since the start of the Industrial Revolution in the 19th century.

Figure 8-46 compares the rise in atmospheric CO2 to the decrease in the ratio of stable carbon isotopes 13C/12C.
The cyclic patterns in the graph is a result of the annual growth of plants in the northern hemisphere. During the summer months plant growth consumes CO2, reducing CO2 concentrations in the air. In the winter months the decay of organic matter increases CO2 concentrations. The overall trend shows that atmospheric concentrations of CO2 is increasing. The cyclic pattern in the 13C/12C also reflects the plant-growth cycles, but also shows the dilution of 13C concentrations by the influx of carbon from fossil fuels. Carbon in fossil fuels (coal and oil) are enriched in12C.

There are many knowns and unknowns about the future of global warming. Highlights include sea-level rise, climate changes, changes in storm intensity and regional precipitation, changes in air and ocean chemistry (acidification), and other impacts on humanity and natural ecosystems.

Select resources about Carbon's role in the global environment:
Atmospheric Carbon Tracker Animation (NOAA)
The Carbon Cycle (NASA)
Ocean acidification: Issue briefs (United Nations)

Camparison of carbon dioxide concentrations to global temperature changes
Fig. 8-45.
Changes in global temperature with the rise in atmospheric CO2.

Greenhouse trends of carbon dioxide and carbon-13
Fig. 8-46.
Changes in atmospheric 13C/12C concentrations.

Atmospheres on Other Planets

The processes affecting Earth's atmosphere can also be seen on other planets. For instance, Jupiter, a planet about 318 times more massive than Earth has similar atmospheric circulation zones and bands (Figure 8-47). Figure 8-48 is an animation that shows the movement of the wind belts on Jupiter. On Jupiter, the bright zones are regions of rising cloud tops, and the dark zones are regions of sinking air. Bright spots on Jupiter are massive cyclones (some are larger than planet Earth!). Jupiter's upper atmosphere is composed of hydrogen, helium, and has clouds composed of ammonia ice crystals (NH3).
Fig. 8-47.
Wind belts on Jupiter. Just like on Earth, Jupiter's wind belts are controlled by the Coriolis Effect.

Fig. 8-48. Click on image to see Jupiter winds in motion (animated file).
Chapter 8 quiz questions
https://gotbooks.miracosta.edu/oceans/chapter8.html 10/8/2021