Ch. 6Weather and ClimateMrs. Sealy APES

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

Weather Changes– Air Masses: large lump of air that

similar temperature and moisture level throughout.

– Air Masses that effect the US are

When air masses meet 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

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

Weather Extremes– Hurricanes:

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

• The bad: loss of life and property

• The good: flushes out coastline

Tornadoes:• Form when cold dry air collides

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

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

Climate

– Climate is the long term average precipitation and temperature of an area

– Climate is determined by global wind patterns, latitude, altitude and ocean currents

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

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

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: global circulation

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

Climate type

Cold

Cool Temperate

Warm Temperate

Tropical

(equator)

Tropical

Warm Temperate

Cool Temperate

Cold

Fig. 6.6a, p. 125

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:

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 in what is called:

Ocean Currents

• Long term variations in the amount of incoming solar radiation

• Heat from the sun evaporates water and transfers energy from the ocean to the atmosphere. This creates convection cells that transport heat to different latitudes. This leads to: ocean currents and weather

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 climate• Differences in water temp, winds and

the rotation of the earth create currents.

• Currents redistribute heat. For example the gulf stream brings heat to Europe

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

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 weaken or stop

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

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

species– Also causes severe weather, storms in

the US especially CA, and drought in southeast Asia

Normal Conditions

Cold water

Warm water

Thermocline

SOUTHAMERICA

Warm waterspushed westward

AUSTRALIA

EQUATOR

Surface windsblow westward

Fig. 6.10a, p. 127

1982–83 1997–98

Year

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

El Nino conditions

La Nina conditions

+3

+2

+1

0

-1

-2

Tem

per

atur

e/C

han

ge (

°F)

Fig. 6.12, p. 128

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, and an increase in tornadoes.

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

Ozone Layer• The ozone layer is located in the

stratosphere. It is created when ultraviolet light turns oxygen into ozone. The chemical reactions is:– Ozone blocks all short wavelength UV-C radiation,

50% of the UV-B radiation and almost no long wavelength UV-A radiation.

– Ozone also forms a thermal cap which: traps heat

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

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

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