The Earth’s Radiation Balance

Incoming Energy = Outgoing Energy

(absorbed sunshine)(area) = (thermal loss)(area)

S(1-a)pr2 = s T4 (4 pr2)

Solve for TT = -18° C; (0°F)

The radiative equilibrium temperature

The equilibrium surface temperature should be -18°C (~0°F) .

But, the observed surface temperature is

+15°C (~60°F).

Why?

We have an atmosphere

Solar Radiation

30% reflected

~50% absorbed by the surface

20% absorbed by the

atmosphere

Greenhouse Effect

Heat radiated by the Earth’s surface is absorbed by gases in the atmosphere and

reradiated back to the Earth

Carbon Dioxide (CO2) ConcentrationsCombustion of fossil fuels (coal and oil) by humans has increased concentrations of some greenhouse gases

Scattering: Why is the sky blue?

• Sunlight is scattered by air molecules

• Shorter wavelengths (green, blue, violet) scattered more efficiently

• Unless we are looking directly at the sun, we are viewing light scattered by the atmosphere

• Finally, blue dominates over violet because our eyes are more sensitive to blue light

Why are Sunsets Red?• The sun appears fairly white

when it’s high in the sky

• Near the horizon, sunlight must penetrate more atmosphere and therefore, there is more scattering of blue and other shorter wavelengths

• The orange sunset occurs because longer wavelengths, such as orange and yellow, make it through

• If there are a lot of particulates, yellow will be scattered as well

Weather and Climate

What is the difference?

Time Scalesweather vs climate

Weather

The conditions at a specific location at a specific time

• Hourly

– e.g., this afternoon

• Diurnal

– The day-night cycle -24 hours

• Weekly

– Standard long range TV weather forecast

Climate

The average conditions and their variability (includes extremes)

– Seasonal

– Annual

– Decadal

– Century

– Geologic Timescales

Spatial Terms of Reference

• Global - The planet as a whole

• Planetary - as in planetary waves

• Hemispheric - e.g. northern hemisphere

• Zonal- implies East-West

– a latitude band

• e.g. subtropics 20-30o lat

• Meridional - implies North-South

• Regional– High Plains

– The Mountains

• Local– Billings

• Synoptic scale

– 500 to 3000 Kilometers

• midlatitude cyclones

• Mesoscale

– 20 to 200 Kilometers

• Thunderstorms

• Microscale

– centimeters to 1 Kilometer

• In-Cloud updraft

Spatial Terms of Reference

Seasons & Sun's Distance

Earth is 5 million kilometers further from the sun in July than in January, indicating that seasonal warmth is controlled by more than solar proximity.

Figure 3.1

Solstice & Equinox

• Earth's tilt of 23.5° and revolution around the sun creates seasonal solar exposure and heating patterns

• At solstice, tilt keeps a polar region with either 24 hours of light or darkness

• At equinox, tilt provides exactly 12 hours of night and 12 hours of day everywhere

Solar intensity, defined as the energy per area, governs Earth's seasonal climate changes

A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly.

Seasons & Solar Intensity

Midnight Sun

The region north of the Arctic Circle experiences a period of 24 hour sunlight in summer, where the Earth's surface does not rotate out of solar exposure

Take home message

• Seasons are regulated by the amount of solar energy received at Earth’s surface, which depends upon:

– angle at which sunlight strikes Earth’s surface.

– how many hours the sun shines per day.

• Seasons are NOT due to the elliptical nature of the earth’s orbit, i.e. changes in distance from the Sun.

Questions

• Since the North Pole has 24 hours of sunlight during summer, why isn’t it the hottest place on the planet?

• Why aren’t daily temperatures highest on June 15, summer solstice, when the Sun is at its highest and the days are longest?

Temperature Lags

Earth's surface temperature is a balance between incoming solar radiation and outgoing terrestrial radiation.

Peak temperature lags after peak insolation because surface continues to warm until infrared radiation exceeds insolation.

Radiation Budget at the top of the Earth’s Atmosphere

Red Line is incoming radiation

from the sun

Blue Line is outgoing radiation

emitted by the earth

The Job of the Atmosphereis to let the energy out!

“Piles up” in tropics “Escapes” near poles and aloft

The movement of the air (and oceans) allows energy to be transported to its “escape zones!”

What a single cell convection model would look like for a non-rotating earth

• Thermal convection leads to formation of convection cell in each hemisphere

• Energy transported from equator toward poles

• What would prevailing wind direction be over N. America with this flow pattern on a rotating earth?

What’s wrong with the 1-cell model?

Answer: The Earth Spins and ultimately it is not stable.

What is stable?

Climate “Zones”• Circulation features are

tied to regional climate

• Rising air associated with lots of precipitation

Climates of the World• Deep Tropics: hot and wet, with little seasonal variation• Seasonal tropics: hot, with “summer” rain and “winter” dry

(monsoon)• Subtropics: dry and sunny, deserts and savannas, often with a

well-defined rainy season (summer or winter)• Midlatitude temperate zone: warm summers, cold winters,

moisture varies by location but often comes in episodes throughout the year

• Polar regions: very cold, generally very dry, dark in the winter

Other Influences:Ocean currents, “continentality,” vegetation, mountain ranges

(altitude and orographic precipitation)

Orographic = lift due to the presence of mountains

Coriolis Force acts to the right in the Northern Hemisphere

Physics

Coriolis Effect

The Coriolis Effect deflects moving objects to the right in the northern hemisphere and to the left in the southern.

The atmosphere’s water

The Earth’s Hydrologic Cycle

Properties of Water

• Physical States– only natural substance that occurs naturally in three states on the earth’s

surface

• Heat Capacity– Highest of all common solids and liquids

• Surface Tension– Highest of all common liquids

• Latent Heat of Fusion– Highest of all common substances

• Compressibility– Virtually incompressible as a liquid

• Density– Density of seawater is controlled by temperature, salinity and pressure

– Liquid has maximum density at +4oC; solid phase has lower density!

Molecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.

water moleculeice

Molecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.

ice++

--

Energy associated with phase change

80 calories 100 calories 540 calories!

Water vapor pressure

• Molecules in an air parcel (e.g., N2, O2, H2O) all contribute to pressure

• The VAPOR PRESSURE, e, is the pressure exerted by water vapor molecules in the air

Water vapor saturation• Water molecules move

between liquid and gas phases

• When the rate of water molecules entering the liquid equals the rate leaving the liquid, we have equilibrium– The air is said to be

saturated with water vapor at this point

– Equilibrium does not mean no exchange occurs

Expressing the water vapor pressure

• Relative Humidity (RH) is ratio of actual vapor pressure to saturation vapor pressure

– 100 * e/eS (e = vapor pressure, es = saturation vapor pressure)

– Range: 0-100% (+)

– Air with RH > 100% is supersaturated

• RH can be changed by

– Changes in water vapor content, e

– Changes in temperature, which alter eS

e = vapor pressure; es = saturation vapor pressure

Warm air can hold more water vapor

Dewpoint Temperatures

Dew point temperaturethe temperature at which dew forms, i.e. condensation

• Dewpoint temperature is a measure of the water vapor content of the air

• It is not a measure of temperature!

Condensation

• Condensation is the phase transformation of water vapor to liquid water

• Water does not easily condense without a surface present

–Vegetation, soil, buildings provide surface for dew and frost formation

–Particles act as sites for cloud and fog drop formation

Where do Cloud Condensation Nuclei (CCN) come from?

• Good CCN are hygroscopic (“like” water,)

• Salt and acid particles in the atmosphere

• Natural CCN– Sea salt (Na+, Cl-, SO4

=, K+, Mg2+)

– biogenic sulfur emissions

– vegetation burning

• CCN from human activity– Pollutants from fossil fuel combustion (CO2, SO2)

Smog over China

Fogs

• Fogs are clouds in contact with the ground

• Several types of fogs commonly form– Radiation fog

– Advection fog

– Upslope fog

– Evaporation (mixing) fog

Clouds• Clouds result when

air becomes saturated away from the ground

• They can– be thick or thin,

large or small

– contain water drops and/or ice crystals

– form high or low

• Clouds impact the environment in many ways– Temperature,

precipitation (snow/rain), pollutant processing, climate change, kill?, etc….

Cloud Classification

• Clouds are categorized by their height, appearance and vertical development

– High Clouds - generally above 16,000 ft at middle latitudes• Main types - Cirrus, Cirrostratus, Cirrocumulus • (wispy; high altitude ice)

– Middle Clouds – 7,000-23,000 feet• Main types – Altostratus, Altocumulus

– Low Clouds - below 7,000 ft• Main types – Stratus, stratocumulus, nimbostratus • (layered; stable)

– Vertically “developed” clouds (via convection)• Main types – Cumulus, Cumulonimbus• (cottony; beautiful day, but it might rain)

Cloud type summary

Cirrus

Cirrus

Cirrus Display at Dawn

Stratus from below

Stratus from above

Stratus over the ocean

Puffy cumulus

Vertically “developed”

clouds• Cumulus

– Puffy “cotton”

– Flat base, rounded top

– More space between cloud elements than stratocumulus

• Cumulonimbus– Thunderstorm cloud

– Very tall, often reaching tropopause

– Individual or grouped

– Large energy release from water vapor condensation

Cumulonimbus

Cumulonimbus with Pileaus caps

Cumulonimbus Clouds Spawn Tornadoes

Cirrocumulus

Cirrocumulus at Sunset

Altostratus

Alto Stratus Castellanus

Lenticular clouds downwind of mountains

Stability and Cloud Development

Why is stability important?

• Vertical motions in the atmosphere are largely controlled by vertical stability

• There are two types of vertical motion:– forced motion such as forcing air up over a hill,

over colder air, or from horizontal convergence

– buoyant motion in which the air rises because it is less dense than its surroundings - stability is especially important here

Stability & Instability

A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed.

If the rock instead accelerates in the direction of the push, it was in unstable equilibrium.

Stability in the atmosphere

Stable Unstable Neutral

If an air parcel is displaced from its original height it can:Return to its original height - StableAccelerate upward because it is buoyant - UnstableStay at the place to which it was displaced - Neutral

An InitialPerturbation

“Lapse rate”

• The lapse rate is the change of temperature with height in the atmosphere

• There are two kinds of lapse rates:– Environmental Lapse Rate (~ 4°/1000 m)

• What you would measure with a weather balloon

– Parcel Lapse Rate• The change of temperature that an air parcel would

experience when it is displaced vertically

• This is assumed to be an adiabatic process (i.e., no heat exchange occurs across parcel boundary)

Stability and the dry adiabatic lapse rate

• Atmospheric stability depends on the environmental lapse rate– A rising unsaturated air parcel

cools according to the dry adiabatic lapse rate

– If this air parcel is• warmer than surrounding air

it is less dense and buoyancy accelerates the parcel upward

• colder than surrounding air it is more dense and buoyancy forces oppose the rising motion

A saturated rising air parcel cools less than an unsaturated parcel

• If a rising air parcel becomes saturated condensation occurs

• Condensation warms the air parcel due to the release of latent heat

• So, a rising parcel cools less if it is saturated

• Define a moist adiabatic lapse rate– ~ 6 C/1000 m

– Not constant (varies from ~ 3-9 C)

– depends on T and P

Stability and the moist adiabatic lapse rate

• Atmospheric stability depends on the environmental lapse rate– A rising saturated air parcel cools

according to the moist adiabatic lapse rate

– When the environmental lapse rate is smaller than the moist adiabatic lapse rate, the atmosphere is termed absolutely stable• Recall that the dry adiabatic lapse

rate is larger than the moist

– What types of clouds do you expect to form if saturated air is forced to rise in an absolutely stable atmosphere?

dry

Absolute instability

• The atmosphere is absolutely unstable if the environmental lapse rate exceeds the moist and dry adiabatic lapse rates

• This situation is not long-lived– Usually results from surface heating and is confined to a

shallow layer near the surface

– Vertical mixing can eliminate it

• Mixing results in a dry adiabatic lapse rate in the mixed layer, unless condensation (cloud formation) occurs (in which case it is moist adiabatic)

Absolute instability (examples)

Conditionally unstable air

• What if the environmental lapse rate falls between the moist and dry adiabatic lapse rates?– The atmosphere is

unstable for saturated air parcels but stable for unsaturated air parcels

– This situation is termed conditionally unstable

• This is the typical situation in the atmosphere