Overview

Air Temperature Air Pressure and Winds Ocean Currents Moisture in the Atmosphere Climate Regions Climatic Change

Weather vs. Climate

Weather State of the atmosphere at a given time and place

Climate Long-term average weather conditions in a place

Troposphere is of particular concern Atmospheric layer closest to the earth Contains virtually all of the air, clouds, and

precipitation of the earth

Air Temperature

Insolation Solar radiation received at the earth’s surface

Determined by angle of the sun’s rays and number of daylight hours

Modifying variables Amount of water vapor in the air Cloud cover Nature of the surface of the earth Elevation Degree and direction of air movement

Earth Inclination

Axis of the earth tilts at ≈ 23.5° Summer Solstice (about June 21)

Northern hemisphere tilted toward the sun Vertical rays of the sun at 23.5° N

Winter Solstice (about December 21) Northern hemisphere tilted away from the sun Vertical rays of the sun at 23.5° S

Spring and fall equinoxes (about March 21 and September 21) Vertical rays of the sun at equator

Earth Inclination

Variation in length of days and nights At the equator

12 hours of light each day of the year Inside the Arctic Circle and Antarctic Circle

24 hours of daylight/darkness on solstice

Angle of the sun’s rays More direct angle = more energy available

Reflection and Reradiation

Clouds and light colored surfaces reflect solar energy

Reradiation Shortwave solar energy absorbed, returned into the

atmosphere as longwave terrestrial radiation Water heats and cools more slowly

Marine environment Cooler summers, warmer winters

Land heats and cools more rapidly Continental environment

Hotter summers, colder winters

The Lapse Rate

Temperature generally decreases as altitude increases Lapse rate

Average of 3.5° F per 1000 feet (6.4° C per 1000 m)

Temperature inversion Cooler air trapped below warmer air

Contributes to smog problems

Air Pressure and Winds

Weight of the atmosphere Air pressure is higher closer to the earth’s

surface Temperature and air pressure

Cold air is denser: high pressure Warm air is lighter: low pressure

Air pressure is measured by a barometer

Air Pressure and Winds

Zones of high and low air pressure Pressure gradient force

Causes air to flow from high to low pressure areas Wind

Velocity is in direct proportion to pressure differences

Convection Circulatory movement of rising warm air and

descending cool air

Air Pressure and Winds

Land and sea breezes Day: from sea to land Night: from land to sea

Mountain and valley breezes Day: from valley to mountains Night: from mountains to valley

Air Pressure and Winds

Coriolis effect Apparent deflection relative to the earth’s surface

Northern Hemisphere: wind veers toward the right Southern Hemisphere: wind veers toward the left

Spiral wind patterns Frictional effect

Decreases wind speed Changes wind direction

Global Air-Circulation Pattern

Equatorial low pressure Subtropical high pressure

About 30° N and 30° S of the equator Trade winds

In the tropics Westerlies

In the midlatitudes Subpolar low Polar easterlies Polar high

Global Air-Circulation Pattern

Jet streams Belts of strong winds in the upper atmosphere

From west to east Guide the movement of weather systems

Monsoon Wind system that reverses direction seasonally

Produces wet and dry seasons Significant effect on parts of southern and eastern Asia

Farm economy is dependent upon summer monsoon rains Flooding

Ocean Currents

Movement due to winds and differences in water density Direction also influenced by landmasses and shape

of ocean basins North Atlantic drift Ocean currents affect temperature and

precipitation on adjacent land areas Cold currents

Dry conditions Warm currents

Moist conditions

Moisture in the Atmosphere

Ascending air expands and cools Less able to hold water vapor

Supersaturated air Water vapor condenses around condensation nuclei

Clouds Rain droplets or ice crystals supported by upward

movements of air Droplets may coalesce and fall as precipitation

Form and altitude depends on: Water vapor content, temperature, wind movement

Moisture in the Atmosphere

Relative humidity Percentage measure of the moisture content of the air

Amount present relative to the maximum that can exist at the current temperature

Dew point Temperature at which condensation forms

Types of Precipitation

Convectional precipitation Heated, moisture-laden air rises and then cools

below the dew point Summer in tropical and continental climates

Orographic precipitation Warm, moisture-laden air is forced to rise over hills

or mountains and is thereby cooled Windward side

Receives a great deal of precipitation Leeward side

Very often dry (rain shadow)

Types of Precipitation

Cyclonic (frontal) precipitation Cool and warm air masses meet Common to the midlatitudes In the tropics—originator of hurricanes and typhoons Air mass

Body of air with similar temperature, pressure, and humidity characteristics throughout

Form over a source region Front

Zone of separation between two air masses

Storms

Cyclone Masses of air circulate rapidly about a region of low

atmospheric pressure Can develop into a storm

Hurricane Severe tropical cyclone with winds exceeding 75 mph In the Atlantic, Caribbean, or Gulf of Mexico

Typhoon Hurricane in the western Pacific

Storms

Blizzard Heavy snow and high winds

Tornado Funnel-shaped cloud of whirling winds that can form

beneath a cumulonimbus cloud and moves at speeds as high as 300 mph

Esp. Central U.S.

Climate Regions

Generalizations based on daily and seasonal weather conditions

Köppen system Based on temperature, precipitation, and natural

vegetation criteria A: tropical B: dry C: mild midlatitude D: midlatitude with cold winters E: polar H: highland

Tropical Climates

Tropical rainforest Equatorial low pressure High temperatures and heavy convectional rainfall

all year Dense forests Lack of soil nutrients

Tropical Climates

Tropical savanna To the north and south of rain forests High temperatures Heavy convectional rainfall in summer, dry winters Forests to grasslands

Tropical monsoon Significant increase in rainfall when summer

monsoon winds bring water-laden air Dense forests

Dryland Climates

Hot deserts Subtropical high pressure Considerable sunshine, high temperatures Very little precipitation Shrubs in gravelly or sandy environments

Midlatitude deserts and semideserts Warm/hot summers and cold winters Some convectional or frontal rainfall in summer,

some snowfall in winter Grasslands, desert shrubs Steppes have fertile soils

Humid Midlatitude Climates

Mediterranean Transition zone between subtropical high and

westerlies Warm/hot summers and mild/cool winters Dry summer, frontal precipitation in winter Shrubs and small deciduous trees

Humid subtropical Hot, moist summers and moderate, moist winters Convectional rainfall in summer, frontal precipitation

in winter Deciduous and coniferous forests

Humid Midlatitude Climates

Marine west coast Prevailing winds from the sea Moderate temperatures in both summer and winter Frontal and orographic precipitation Deciduous and coniferous forests

Humid continental Prevailing winds from the land Hot/mild summers and cool/cold winters Frontal and occasionally convectional rainfall in

summer, snow in winter Deciduous and coniferous forests

Arctic and Subarctic Climates

Subarctic Cool/cold, short summers and very cold winters Coniferous forest to mosses and lichens Tundra

Treeless area between the Arctic tree line and the permanently ice-covered zone

Arctic Ice cap near the poles Extremely cold with light precipitation

Highland Climates

Lower temperatures than lowlands at the same latitude

Variety of conditions based on: Elevation Prevailing winds Orientation of slope relative to the sun Valley, slope, or peak Ruggedness

Climatic Change

Long-term climatic change Significant variations over geologic time

Ice ages Medieval warm period and “little ice age”

May be due to variations in: shape of Earth’s orbit, tilt of the axis, gyration of the rotation axis

Short-term climatic change Natural processes

Volcanic eruptions, oceanic circulation, sunspot activity Human processes

Enhanced greenhouse effect

Climatic Change

Greenhouse effect Certain gases in the atmosphere function as an

insulating barrier, trapping infrared radiation Global warming

Caused by human activities that have increased the amount of greenhouse gases in the atmosphere Carbon dioxide: burning fossil fuels, deforestation Methane: natural gas and coal mining, agriculture and

livestock, swamps, landfills Nitrous oxides: motor vehicles, industry, fertilizers Chlorofluorocarbons: industrial chemicals

Climatic Change

Evidence of global warming 20th century was the warmest in 600 years

Average surface temp rose over 1° F during the century Winter temps in the Arctic have risen about 7° F since

the 1950s Loss of Arctic ice cap

Glaciers are thinning and retreating Consequences of global warming include:

Rising sea levels Changes in temperature and precipitation patterns

Impact on soils, vegetation, agriculture