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Physical Geography:Weather and Climate

Chapter 4

Weather vs. Climate

Weather – short-term, day-to-day expression of atmospheric processes Ex. - Today is clear, cold and sunny

Climate – long-term, average conditions Usually at least 30 years of daily weather data

(temperatures and precipitation) Ex. - NY – Humid Continental, warm summer

Meteorology – the scientific study of the atmosphere

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

Reasons for Seasons

Circle of illumination – the travelling boundary that divides daylight and darkness ½ the world is in darkness and ½ in sunlight at

any moment

Earth rotates counterclockwise

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

Annual March of the Seasons

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Annual March of the Seasons

Earth-Sun Relations

Seasons of the Year

Oceanic & Continental Effects

Water heats and cools more slowly Marine environment

Cooler summers, warmer winters Temperatures are moderated

Land heats and cools more rapidly Continental environment

Hotter summers, colder winters Wider temperature variation

Oceanic & Continental Effects

Marine effect – exhibit moderate influences of the ocean (ex. Vancouver, BC)

Vancouver - 28°F temp. range

Oceanic & Continental Effects

Continental effects – areas less affected by the sea, therefore have greater range between max and min temperatures (Winnipeg, Manitoba - 49°N)

Winnipeg - 64°F Temp range

Temperature generally decreases as altitude increases Lapse rate

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

Temperature inversion Decrease in temperature less than expected (or an increase)

Main cause - air near the ground rapidly loses its heat on a clear night - the ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day. Contributes to smog problems

The Lapse Rate

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Moisture in the Atmosphere

Humidity – water vapor in the air A function of temperature

Relative humidity - the ratio of water vapor in the air, compared to maximum water vapor possible Tells us how close we are to saturation

Dew point – temperature at which air becomes saturated

Further cooling results in condensation (dew)

Moisture in the Atmosphere

Relative humidity – Highest at dawn – when temperature is lowest

Lowest in late afternoon – temperatures highest

Water Vapor

11 a.m.50%

Relative Humidity

Water Vapor

5 p.m. 20%

Relative Humidity

Warmer air = greater maximum water vapor possible

Cooler air = lesser maximum water vapor possible

Water Vapor

5 a.m. 100%

Relative Humidity

Maximum water-vapor

possible

Atmospheric Lifting

Orographic Lifting - air is forced over a barrier (like a mountain range)

Windward side – wet

Leeward side – dry

Rain Shadow

Types of Precipitation

Orographic rainfallit occurs in the Western Ghats and Himalayas in South Asia – resulting in Rain-shadow effect: the area of low rainfall found on the leeward (or downwind side) of a mountain range

Rain Shadow

Orographic LiftingTypes of Precipitation

Frontal Lifting (Cold and Warm Fronts) - along the leading edges of contrasting air masses.

Front – Zone of separation between two air masses

Leading edge of cold air mass is a cold front

Leading edge of warm airmass is a warm front Front

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Frontal Lifting - Cold Front

Cold Fronts Cumulonimbus clouds may produce large raindrops, heavy

showers, lightning and thunder, and hail.

Frontal Lifting – Warm Front

Gentle lifting of the warm, moist air produces nimbostratus and stratus clouds and drizzly rain showers,

Weather Phenomena

Lake Effect Snow

Sleet

Freezing Rain

Thunderstorms

Hail

Tornados

Lake-Effect Snow

cP air masses move south and east Cold air passes over warmer Great Lakes

Air masses are warmed and water vapor added

Lake-Effect Snow Dangerous Weather

Sleet – frozen raindrops or partially refrozen snowflakes that bounce when they reach the ground

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Dangerous Weather

Freezing Rain – precipitation that starts as snow at high altitudes, melts and freezes after it hits the ground

Violent Weather

Thunderstorms - condensation of large amounts of water vapor creates lots of energy, heating the air - Causes updrafts Raindrops create friction - causing

downdrafts

Giant cumulonimbus cloudscause dramatic weather Heavy rain, lightning,

thunder, hail, heavy winds

Hailstones

Hail is a form of precipitation which consists of balls or irregular lumps of ice Form in strong thunderstorm clouds, particularly

those with intense updrafts

Falls - updrafts

Once it’s too heavy for updraft, it falls to ground Stronger the updrafts – larger the hail stones

Hailstones

Figure 8.21

Violent Weather

Birth of a Tornado Air at higher altitudes moves faster than surface

air Creating rotation in the air parallel to the ground

Violent Weather

Birth of a Tornado Updrafts create a shift in

the axis of rotation Creating rotation in the air

perpendicular to the ground

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Violent Weather

Birth of a Tornado Mesocyclone forms as a rotating updraft within

the thunderstorm. If one forms, a tornado will descend from the lower

portion of the mesocyclone.

Mesocyclone –a large, rotatingatmosphericcirculation

Violent Weather

Fujita Scale – scale for rating tornado intensity based on damage to human-made structures, vegetation

Category Wind Speed Potential Damage

F0 65-85 mph

39%

Light Damage: Minor roof damage, gutters, sidingdamagedBranches broken off trees

F1 86-110 mph36%

Moderate Damage: severe roof damage, mobile homes overturned, broken glass

F2 111-135 mph19%

Considerable Damage: roofs torn off, foundations shifted, large trees snapped, cars lifted off ground

F3 136 -165 mph

5%

Severe Damage: severe damages to large buildings (malls), trains overturned, structures withweak foundations blown great distances

F4 166-200 mph1%

Devastating Damage: homes completely leveled, cars thrown, small missiles generated

F5 >200 mph<0.1%

Incredible Damage: houses swept away, cars carried 100 meters, structural damage to high-rises

Top Ten Deadliest Tornadoes

Rank States Date F-Scale Dead Injured Towns

1 MO-IL-IN March 18, 1925 F5 695 2027 Murphysboro, DeSoto

2 LA-MS May 7, 1840 317 109 Nachez

3 MO-IL May 27, 1896 255 1000 St. Louis, E. St. Louis

4 MS April 5, 1936 F5 216 700 Tupelo

5 GA April 6, 1936 203 1600 Gainesville

6 TX-OK-KS April 9, 1947 F5 181 970 Glazier, Higgins

7 LA-MS April 24, 1908 143 770 Amite,. Pine, Purvis

8 WI June 12, 1899 F5 117 200 New Richmond

9 MI June 8, 1953 F5 115 844 Flint

10 TX May 11, 1953 F5 114 597 Waco

Tornado Damage

Figure 8.23

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Climate Classification

Köppen system the A climates are tropical climates

Tropical – equatorial and tropical latitudes the B climates are dry

Desert Arid – tropical/midlatitudes Semi-arid – tropical/midlatitudes

the C climates are generally moderate and are found in the middle latitudes Temperate– mid-latitudes, mild winters

the D climates are associated with continental and high-latitude locations. Continental – midlatitudes/high latitudes, cold winters

The E climates are polar High latitudes/polar regions

Highland climates – lower temps than similar latitudes (lapse rate)

Generalized Climate Regions

Tropical Climates (A)

Tropical Rain forest – high temps, high precipitation all year Dense vegetation

Tropical Monsoon – high temperatures,season variation in precipitation

Dryland Climates (B)

Hot deserts – high temperatures, Limited rainfall Drought resistant plants/sand

Steppe – transition zones adjacent to deserts Higher rainfall, more seasonal

temperature variation grasslands

Humid Midlatitude(C,D)

Mediterranean – Mild winters, warm summers Most rain in low sun (winter)

Humid Subtropical – warm temps in summer East side of continents

Marine West Coast – mild winters, Cool summer West coast of continents

Humid Continental – warm temps in summer East side of continents

Tropical Climates (A)

Tundra – temperature - warmest month between 32o & 50o, - winter very coldlow plants & many wild flowers in summer

•Subarctic – 7-8 months continuous snow cover

•Coniferous forest – called taiga

• Ice Cap – temperature – extreme cold, summers average below freezing

•No vegetation•<5” precipitation/year

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Highland Climates (H)

Highland (H)

Climates change rapidly on mountains

related to the climate of the surrounding climate. The highlands have the same seasons and wet and dry periods as the climate zone

Mountain climates are very important to mid-latitude climates.

Polar Climate

Highland (H) Climates change rapidly on mountains, becoming colder the

higher the altitude gets.

Closely related to the climate of the surrounding climate. The highlands have the same seasons and wet and dry periods as

the climate zone

Mountain climates are very important to mid-latitude climates. Snow is kept back until spring and summer when it is released slowly as water

through melting

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

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