The Atmosphere

Earth radius 6,370 km (3,981 miles)The atmosphere extends upward to 500 km (321 miles),HOWEVER, 99% of all atmosphere gasses are below 32 km (20 miles)

ThereforeAlthough the entire atmosphere = 8% of earth’s solid radius 99% of gasses 0.005 = 0.5% (one half of one percent) of earth’s radius

The atmosphere is very thin

TroposphereTropopause barrier!

Cloud charts, radiosonde. instruments

Dry Air

Two common gasses, N2 (78%) and O2 (21%), make up 99% of dry air. Other gasses, e.g. CO2 CH4 NO2 and water vapor H2O also play an important role by keeping the atmosphere warm, the “greenhouse effect”.

For Wet Air add water vapor (up to 4% around here)

Heat vs. Temperature• Atoms in air are in constant motion, the energy of

their motion is known as kinetic energy. Kinetic energy increases as the speed of atomic motion increases. Ek = 1/2mv2 (identify symbols)

• Heat energy is the total kinetic energy of all the atoms in a substance. The more atoms present, the greater the heat.

• Temperature represents the average kinetic energy of the atoms in a substance. A few atoms with rapid motion will have a higher temperature than many atoms with slow motion.

The atmosphere consists of four distinct layers

(Tropopause)

top of mesosphere

thermosphere Ionized Gas Auroras in the Thermosphere

Atmosphere protects us from incoming comets, asteroids burn up ; blocks short wavelength radiation from the Sun.

Ozone

Temperature changes in

predictable ways with increasing altitude The thermosphere

has very few atoms, but they

are moving fast, so it has high

Temperature

Lapse Rates

1. Atmosphere Layers w/ Pauses 2. Pressure the weight/area of air above

4. 75% of gasesIn Troposphere

6.5oC/km

equatorpoles

Ozone layer

6. Note change of sign of lapse rate at Tropopause (next slide)5. lapse rate

3. Tropopause higher at equator

Lapse Rate in Troposphere When rising air hits the tropopause it cannot go much higher, so it spreads out.What does that remind you of? Mantle convection under lithosphere

The apparent position of the sun at the NH Winter SolsticeTropic of Capricorn gets the most direct sunlight on about Dec 21st

North Pole Dark 24hours/day, South Pole daylight 24 hours per day

Earth’s Spin Axisis inclined 23½o

to its orbit around Sun

Proportions of Solar Radiation Reaching Earth

Less than 10% of UV reaches the surface

7% is a good average

(albedo)

100% in from sun – 30%reflected -19% absorbed = 51% reaches the earth

Of that 51%, 23% used to evaporate water, and about 28% heats the Earth

Radiation Penetration

300 meters

Hydrologic Cycle

The volume of water falling as precipitation is approximately

4.2 x1014 m3 (420 trillion m3) per year, many times greater than the moisture stored in

the atmosphere. Water must be constantly cycled through the

atmosphere to maintain such high precipitation volumes.

Water only compound in three states (liquid, gas, solid) on Earth’s surface. Heat energy is transferred through the atmosphere as water changes fromone state to another.

Heat from water is lost to the atmosphere during freezing, condensation, and “precipitation”.This heats the air, causing it to expand and, if possible, rise.

The atmosphere’s heat is absorbed by water in processes such as melting, sublimation, and evaporation.

“ “ These two transfer the most energy, are less common,

don’t cause storms

Evaporation puts moisture (water vapor gas) into the atmosphere

Condensation releases heat to atmosphere & forms cloud droplets

“water vapor”

Latent HeatLatent Heat• Latent heat is released to atmosphere as Latent heat is released to atmosphere as

water changes from a less-ordered state to a water changes from a less-ordered state to a more-ordered state more-ordered state

“ “Latent heating of condensation” (gas to Latent heating of condensation” (gas to liquid).liquid).

• Atmosphere’s Heat is absorbed by water as it Atmosphere’s Heat is absorbed by water as it changes to a less-ordered statechanges to a less-ordered state

“ “Latent cooling of evaporation” (liquid to gas) Latent cooling of evaporation” (liquid to gas)

• The amount of heat lost or gained per gram of The amount of heat lost or gained per gram of water is expressed in calories of latent heat. water is expressed in calories of latent heat.

Some useful unitsSome useful units• One Gram is the mass of liquid water in a little One Gram is the mass of liquid water in a little

cube, one centimeter on a side. cube, one centimeter on a side.

• A centimeter is less than half an inch. It is 1/100 of A centimeter is less than half an inch. It is 1/100 of a meter. a meter.

• A meter is 39.37 inches, so a centimeter is about A meter is 39.37 inches, so a centimeter is about 0.394 inches0.394 inches

• A mole of anything is 6.023 x 10A mole of anything is 6.023 x 102323

• 101023 means 23 means 10x10x10x10x10x … x10 twenty three times10x10x10x10x10x … x10 twenty three times

Latent heat amount the same either Latent heat amount the same either wayway• EXAMPLE: The EXAMPLE: The latent heat of fusionlatent heat of fusion, the heat released as , the heat released as

water FREEZES, i.e. goes from liquid to solid, is 80 calories water FREEZES, i.e. goes from liquid to solid, is 80 calories per gram of water.per gram of water.

• The reverse reaction, the conversion of ice to water The reverse reaction, the conversion of ice to water absorbs 80 calories of heat for each gram of water MELTED.absorbs 80 calories of heat for each gram of water MELTED.

Changes of State and Changes of State and WeatherWeather

• Changes in state where latent heat is released Changes in state where latent heat is released ((freezing, freezing, condensationcondensation, “precipitation” , “precipitation” [vapor to [vapor to

ice] ice] ) )

• Changes in state where latent heat is absorbed Changes in state where latent heat is absorbed ((melting, melting, evaporationevaporation, sublimation , sublimation [ice to vapor] [ice to vapor] ). ).

• Evaporation and condensation occur over large Evaporation and condensation occur over large areas of Earth's surface contribute significantly to areas of Earth's surface contribute significantly to the generation of weather phenomena and the the generation of weather phenomena and the redistribution of heat on Earth’s surface. We will redistribution of heat on Earth’s surface. We will spend most of our time considering these two.spend most of our time considering these two.

EvaporationEvaporation• Liquid water is converted to water vapor during Liquid water is converted to water vapor during

evaporationevaporation. Heat is absorbed . Heat is absorbed from the atmospherefrom the atmosphere to to convert the liquid water to a less-ordered form, a gas, convert the liquid water to a less-ordered form, a gas, called “water vapor”.called “water vapor”.

• ““Latent Cooling of Evaporation”585 calories per gram Latent Cooling of Evaporation”585 calories per gram areare

absorbed by water as it changes to gas. absorbed by water as it changes to gas.

• Anything touching the water loses heat and cools. Anything touching the water loses heat and cools. So do we as our water (“sweat”) evaporates So do we as our water (“sweat”) evaporates

We are mostly interested in the atmosphere We are mostly interested in the atmosphere losing heat.losing heat.• Tusker Beer AnecdoteTusker Beer Anecdote

Note the LARGE number 585 calories

CondensationCondensation• Water vapor is converted to liquid water during Water vapor is converted to liquid water during

condensationcondensation. Heat is released to the air as the vapor . Heat is released to the air as the vapor converts to the more-ordered liquid form. converts to the more-ordered liquid form. Nearby air Nearby air heats up, expands, and usually rises.heats up, expands, and usually rises.

• ““Latent Heating of Condensation” Latent Heating of Condensation” • Condensation starts at the cloud base. Cloud bases are Condensation starts at the cloud base. Cloud bases are

made of tiny droplets of LIQUID water. made of tiny droplets of LIQUID water. (These may also freeze)(These may also freeze)

• 585 calories per gram are released as water vapor is 585 calories per gram are released as water vapor is converted to liquid water . Objects nearby (e.g. atoms of converted to liquid water . Objects nearby (e.g. atoms of NN22 and O and O22 gas in the air) gain the heat that is released. gas in the air) gain the heat that is released.

• 585 calories/gram of water is really a lot of heat585 calories/gram of water is really a lot of heat

Evaporation/Condensation transfers a lot of energyEvaporation/Condensation transfers a lot of energy

• Much more latent heat is lost/gained during Much more latent heat is lost/gained during changes between liquid and gas states than changes between liquid and gas states than during changes between solid and liquid states. during changes between solid and liquid states.

• Depends on the number of bonds that must be Depends on the number of bonds that must be broken or modified between water molecules. broken or modified between water molecules.

• During freezing/melting these bonds are altered During freezing/melting these bonds are altered butbut

generally do not break as the atomic structure generally do not break as the atomic structure changes slightly.changes slightly.

• In contrast, during evaporation/condensation all In contrast, during evaporation/condensation all the bonds between the molecules must be broken the bonds between the molecules must be broken or formed, requiring much more energy.or formed, requiring much more energy.

HumidityHumidity• The presence of moisture (water vapor, The presence of moisture (water vapor,

an invisible gas) in the atmosphere is an invisible gas) in the atmosphere is measured by the measured by the humidity humidity of the air. of the air.

• Humidity and condensation are closely Humidity and condensation are closely related as condensation inevitably related as condensation inevitably occurs when the air is saturated with occurs when the air is saturated with moisture (100% humidity).moisture (100% humidity).

“Latent Heat of Condensation”Gas to liquid droplet, heat is released to the atmosphere, air molecules move faster, move apart, less dense, rise

Relative Humidity and Dew Relative Humidity and Dew PointPoint• Absolute humidityAbsolute humidity measures the amount of water measures the amount of water

vapor in air. Grams Hvapor in air. Grams H22O/mO/m33 of air of air

• Relative humidityRelative humidity measures the amount of water measures the amount of water vapor in air relative to the maximum amount of vapor in air relative to the maximum amount of water vapor the air could hold at that temperature.water vapor the air could hold at that temperature.

• Relative humidity increases with increasing water Relative humidity increases with increasing water vapor or decreasing temperature.vapor or decreasing temperature.

• Cold air can’t hold as much water vapor as warm Cold air can’t hold as much water vapor as warm air.air.

• The The DewDew PointPoint is the temperature at which air is the temperature at which air becomes saturated with moisture, i.e. it can’t hold becomes saturated with moisture, i.e. it can’t hold any more.any more.

Absolute HumidityAbsolute Humidity• Absolute humidity measures the amount Absolute humidity measures the amount

(mass) of water in a volume of air. Units are (mass) of water in a volume of air. Units are gramsgramsH2OH2O/meters/meters33

• The absolute humidity of air The absolute humidity of air varies with temperature; varies with temperature; warm air can hold more warm air can hold more moisture (water vapor, a gas) moisture (water vapor, a gas) than cold air.than cold air.

Heat flows from hot to cold: Heat flows from hot to cold: Why?Why?

• Warm air overlying cooler Warm air overlying cooler water? The air will warm water? The air will warm the water. Example: near the water. Example: near the equatorthe equator

• Cold air over warm water? Cold air over warm water? Water warms air. Example: Water warms air. Example: over the Gulf stream near over the Gulf stream near BritainBritain

Heat: total kinetic (motion) energy of molecules in a packet of air of specified volume

Demo: Collisions

EvaporationEvaporation• When water is warmed, the bonds When water is warmed, the bonds

between the water molecules break as the between the water molecules break as the velocity of the molecules increases and velocity of the molecules increases and the liquid is converted to a gas phase. the liquid is converted to a gas phase.

• This addition of water molecules to the air This addition of water molecules to the air increases the vapor density, and thus the increases the vapor density, and thus the absolute humidity, gramsabsolute humidity, gramsH2OH2O/meters/meters33 of the of the air mass.air mass.

• EVAPORATION INCREASES HUMIDITY EVAPORATION INCREASES HUMIDITY

Moist Air vs. Dry Air 1Moist Air vs. Dry Air 1• Air with water vapor in it (Moist Air) is Air with water vapor in it (Moist Air) is

lighter than dry airlighter than dry air Here’s Why:Here’s Why:

• When water vapor HWhen water vapor H22O is added to air, O is added to air, other gases are pushed aside. other gases are pushed aside.

• Recall that dry air is mostly Nitrogen NRecall that dry air is mostly Nitrogen N22 and Oxygen Oand Oxygen O22 molecules. molecules.

Moist Air vs. Dry Air 2 Moist Air vs. Dry Air 2 OR “why moist air rises” OR “why moist air rises”

• Water HWater H22O “weighs” 18 grams per mole. O “weighs” 18 grams per mole. Nitrogen NNitrogen N22 “weighs” 28 grams per mole “weighs” 28 grams per mole

Oxygen OOxygen O22 “weighs” 32 grams per mole “weighs” 32 grams per mole

• The number of moles of molecules in air at The number of moles of molecules in air at constant T and P is constant. constant T and P is constant.

• Since light water molecules displace much Since light water molecules displace much heavier molecules, air with water vapor in heavier molecules, air with water vapor in it is “lighter”, less dense, more bouyant.it is “lighter”, less dense, more bouyant.

Relative HumidityRelative Humidity• Relative humidity Relative humidity is expressed as a is expressed as a

percentage. percentage.

• Relative humidity measures the amount Relative humidity measures the amount of moisture in air in comparison to the of moisture in air in comparison to the maximum mass/volume of moisture the maximum mass/volume of moisture the air would contain when saturated. air would contain when saturated.

• Saturation is Saturation is the point where the point where increasing vapor density increasing vapor density results in condensation (clouds)results in condensation (clouds)

25oC 72oF12oC

53.6oF

Various Temperature Scales

Warm air can hold more water vapor than Warm air can hold more water vapor than cold aircold air• For example, air with a temperature of 25For example, air with a temperature of 25ooC C

and an absolute humidity of 11.5 g/mand an absolute humidity of 11.5 g/m33 has a has a relative humidity of 50% because air at that relative humidity of 50% because air at that temperature can hold up to 23 g/mtemperature can hold up to 23 g/m33. .

• In contrast, the relative humidity of the same In contrast, the relative humidity of the same air would be 100% if the air was cooled to air would be 100% if the air was cooled to

1212ooC and the moisture content remains C and the moisture content remains constant (11.5 g/m3), because 11.5 g/mconstant (11.5 g/m3), because 11.5 g/m33 is all is all the water vapor the cold 12the water vapor the cold 12ooC air can hold.C air can hold.

Dew PointDew Point• Condensation Condensation occurs when the air becomes saturated occurs when the air becomes saturated

with moisture with moisture (relative humidity = 100%). (relative humidity = 100%).

• As temperature falls the As temperature falls the relative humidity of the air rises.relative humidity of the air rises.

• The temperature at whichThe temperature at which condensation begins is condensation begins is termed the termed the dew pointdew point..

• Condensed water forms clouds.Condensed water forms clouds. A million cloud droplets mayA million cloud droplets may clump together to form a rain drop.clump together to form a rain drop.

TEMPdry lapse rate

wet lapse rate

Air Pressure and AltitudeAir Pressure and Altitude• Air (atmospheric) pressureAir (atmospheric) pressure is the pressure exerted by the is the pressure exerted by the weight of the overlying column of airweight of the overlying column of air

• 50% of all air lies below 5.5 km 50% of all air lies below 5.5 km (3 miles) of altitude, therefore air (3 miles) of altitude, therefore air pressure at this altitude (~500mb)pressure at this altitude (~500mb) is half of the air pressure is half of the air pressure at sea level (~1000 mb).at sea level (~1000 mb).

Coalescence - Making RainCoalescence - Making Rain• Condensation occurs on surfaces such as dust Condensation occurs on surfaces such as dust

particles to form tiny cloud droplets.particles to form tiny cloud droplets. • The droplets are readily kept airborne by air The droplets are readily kept airborne by air

turbulence. When they become so common that turbulence. When they become so common that they collide and coalesce, the larger droplets fall, they collide and coalesce, the larger droplets fall, colliding with other droplets to eventually form a colliding with other droplets to eventually form a rain-drop.rain-drop.

• Each rain drop contains approximately one million Each rain drop contains approximately one million cloud droplets. cloud droplets.

• With decreasing temperatures (less than -10With decreasing temperatures (less than -10ooC) C) ice crystals replace water droplets.ice crystals replace water droplets.

LiftingLifting• When air is lifted, it expands, cools.When air is lifted, it expands, cools.• Density lifting (Buoyancy Lifting) occurs when a Density lifting (Buoyancy Lifting) occurs when a

warm air mass, surrounded by cooler air, rises. warm air mass, surrounded by cooler air, rises. The cold, denser air pushes under the warm, low The cold, denser air pushes under the warm, low density air. The warm low density air is forced up.density air. The warm low density air is forced up.

• Frontal lifting occurs when warm air rises over Frontal lifting occurs when warm air rises over cold air along a warm or cold front.cold air along a warm or cold front.

• Orographic lifting takes place Orographic lifting takes place when air is forced to rise over when air is forced to rise over a mountain range.a mountain range.

http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm

PV=nRT

Like oil & water

Lapse RatesLapse Rates• For stable air, not rising or falling, the For stable air, not rising or falling, the normal lapse normal lapse

rate rate is 4°C to 6.5°C per 1000m (2°F per 1000ft)is 4°C to 6.5°C per 1000m (2°F per 1000ft)

• When air rises it cools at a relatively constant rate.When air rises it cools at a relatively constant rate. If the air is unsaturated, this rate, called the If the air is unsaturated, this rate, called the dry dry

adiabatic rateadiabatic rate, is, is 10°C per 1000m (5.5°F per 10°C per 1000m (5.5°F per 1000ft),1000ft),

• For saturated cold air the wet adiabatic rate ~ dry For saturated cold air the wet adiabatic rate ~ dry adiabatic rate. For warm air the wet adiabatic rate adiabatic rate. For warm air the wet adiabatic rate is less than the dry adiabatic rate. An average is less than the dry adiabatic rate. An average value of 6°C per 1000m (3.3°F per 1000ft) is value of 6°C per 1000m (3.3°F per 1000ft) is commonly used. commonly used. http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm

Adiabatic ProcessesAdiabatic Processes• An adiabatic processAn adiabatic process takes place without a takes place without a

transfer of heat between the air parcel and its transfer of heat between the air parcel and its surroundings. In an adiabatic process surroundings. In an adiabatic process compression always results in warming, and compression always results in warming, and expansion results in coolingexpansion results in cooling

• A mass of warm dry air will rise through the A mass of warm dry air will rise through the stable air as long as the temperature of the stable air as long as the temperature of the warm air mass remains above that of the warm air mass remains above that of the surrounding air. As it rises, it expands and cools.surrounding air. As it rises, it expands and cools.

http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm

•This is because the density of warm air is less that the density of cool surrounding air

Assume that a warm air mass begins to rise with a temperature of 20oC and that the surrounding (stable) air has a temperature of 10oC. The temperature of the air mass and the stable air will be equalized at –5oC at an altitude of 2.5 km.NOTE: This is well below the tropopause

Example:

1. If the air reaches the dewpoint . . . . . . before it stops, it will continue to rise at the wet adiabatic rate

2. Above cloud base, water vapor condenses to liquid, releases heat, parcel rises faster!

Sometimes surface air is saturated with water, and a cloud formsat the surface

FOG

•Frontal lifting occurs when two large air masses of contrasting density (temperature, moisture content) meet.

•The boundary between the air masses is termed a front and may be 10 to 150 km (6-94 miles) across and hundreds of kilometers in length.

Fronts

http://www.irkutsk.com/home/meteo/warmfront.jpg

A warm front forms when a warm air mass displaces a cold air mass. The warm air rises above the colder air while pushing it. Condensation causes storms

Cold FrontA cold front forms when a cold air mass displaces a warm air mass. The cold air

wedges under the warmer air while pushing up. The warm air holds more moisture, which condenses,

resulting in storms

http://www.irkutsk.com/home/meteo/warmfront.jpg

Warm air is also forced upward when cold air approaches a warm air mass along a cold front. Cold fronts are steeper than warm fronts and cause cloud formation and precipitation tooccur across a narrower area.

Sea Breeze and Land Breeze Land heats and cools faster than water

Convergence lifting occurs when two air masses collide, forcing some air upward as both air masses cannot occupy the same space. Thunderstorms result.

Florida Sea Breeze

Ha “ More thunderstorms pass over Miami than New York in a year”

Cloud names and meanings

High (>6 km) Cirrus, cirrostratus, cirrocumulus

Middle (2-6 km) Altostratus, altocumulus

Low (< 2 km) Cumulus, stratocumulus, nimbostratus

Rule of Thumb

Highs and LowsHighs and Lows• High-pressure regions are dominated by High-pressure regions are dominated by

cool or cold, descending air.cool or cold, descending air.

• Low-pressure areas are associated with Low-pressure areas are associated with warm, rising air masses.warm, rising air masses.

• Good weather is associated with high-Good weather is associated with high-pressure, poor weather with low-pressure.pressure, poor weather with low-pressure.

Surface winds in Northern Hemisphere

converge CCW on low-pressure cyclones

diverge CW from high-pressure anticyclones

Low (Cyclone) and High (Anticyclone) Divergence and Convergence

These are not usually aligned vertically; a Low will follow its divergence aloft: good for storm forecasting

Pressure gradient is the difference in pressure between two points divided by the distance between those points. The greater the contrast in pressure the faster the wind will blow. These red isobars are lines of equal pressure

Pressure Gradients

Coriolis turning

Initially wind flows from high to lowbut Coriolis turns it nearly parallel to lines of equal pressure (isobars)

Winds blowing parallel to isobars are called geostrophic windsThis occurs well above the surface where there is no friction

Again: above the surface, Coriolis turns the winds until they blow parallel to the isobars

Winds blowing parallel to isobars are called geostrophic winds

Winds Aloft, maybe 3 km up

Polar Jet FormationPolar Jet Formation

Steep gradients of Steep gradients of temperature temperature change at the change at the Polar front Polar front trigger steep trigger steep pressure pressure gradients, which gradients, which then forces then forces higher velocity higher velocity geostrophic geostrophic winds.winds.

This is the trigger This is the trigger for jet stream for jet stream flow.flow.

Figure 11.13AFigure 11.13A

Coriolis turns these fast winds to the right in the northern hemisphere, along the cell boundaries.

Friction turns surface winds back toward the pressure gradient.Near the surface, winds almost move from High to Low pressureThey spiral counterclockwise into a Low in Northern Hemisphere

AT THE SURFACE

That’s enough for That’s enough for now.now.Please read the book Please read the book chapter on Weatherchapter on Weather