Next quiz: this Friday, 10-20 questions from Plus more on Wednesday. Chapter 19: 1, 6, 7, 8, 10, 11, 15, 16, 29, 20, 21, 27, 42, 46 Chapter 20: 1, 2, 7, 13, 14, 16, 19, 20, 34, 37, 40, 41, 42, 43, 47, 48 Chapter 21: 1, 2, 3, 7, 10, 12, 13, 14, 16, 19, 20, 21, 27, 29, 30, 32, 34, 38, 39, 44, 45, 46 Chapter 22:1, 3, 4, 6, 9, 10, 11, 12, 13, 16, 18, 24, 25, 27, 28, 31, 32, 34, 36, 39, 48
The atmosphere is layered according to its temperature structure In some layers the temperature increases with height In others it decreases with height or is constant Why? pause is a level sphere is a layer
Heat transfer processes Conduction - Where molecules transfer energy by coming into contact with one another. Convection - Where a fluid moves from one place to another, carrying it s heat energy with it. In atmospheric science, convection is usually associated with vertical movement of the fluid (air or water). Advection is the horizontal component of the classical meaning of convection. Radiation - The transfer of heat by radiation does not require contact between the bodies exchanging heat, nor does it require a fluid between them.
Conduction Direct Heat Transfer Conduction of heat energy occurs as warmer molecules transmit vibration, and hence heat, to adjacent cooler molecules. Warm ground surfaces heat overlying air by conduction.
Heat driven convection 1. Bottom water is warmed 2. It expands an is therefore less dense 3. It rises to the surface and then spreads out 4. Cooler water at the sides descends to fill the void
A convective thunderstorm
Temperature, Density, and Convection Heating of the Earth s surface during daytime causes the air to mix
Warmed from Below Solar radiation passes first through the upper atmosphere, but only after absorption by earth's surface does it generate sensible heat to warm the ground and generate longwave energy. This heat and energy at the surface then warms the atmosphere from below.
Latent (hidden) Heat: Water phase changes
Moist Convection A daily occurrence in summer along the high plains -- caused by surface heating, rising buoyant plumes, and the release of latent heat in clouds
Electromagnetic Radiation Radiation travels as waves or photons Waves do not require molecules to propagate Rate of energy loss is proportional to the 4 th power of temperature
Ways to label radiation By its source Solar radiation - originating from the sun Terrestrial radiation - originating from the earth By its proper name ultra violet, visible, near infrared, infrared, microwave, etc. By its wavelength short wave radiation 3 micrometers long wave radiation > 3 micrometers
Basic Radiation Laws Stefan-Boltzmann law: (E = T 4 ) (energy flux in Watts/m 2 ) As T increases, E increases by a power of 4. If T doubles, E increases by 16 times! Wien s law: Output wavelength ~ 3000/T Wavelength of peak radiation emitted by an object is inversely related to temperature
Solar Spectrum Solar radiation has peak intensities in the shorter wavelengths, dominant in the region we know as visible, but extends at low intensity into longwave regions.
Shortwave and Longwave Radiation The hot sun radiates at shorter wavelengths that carry more energy, Energy absorbed by the cooler earth is then re-radiated at longer wavelengths, as predicted by Wein's law.
The Earth s Radiation Balance Incoming Energy = Outgoing Energy (absorbed sunshine)(area) = (thermal loss)(area) S(1-a)pr 2 = T 4 (4 pr 2 )
The radiative equilibrium temperature Incoming radiation is balanced by outgoing radiation. For Earth the equilibrium surface temperature should be -18 C (~0 F). But, the Earth s observed surface temperature is ~ +15 C (~60 F). Why? Answer: layers
Solar Radiation 30% reflected 50% absorbed by the surface 20% absorbed by the atmosphere
FAQ 1.3, Figure 1
Without natural levels of greenhouse gases absorbing and emitting, this surface temperature would be 33 C cooler than the observed temperature. Greenhouse Effect Earth's energy balance requires that absorbed solar radiation is emitted to maintain a constant temperature.
Greenhouse gas emissions Human activities have caused dramatic increases in greenhouse gas concentrations
Scattering: Why is the sky blue? Sunlight is scattered by air molecules Air molecules are much smaller than the light s Shorter wavelengths (green, blue, violet) scattered more efficiently Unless we are looking directly at the sun, we are viewing light scattered by the atmosphere, so the color we see is dominated by short visible wavelengths blue dominates over violet because our eyes are more sensitive to blue light
The sun appears fairly white when it s high in the sky Near the horizon, sunlight must penetrate a much greater atmospheric path More scattering In a clean atmosphere, scattering by gases removes short visible s from the line-of-sight Sun appears orange/yellow because only longer wavelengths make it through When particle concentrations are high, the slightly longer yellow s are also scattered - Sun appears red/orange Why are Sunsets Red?
Weather and Climate What is the difference?
Time Scales - Terms of Reference Weather The conditions at a specific location at a specific time Minutes to weeks - the time period for which a specific event may be forecast Diurnal The day-night cycle 24 hours midnight to midday to midnight Climate The average conditions and their variability (includes extremes) Seasonal Annual Decadal Century * Age - as in Ice Age * Epoch - as in Holocene * Period - as in Quaternary * Borrowed from Geology
Spatial Terms of Reference Global - The planet as a whole Planetary - as in planetary waves Hemispheric - eg northern hemisphere Zonal- implies East-West a latitude band eg subtropics 20-30 o lat Meridional - implies North-South along a meridion Regional eg - High Plains - Front Range Local eg - Fort Collins - DIA Synoptic scale 500 to 3000 Kilometers midlatitude cyclones Mesoscale 20 to 200 Kilometers Thunderstorms Microscale centimeters to 1 Kilometer In-Cloud updraft
Seasons & Sun's Distance Figure 3.1 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.
Solstice & Equinox 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 Earth's tilt of 23.5 and revolution around the sun creates seasonal solar exposure and heating patterns
Seasons & Solar Intensity 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.
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 long 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 to Think About Since polar latitudes receive the longest period of sunlight during summer, why aren t temperatures highest there? Why aren t temperatures highest at the summer solstice?
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 Atmosphere is 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 nonrotating 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 hysics
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
Water vapor pressure Molecules in an air parcel all contribute to pressure Each subset of molecules (e.g., N 2, O 2, H 2 O) exerts a partial pressure The VAPOR PRESSURE, e, is the pressure exerted by water vapor molecules in the air similar to atmospheric pressure, but due only to the water vapor molecules often expressed in mbar (2-30 mbar common at surface)
Water vapor saturation Water molecules move between the 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 e = vapor pressure; e s = saturation vapor pressure Relative Humidity (RH) is ratio of actual vapor pressure to saturation vapor pressure 100 * e/e S (e = vapor pressure, e s = 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 e S
Warm air can hold more water vapor
Dewpoint Temperatures Dew point temperature the 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
Fogs are clouds in contact with the ground Several types of fogs commonly form Radiation fog Advection fog Upslope fog Evaporation (mixing) fog Fogs
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 in the troposphere even form in the stratosphere (important for the ozone hole!) Clouds impact the environment in many ways Radiative balance, water cycle, pollutant processing, earthatmosphere charge balance, 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 Middle Clouds 7,000-23,000 feet Main types Altostratus, Altocumulus Low Clouds - below 7,000 ft Main types Stratus, stratocumulus, nimbostratus Vertically developed clouds (via convection) Main types Cumulus, Cumulonimbus
Cloud type summary
Cirrus
Altostratus Alto Stratus Castellanus
Stratus A Layer of Stratocumulus Cloud viewed from above
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 with Pileaus caps
Cumulonimbus Clouds Spawn Tornadoes
Smog over China