Session 6

Energy budget

Accounting system that measures all the energy entering and leaving the Earth system

First law of thermodynamics

Energy cannot be created or destroyed; so incoming solar energy must be reflected or absorbed and then radiated back to space

Main radiation in sunlight

Visible light (most) plus considerable UV

Solar energy reaching the upper atmosphere

About 82000 J (19500 cal) per square meter per minute

Share of sunlight reflected back to space

About 30% (clouds 20% + atmosphere 6% + surface 4%)

Share of sunlight absorbed

About 70% (land and oceans 51% + atmosphere 16% + clouds 3%)

Annual global energy balance

Energy in equals energy out over a year but a place or time can be out of balance

Daily temperature cycle

Example of local imbalance: net heat gain by day and net heat loss to space at night

Main reason for different climates

Regional differences in the energy budget

Weather

Atmospheric conditions (temperature; humidity; rainfall) at a particular place and time

Climate

Long-term pattern of weather conditions in a region

Warm air

Expands and becomes less dense so it rises

Rising air

Cools with height; reaches the dew point; condenses into cloud and possibly rain; air pressure falls

Dew point

Temperature at which water vapor condenses into liquid

Descending air

Air pressure rises and conditions are usually dry

Low pressure

Associated with rising air and a higher chance of rain

High pressure

Associated with descending air and dry conditions

Air mass

Large area of relatively warm or cold air produced by local differences in heat gain and loss

Front

Boundary where warm and cold air masses meet

Cold front

Cold air pushes under warm air; brief intense storms then cooler clearer weather

Warm front

Warm air slides over cold air; gradual widespread cloud and steady rain then warmer weather

Stationary front

Neither air mass advances; days of cloud and rain along the boundary

Occluded front

Cold front overtakes a warm front; mixed and often prolonged precipitation

Why latitudes receive different solar energy

Earth's surface is curved; so sunlight is spread over more area toward the poles

Radiation per square meter

Highest at the equator and diminishing toward the poles

Wind cells

Large-scale convection currents in the troposphere caused by uneven heating

Intertropical convergence zone (ITCZ)

Band of rising air around the equator where warm humid air cools and brings heavy rainfall; home to the wettest rainforests

Doldrums

Region near the equator (ITCZ) with weak unpredictable winds that stalled sailing ships

Hadley cells

Cells at 0-30 degrees latitude: air rises at the equator and descends at about 25-30 degrees

Tropical zone

Warm and moist area beneath the Hadley cells

Deserts at 25-30 degrees latitude

Descending warmed air has low relative humidity (e.g. the Sahara)

Ferrel cells

Cells at 30-60 degrees latitude: surface air moves poleward and rises between 45 and 60 degrees

Temperate zone

Region of the Ferrel cells with distinct seasonal changes and temperate deciduous forests

Polar cells

Cells at 60-90 degrees latitude: cold dense air sinks at the pole and flows toward the equator along the surface

Circumpolar zone

Very cold region of the polar cells; tundra biome

Coriolis effect

Apparent deflection of moving air or water caused by Earth's rotation at different speeds at different latitudes

Rotation speed of Earth's surface

Fastest at the equator (about 40000 km per day) and zero at the poles

Deflection direction in the slides

Air moving toward the equator is deflected east to west; air moving toward the poles is deflected west to east

Easterly winds

Prevailing winds in the Hadley and polar cells (tropical and circumpolar zones)

Westerly winds

Prevailing winds in the Ferrel cells (temperate zones)

Zones of rising or sinking air

No prevailing winds; winds are weak and unpredictable

Ocean share of Earth's water

Nearly 98%

Surface currents

Currents in the upper ~400 m of the ocean driven by Earth's rotation; winds and temperature differences

Vertical (deep) currents

Circulation exchanging surface water with deep water

Equatorial currents

Warm surface water pushed westward by the prevailing easterly winds

Ocean gyre

Circular current system formed when currents turn poleward at east coasts and return equatorward along west coasts

Gyre rotation

Clockwise in the Northern Hemisphere; counterclockwise in the Southern Hemisphere

North Pacific Subtropical Gyre

Largest contiguous ecosystem on Earth; CO2 sink; contains the Great Pacific Garbage Patch

Great Pacific Garbage Patch

Concentration of floating plastic debris trapped at the center of the North Pacific gyre

West coast vs east coast currents

West-coast currents are colder; so there is less evaporation and west coasts are often drier

Thermohaline circulation

Vertical ocean circulation driven by differences in temperature (thermo) and salinity (haline)

Thermohaline speed and effect

Very slow (hundreds of years per circuit) but moves vast amounts of water and heat so it affects global climate

Cause of the seasons

Earth's axis is tilted 23.5 degrees so each hemisphere tilts toward the sun in summer and away in winter

Summer vs winter energy

Solar energy per unit area is highest in summer and lowest in winter

Solstice

Day when a hemisphere is tilted most toward or away from the sun (longest or shortest day) around 21 June and 21 December

Equinox

Day when neither hemisphere is tilted toward the sun and day and night are about equal around 20 March and 22 September

Seasonal shift of wind cells

Wind cells move north and south over the year as radiation patterns change

Specific heat capacity

Energy needed to raise the temperature of a substance; water's is much higher than land's

Land vs water heating

Land heats and cools faster; water heats and cools slowly

Coastal climate

Moderated by nearby water: milder summers and winters

Continental climate

Interiors become much hotter in summer and much colder in winter

Summer continental low pressure

Hot rising air over continental interiors pulls in moist ocean air and favors summer thunderstorms (strongest near east coasts)

Coldest winter locations

Centers of continents because land cools much faster than adjacent water