Atmosphere and Weather

I. Introduction

A. Weather and Climate

1. Weather is over a short period of time. Constantly changing.

2. Climate is over a long period of time. Generalized composite of weather

3. Elements of weather and climate: properties that are measured regularly.

a. Temperature

b. Precipitation

c. Cloudiness

d. Humidity

e. Air pressure

f. Wind speed and direction

B. The atmosphere is what makes our planet livable

1. Oxygen is what we breathe

2. Characteristics of the atmosphere help retain and circulate heat

C. Where does the energy come from? The Sun

II. Composition of the Atmosphere

A. Air is a mixture of discrete gases: clean dry air is composed of: nitrogen, oxygen, argon, methane, carbon dioxide ****

B. Variable components:

1. Water vapor: Clouds and precipitation, important for absorbing heat energy.

2. Dust (includes pollen and spores), water vapor condenses on it, reflects sunlight

3. Ozone: concentrated 10-50 km above the surface. Absorbs harmful UV radiation.

Human activity is depleting ozone by adding chlorofluorocarbons (CFCs)

III. Structure of the Atmosphere

A. Layers based on temperature

1. 4-layer structure

2. temp. increases or decreases within each layer

B. Pressure changes in the atmosphere

1. Pressure is the weight of the air above

2. Pressure decreases with altitude (90% of the atmosphere is below 10 miles!)

C. Relationship between pressure and temperature: adiabatic change

1. If air moves from low pressure to higher pressure, without additional heat change, adiabatic heating

2. If air moves from high pressure to lower pressure, without additional heat change, adiabatic cooling

IV. Earth-Sun Relationships

A. Earth motions

1. Earth rotates on its axis

2. Axis is tilted and always points in the same direction

3. Earth revolves around the Sun

B. Seasons

1. Result of changing Sun angle and changing length of daylight

2. Changing Sun angle (tilt of the rotational axis of the Earth)

V. Atmospheric Heating: Energy comes from the Sun through radiation!

A. Mini-heat review:

1. Heat is transferred from warmer to cooler objects

2. Heat transfer mechanisms:

a. Conduction through molecular activity

b. Convection: movement of a mass, usually vertical movement

c. Horizontal motions are called advection

d. Radiation (electromagnetic radiation)

1. Travels at the "speed of light" (186,000 mi/sec)

2. Consists of different wavelengths

B. Incoming and outgoing radiation

1. Incoming solar radiation: rel. short wavelength, most gets through atmosphere

2. Outgoing Earth radiation:

a. Earth reradiates energy at the longer wavelengths

b. Longer terrestrial radiation is absorbed by carbon dioxide and water vapor. Retains heat.

c. Lower atmosphere is heated from Earth's surface. "Greenhouse effect"

C. Heat budget for Planet Earth

1. Reflection and absorption are in dynamic equilibrium

2. Important "Actors" in atmospheric heating: Carbon dioxide, water vapor and dust

a. Minor components but very important

b. Greenhouse gases and global warming: dynamic equilibrium

VI. Air Pressure and Wind

A. Air pressure

1. Air pressure is the force exerted by the weight of the air above

2. Decreases with altitude (structure of the atmosphere)

3. Measured in millibars or inches of mercury

B. Wind

1. Movement of wind

a. Wind is the horizontal movement of air

b. Moves from high pressure to low pressure

2. Controls of wind

a. Pressure gradient force

1. Isobars: lines of equal pressure

2. Pressure gradient: pressure change over distance

b. Coriolis Effect

1. Apparent deflection of wind (and ocean currents) direction due to Earth's rotation

2. Deflection is to the right in the Northern Hemisphere; to the left in the Southern Hemisphere

c. Friction with the Earth's surface (only important near the Earth's surface, slows down air movement)

C. General atmospheric circulation

1. Underlying cause is unequal surface heating

2. On the rotating Earth there are 3 pairs of atmospheric cells that redistribute the heat

3. Idealized global circulation

a. Equatorial low-pressure zone: rising air, abundant precipitation

b. Subtropical high-pressure zone: subsiding, stable, dry air, location of great deserts

c. Subpolar low-pressure zone: polar fronts are a stormy area

d. Polar high-pressure zone: cold, subsiding air

4. Influence of continents changes the idealized global circulation

a. Seasonal temperature differences disrupt the global pressure and global wind patterns

b. Influence is most obvious in the Northern Hemisphere (where a higher % of surface area is land

c. Continents heat up faster and cool down faster. Get warmer in the summer and colder in the winter.

D. Circulation in the mid-latitudes is complex

1. Airflow is interrupted by cyclones that move west to east in the Northern Hemisphere

2. The paths of the cyclones and anticyclones are associated with upper-level airflow (jet stream)

3. Upper air winds: the Jet Stream

a. "River" of air

b. High altitude

c. High velocity (120-240 km/hr; 70-140 mph)

VII. Air Masses

A. Characteristics

1. Large body of air (1000 + mi) across

2. Similar temperature and moisture at any given altitude

3. Moves and affects large part of a continent

B. Classification of an air mass

1. By the nature of the source region (the place where an air mass acquires its properties

a. Continental (c): forms over land, likely to be dry

b. Maritime (m): forms over water, likely to contain moist air

2. By the latitude of the source region

a. Polar (P): high latitude, cold air

b. Tropical (T): low latitudes, warm

3. Four basic types of air masses

a. Continental polar (cP)

b. Continental tropical (cT)

c. Maritime polar (mP)

d. Maritime tropical (mT)

D. Air masses and weather

1. cP and mT air masses are the most important in N.A., esp. e. of the Rockies

2. North American (east of the Rockies)

a. Continental polar (cP) "Alberta Clipper"

1. From N. Canada and interior of Alaska

a. Winter - brings cold, dry air

b. Summer - brings cool relief

2. Responsible for lake-effect snows

a. cP air crosses Great Lakes

b. Air picks up moisture from lakes

c. Snow occurs on the leeward shores of the lake

b. Maritime tropical (mT)

1. From the Gulf of Mexico and the Atlantic Ocean

2. Warm, moist, unstable air

3. Brings precipitation to the Eastern U.S.

3. Continental tropical (cT): important in the southwest and Mexico, hot, dry.

4. Maritime polar (mP)

a. Brings precipitation to western mountains

b. Occasional influence in n.e. U.S. (causes the "Northeaster" in New England with its cold temperatures and snows)

VIII. Fronts

A. Boundary that separates air masses

1. Air masses maintain their identities

2. Warmer, less dense air rises

3. Cool, more dense air acts as a wedge (stays low or sinks)

B. Types of Fronts

1. Warm front

a. Warm air replaces cooler air

b. Shown on map with semicircles

c. Clouds become lower as front nears

d. Slow rate of advance

e. Light to moderate precipitation (if any)

f. Gradual temperature increase with the passage of the front

2. Cold Front

a. Cold air replaces warmer air

b. Shown on the map with triangles

c. Advances faster than a warm front

d. Weather more violent than a warm front (intensity of precipitation is greater, duration is shorter)

e. Wall of dark clouds, heavy precipitation, followed by cooler air and clearing conditions.

3. Stationary front

a. Flow of air on both sides of front is almost parallel

b. Surface position of the front does not move

4. Occluded front

a. Active cold front overtakes a warm front

b. Cold air wedges warm air upward

c. Weather is often complex

d. Precipitation is associated with warm air being forced aloft

IX. Middle-latitude cyclones: Primary weather producer in the middle-latitudes

A. Cyclones and Anticyclones

1. Cyclone

a. A center of low pressure

b. Pressure decreases towards the center

c. Winds associated with cyclones spiral inwards: convergence

d. Associated with rising air

e. Often bring clouds and precipitation

2. Anticyclone

a. A center of high pressure

b. Pressure increases towards the center

c. Winds associated with cyclones spiral outwards: divergence

d. Associated with descending air

e. Usually bring "fair" weather

B. Life cycle of a mid-latitude cycle

1. Originate along a front where air masses are moving parallel to the front in opposite directions

a. Continental polar (cP) air is often north of the front

b. Maritime tropical (mT) is often south of the front

2. Warm front and cold front form

4. Cold front catches up to warm front and produces an occlusion

5. Warm sector is forced aloft

6. Fronts discontinue

7. Storm comes to an end

C. General characteristics

1. Large center of low pressure (counterclockwise, inward air circulation)

2. Travels west to east

3. Last a few days to more than a week, stay about 2-4 days in any given area of the country

4. Extending from the center of the low are a cold front and (frequently) a warm front

5. Convergence and forceful lifting cause cloud development and abundant precipitation

6. Maintained by upper-level air flow

7. Aloft (up in the upper-levels), cyclones have divergence, this sucks up the air

and maintains low pressure and convergence near the Earth's surf

8. Largest weather contrasts in the Spring (and Fall) when change of temperatures happening.

X. Moisture in the Atmosphere

A. The physics of water

1. Three states of matter: solid, liquid, gas

2. To change state, heat must be either released or absorbed

3. Heat energy is measured in calories

4. Latent heat ("hidden" or stored heat)

a. Not derived by temperature change but by change of state

b. Very important in atmospheric processes

5. Processes

a. Latent heat is absorbed in evaporation, sublimation (solid to gas), melting

b. Latent heat is released in freezing, condensation, deposition: (gas to solid)

B. Humidity

1. Amount of water vapor in the air

a. Water vapor adds pressure ("vapor pressure")

b. Saturated air is filled with water vapor to capacity

c. Capacity is temperature dependent (warm air can hold much more water vapor)

2. Measurements of humidity

a. Relative humidity

1. Ratio of air's actual water vapor content to its potential (at a given temperature)

relative humidity = actual water vapor/capacity

2. Expressed as a %

3. Saturated air: 100% relative humidity, content=capacity

4. Relative humidity can be changed in 2 ways

a. Adding or subtracting moisture to the air

b. Changing the air temperature

b. Dew point

1. Temperature at which the air is saturated and the relative humidity is 100%

2. Cooling air below the dew point causes condensation

a. Cloud formation, "dew"

b. Water vapor requires a surface to condense on

c. HEAT (ENERGY) is released

C. Stability of air often determines type of clouds that develop and intensity of precipitation

1. Stable air

a. Resists vertical movement

b. Often results in widespread, thin clouds

c. Precipitation, if any, is light to moderate

2. Unstable air

a. Acts like a hot air balloon

b. Clouds are often towering

c. Often results in heavy precipitation

D. Processes that lift air

1. Air moving over a barrier (mountains): rainshadow effect

2. Air masses encountering other air masses at fronts

E. Condensation and cloud formation

1. Condensation: ENERGY, HEAT released

a. Water vapor in the air changes to liquid and forms dew, fog or clouds

b. Water vapor needs a surface to condense on (such as: dust, smoke, ocean salt crystals)

2. Clouds: composed of millions of minute water droplets or tiny ice crystals

3. Fog: cloud with its base at or near the ground

F. Precipitation

1. Formation of precipitation: needs a nucleus (either an ice crystal or some other particle)

2. Forms of precipitation: rain, drizzle, snow, sleet, glaze, hail, rime

G. Global distribution of precipitation is relatively complex

1. Related to global pressure and wind patterns

a. High-pressure regions: subsiding air, divergent winds, dry conditions, deserts (e.g., Sahara and Kalahari Deserts)

b. Low pressure regions: rising air, converging winds, ample precipitation, rain forests (e.g., Amazon and Congo Basins)

2. Related to distribution of land and water

a. Large landmasses in the middle latitudes often have less precipitation toward their centers (e.g. relatively dry Great Plains in N.A.)

b. Mountain barriers also alter precipitation patterns ("orogenic lifting")

1. Windward slopes receive lots of rainfall

3. Leeward slopes are often dry: rainshadow effect

XI. Severe Weather

A. Thunderstorms

1. Features: towering clouds, heavy rainfall, lightning, occasional hail

2. Occurrence: most common in FL and eastern Gulf Coast region.

3. Stages of development

a. All thunderstorms require warm, moist, unstable (lifting) air. Instability is often related to high surface temperatures and occurs late in the afternoon

b. All thunderstorms require a continual supply of warm air

1. Each surge causes air to rise higher

2. Updrafts and downdrafts form

c. Eventually heavy precipitation forms during the most active stage (gusty winds, hail, lightning)

d. Cooling effect of the precipitation marks the end of the thunderstorm activity

B. Tornado

1. Local violent windstorm of short duration. Rotating column of air extending down from towering clouds. Low pressure causes air to rush in; causing winds that can approach 300 mph

2. Occurrence and development

a. Average of 780 each year in the U.S.

b. Most frequent April through June. Can occur any time during the year.

c. Associated with severe thunderstorms

d. Exact cause is not known

e. Conditions for the formation of tornadoes occur most often along a cold front, during the spring months

3. Characteristics

a. Diameter is between 150-600 m, can be narrower or wider.

b. Speed is about 45 mph

c. Normally cuts about a 10 km (6 mi) long path, can be much longer or barely touch down.

d. Most move toward the northeast

e. Maximum winds about 480 km/hr (300 mph)

f. Measured by the Fujita intensity scale

4. Tornado forecasting is difficult because of their small size

a. Tornado watch alerts people when conditions are favorable for tornadoes. Alerts a large area

b. Tornado warning issued when funnels are sighted or indicated by radar

c. Use of Doppler radar helps increase the accuracy by detecting air motion

C. Hurricane

1. Most violent storm on Earth.

a. Lowest pressure on the surface of the Earth.

1. Winds >119 km/hr (74 mph)

2. rotary circulation of rapid, inwardly-spiraling winds: convergence

b. Hurricanes are huge heat engines that convert the warm of the tropics to wind and wave energy

c. Levels of tropical storms: defined by wind speed

1. Tropical depression: winds less than 39 mph

2. Tropical storm: winds >39 mph, < 74 mph

3. Hurricane: winds > 74 mph

2. Parts of a hurricane

a. Eye

1. At the very center

2. About 20 km (12 mi) in diameter

3. Precipitation and wind stop

4. Air gradually descends and heats

b. Eyewall

1. Near the center, surrounding the eye

2. Rising air, intense convection

3. Wall of towering clouds

4. Greatest wind speeds

5. Heaviest rainfall

3. Hurricanes are known by different names: typhoon in western Pacific; cyclone in Indian Ocean

4. Frequency of hurricanes

a. Most (20/yr) occur in the N. Pacific

b. Fewer than 5/yr usually occur in the warm North Atlantic

5. Necessities for hurricane formation

a. Big expanse of warm ocean water (hurricanes form in all tropical waters except the S. Atlantic and E.S. Pacific)

1. Air is warm and humid.

2. Energy for hurricane comes from ascending water vapor

b. Weak upper air winds blowing in the same direction as the surface winds

6. Formation and Decay

a. Develop most often in late summer when warm water temperatures provide energy and moisture

b. Hurricanes diminish in intensity whenever

1. They move over cooler ocean water

2. They move onto land

3. The flow aloft is unfavorable

c. How do they increase in intensity? Longer time over hot water

7. Destruction from a hurricane

a. Factors that affect the amount of storm damage

1. Strength of storm (most important factor)

2. Size and population density

3. Shape of the ocean bottom near the shore

b. Types of hurricane damage

1. Wind damage

2. Storm surge

a. Large dome of water, pulled by low pressure of hurricane

b. 65-80 km long

c. Comes ashore where eye makes landfall

3. Inland freshwater flooding from torrential rains

Climate and the Carbon Cycle

I. Introduction

A. Climate is an aggregate of weather. Most important elements: temp. and precipitation

B. Involves the exchanges of energy and moisture that occur among the spheres

(atmosphere, hydrosphere, solid earth and biosphere)

C. World climates: every location has a distinct climate

II. Climate Classification

A. We classify climate so that we can describe and compare climates in widely separated areas (or widely separated in time)

B. Many schemes have been developed to organize the huge amt. of climate data worldwide

C. Koppen classification:

1. Best known and most used

2. Uses mean monthly and annual values of temp. and precipitation

3. Boundaries were chosen largely on the limits of certain plant associations: BIOSPHERE

4. Five principal climate groups:

a. Humid Tropical

1. Winterless

2. Two main types

a. Wet tropics (tropical rain forest), influenced by equatorial low pressures

b. Tropical wet and dry (tropical grassland or savanna), seasonal rainfall

b. Dry

1. Evaporation exceeds precipitation. Constant water deficiency.

2. Two main types

a. Arid or desert

b. Semi-arid or steppe (somewhat more humid than deserts and surrounds them

3. Causes of deserts and steppes

a. Subtropical high pressure systems (dry, stable, subsiding air)

b. Interior deserts (located in middle latitudes near the middle of large landmasses, often near high mountains)

c. Humid middle-latitude with mild winters

1. Mild winters

2. Three main types

a. Humid subtropics (eastern sides of continents, hot summers, mild winters)

b. Marine west coast (western - windward - side of continents, mild winters and cool summers)

c. Dry summer tropics: Mediterranean climate

d. Humid middle-latitude with severe winters

1. Severe winters, land-controlled climates

2. Absent in the Southern Hemisphere (WHY?)

2. Two main types

a. Humid continental (confined to central and eastern portions of N.A. and Eurasia; severe winters; hot, humid summers; more precip. in summer than winter. Snow stays on ground for extended periods)

b. Subarctic

e. Polar

1. Enduring cold, very little precipitation

2. Two main types

a. Tundra climate (treeless; almost entirely in N. Hemisphere; severe winters, cool summers)

b. Ice cap climate (very cold, permanent ice and snow)

5. Highland (mountain) climate

a. Usually cooler and wetter than adjacent (remember rainshadow effect?)

b. Great diversity of climatic conditions

III. Human Impact on Global Climate

A. Humans have been modifying the environment over extensive areas for 1000s of years (esp. important: clearing land with fire and overgrazing of marginal lands)

B. Most hypotheses of climatic change are controversial (to some degree)

C. Global warming: jigsaw exercise

IV. The Carbon Cycle

A. Only one of several important cycles where biology, chemistry and geology intersect

B. Carbon, nitrogen and phosphorus are critical components of DNA and RNA

C. The carbon cycle

1. Storage:

a. C is stored in atmosphere as carbon dioxide

b. Stored in biosphere as organic matter

c. Stored in solid earth in rocks, esp. carbonates (limestones)

d. Stored in the ocean as dissolved carbon dioxide, precipitated or organic lime. Ocean is a huge carbon storage area