Ch. 7 and Supplement Ch. 7 and Supplement 77

Weather and Climate Weather and Climate

Adapted from Ms. Sealy’s ppt

Weather– The short-term day-to-

day changes in temperature, air pressure, humidity, precipitation, sunshine, cloud cover and wind direction and speed.

– Most weather is predicted using:

weather balloons, aircraft, radar, and satellites

Climate– is the long term average precipitation

and temperature of an area– is determined by air pressure,

albedo, angle of sunlight, clouds, distance to oceans, fronts, heat (convection), land changes, latitude, location, humidity, mountain ranges, pollution, rotation, wind patterns and human

– Climate is what you expect, weather is what you get!

Climate

the average weather patterns for an area overa long period of time (30 - 1,000,000 years).

is

It is determined by

Average Precipitation Average Temperatureand

which are influenced by

latitude altitude ocean currents

and affects

where people live how people livewhat they

grow and eat

Fig. 6.3, p. 123

• Shows the precipitation and temperature on one graph. Easy to compare biomes

Climatograms

• KEY PROPERTIES OF AIRKEY PROPERTIES OF AIR– Cold air is more dense than warm

air– Cold air sinks and warms– Warm air rises and cools– Warm air can hold more water vapor

than cold air– As air cools, it cannot hold as much

water vapor, so it may precipitate

ATMOSPHERIC CIRCULATION PATTERNS

Weather Changes• Air Mass: a large body of air that has

similar temperature and moisture level throughout.

• Air Masses that affect the US are cA, cP, mP, mT and cT

When air masses meet, the boundary between them is called a front and it causes changes in weather

• Cold front: when a cold air mass collides with a stationary warm air mass. The result is: thunderstorms, short bursts of heavy rain

Warm Front:

• when a warm air mass collides with a stationary cold air mass. The result is: warm steady rain

Air PressureAtmospheric pressure is highest near

Earth’s surface due to weight of air above.

Plotting areas of same pressure (isobars, usually measured in mm Hg) shows high and low pressure areas.

High and Low Pressure

High pressure = cold, dense air, descends toward surface; usually means fair weather.

Low pressure = warm, less dense air; rises, expands then cools; often cloudy, stormy. If there’s enough water and condensation nuclei present, rain results.

Weather is also affected by changes in atmospheric pressure

• High pressure has descending air that moves outward from the center of the high-pressure system. Descending air is warm and dry. The result is: nice dry weather

Low pressure• has ascending air that flows

towards the center of the low-pressure area. Ascending air-cools and condenses as it rises. The result is: clouds, rain

Energy Transfer by Convection in the Atmosphere

Convection Cell

Global Air currents affect regional climates

• Uneven heating of the Earth’s surface causes the equator to receive more sunlight making it hotter

• The poles receive less light making them cooler.

• This causes areas of high and low pressure and global circulation of winds as air moves from high to low pressure

Climate type

Cold

Cool Temperate

Warm Temperate

Tropical

(equator)

Tropical

Warm Temperate

Cool Temperate

Cold

Fig. 6.6a, p. 125

Fig. 6.6b, p. 125Initial pattern of air circulation

Deflections in the paths of air flownear the earth’s surface 

30°S

Easterlies

Westerlies

Southeasttradewinds

(Doldrums)

Northeasttradewinds

Westerlies(from the west)

Easterlies(from the east)

60°S

equator

30°N

60°N

Seasons

• Seasonal changes in temp and precipitation affect climate because the earth is tilted on its axis. It is colder in the winter and warmer in the summer because:

• The earth is tilted 23.5° on its axis

Fall(sun aims directly at equator)

Summer(northern hemisphere

tilts toward sun)

Spring(sun aims directly

at equator)

23.5 °Winter(northern hemispheretilts away from sun)

Solarradiation

Fig. 6.5, p. 124

Coriolis Effect• Rotation of

the Earth on its axis prevents air currents from moving directly north or south causing the winds to curve

• http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm

http://www.science-house.org/nesdis/upwelling/background.html

Albedo

• Is the reflectivity of a surface to solar radiation.

• Snow= high albedo (0.8)

• Water= low (0.07)

• Forest = .05-0.1

• Average albedo of the earth is 0.37

Altitude

• For every 1000 feet, there is a 3°F drop in temperature.

• Every 300 feet in elevation is equivalent to a shift of 62 miles north in latitude and biome similarity.

CLOUDS

Ocean currents

• Surface currentsSurface currents– Driven by wind (10%)

• Deep water currentsDeep water currents– Driven by density and gravity

Influenced by

• Solar heating-

• Winds

• Gravity

• Coriolis

• http://www.montereyinstitute.org/noaa/lesson08.html

• http://www.montereyinstitute.org/noaa/lesson08/l8la1.htm view at home

Polar (ice)

Subarctic (snow)

Cool temperate

Warm temperate

Dry

Tropical

Highland

Major upwelling zones

Warm ocean current

Cold ocean current

River Fig. 6.4, p. 124

– Ocean Currents Affect climateOcean Currents Affect climate• Differences in water temp, winds and

the rotation of the earth create currents.

• Currents redistribute heat. For Currents redistribute heat. For exampleexample the Gulf Stream brings the Gulf Stream brings heat to Europe and moderates the heat to Europe and moderates the climate climate

• Water holds a lot of heat – has Water holds a lot of heat – has a high heat capacity. a high heat capacity.

• This means water takes a long This means water takes a long time to heat up and a long time time to heat up and a long time to cool offto cool off

• Oceans moderate climatesOceans moderate climates– Coastal areas have warmer winters

and cooler summers

OCEAN CIRCULATION PATTERNS

Cool airdescends

Land warmer thansea; breeze flowsonshore

Warm air ascends

Fig. 6.15a, p. 130

Water also changes climate by causing land breezes and sea breezes

Warm air ascends Land cooler than

sea; breeze flowsoffshore

Cool airdescends

Fig. 6.15b, p. 130

Upwelling

• Upwelling is created when the Upwelling is created when the trade winds blow offshore trade winds blow offshore pushing surface water away pushing surface water away from land. The outgoing from land. The outgoing surface water is replaced by surface water is replaced by nutrient bottom waternutrient bottom water

Wind

Movement ofsurface water

Diving birds

Nutrients

Upwelling

Fish

Zooplankton

Phytoplankton

Fig. 6.9, p. 126

The El Nino Southern Oscillation occurs every few years in the Pacific Ocean

– In an ENSO, prevailing westerly winds In an ENSO, prevailing westerly winds weaken or stopweaken or stop

– Surface waters along the coast of Surface waters along the coast of North America and South America North America and South America (west) become warmer(west) become warmer

– Normal upwelling stopsNormal upwelling stops– This reduces the population of some This reduces the population of some

fish speciesfish species– Also causes severe weather, storms in Also causes severe weather, storms in

the US especially CA, and drought in the US especially CA, and drought in southeast Asiasoutheast Asia

Normal Conditions

Cold water

Warm water

Thermocline

SOUTHAMERICA

Warm waterspushed westward

AUSTRALIA

EQUATOR

Surface windsblow westward

Fig. 6.10a, p. 127

El Niño Conditions

Cold water

Thermocline

Warm waterWarm water deepens offSouth America

SOUTHAMERICA

Warm waterflow stoppedor reversed

AUSTRALIA

EQUATOR

Drought inAustralia andSoutheast Asia

Winds weaken,causing updraftsand storms

Fig. 6.10b, p. 127

El Niño

Unusually warm periods

Unusually high rainfall

Drought

Fig. 6.11, p. 127

La Nina• La Ninas follow an El Nino and

are characterized by cooling trends. La Nina brings more Atlantic hurricanes, colder winters in the north and warmer winters in the south.

• Can lead to wetter winters in the Pacific NW, torrential rains in SE Asia, lower wheat yields in Argentina and more wildfires in Fla.

The chemical makeup of the atmosphere affects the weather.

Small amounts of water vapor, carbon dioxide, ozone, methane, nitrous oxide and chlorofluorocarbons trap heat in the atmosphere warming the planet. These gases are called: greenhouse gases

• The greenhouse effect is when greenhouse gases allow light, infrared radiation and UV radiation through to the surface of the earth where it is reflected back into space. The greenhouse gases trap some reflected infrared radiation

Rays of sunlight penetrate the lower atmosphere and warm the earth's surface.

The earth's surface absorbs much of the incoming solar radiation and degrades it to longer-wavelength infrared radiation (heat), which rises into the lower atmosphere. Some of this heat escapes into space and some is absorbed by molecules of greenhouse gases and emitted as infrared radiation, which warms the lower atmosphere.

As concentrations of greenhouse gases rise, their molecules absorb and emit more infrared radiation, which adds more heat to the lower atmosphere.

(a) (b) (c)

Fig. 6.13, p. 128

Topography of the earth also creates microclimates

A microclimate is small area that has a different climate than the general climate of an area.– Vegetation in an area influences climate:

forests stay warmer in the winter and cooler in the summer because of the trees

– Cities create heat islands that trap heat and decrease wind

speeds

• Mountains Affect Precipitation• Air rises, cool and rains• Air descends, warms and draws up moisture• This creates a

rain shadow effect where one side

of the mountain

receives most of

the rain and the

other side is very dry

LOCAL GEOGRAPHY

a Winds carrymoisture inland

from Pacific Ocean

b Clouds, rain onwindward side ofmountain range

c Rain shadow onleeward side ofmountain range

Moist habitats 15/25

1,000/85 1,800/125

3,000/854,000/75

1,000/252,000/25

Fig. 6.14, p. 129

The rain shadow effect changes climate

Mojave Desert is in the rain shadow of the Sierra Nevada Mts.

Weather Extremes– Hurricanes: most severe of all

• What is it? Tropical storm with winds greater than 75 mph

• The bad: loss of life and property

• The good: flushes out coastline

• Hurricane Katrina- August 2005

• The most destructive hurricane (economically ) ever. Landed in Louisiana as a category 3

• $75 billion and 1,830 deaths

• 1900 Galveston Hurricane 6,000-12,000 died

Tornadoes:• Form when cold dry air collides

with warm moist air, which causes the warm air to rise quickly making a funnel cloud

• Winds up to 300 mph

• Classified as F1-F6 (Fujita)

• Most common in US

• Usually occur between April-July and often in the center of the country (Tornado Alley)

Risk of Tornadoes

Highest

High

Medium

Low

Hurricane Frequency

High

Moderately high

Gulf of Alaska

Prince Williams Sound

CANADA

UNITED STATES

Grand Banks

Atlantic OceanMEXICO

Fig. 6.2, p. 122

• Deforestation

• Urbanization

• Release of pollutants

• Burning of fossil fuels

HOW DO HUMANS AFFECT CLIMATE?