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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