The Atmosphere

1
The atmosphere is very thin
The Atmosphere
Troposphere
barrier!
Tropopause
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) Therefore Although
the entire atmosphere 8 of earths solid
radius 99 of gasses 0.005 0.5 (one half of
one percent) of earths radius
Cloud charts, radiosonde. instruments
2
Dry Air
For Wet Air add water vapor (up to 7 in moist
hot tropics) 4 is more typical around here.
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.
3
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.

4
The atmosphere consists of four distinct layers
thermosphere
Ionized Gas Auroras in the Thermosphere
top of mesosphere
(Tropopause)
5
Ozone
Atmosphere protects us from incoming comets,
asteroids burn up blocks short wavelength
radiation from the Sun.
6
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
7
1. Atmosphere Layers w/ Pauses
2. Pressure the weight of air above
3. Tropopause higher at equator
Ozone layer
equator poles
4. 75 of gasses In Troposphere
5. lapse rate
6.5oC/km
6. Note change of sign of lapse rate at
Tropopause (next slide)
8
When rising air hits the tropopause it cannot go
much higher, so it spreads out. What does that
remind you of?
Lapse Rate in Troposphere
9
The apparent position of the sun at the NH Winter
Solstice Tropic of Capricorn gets the most direct
sunlight on about Dec 21st North Pole Dark
24hours/day, South Pole daylight 24 hours per day
Earths Spin Axis is inclined 23½o to its orbit
around Sun
DEMO a years orbit
10
Less than 10 of UV reaches the surface 7 is a
good average
Proportions of Solar Radiation Reaching Earth
11
(albedo)
100 in from sun 30reflected -19 absorbed
51 reaches the earth Of that 51, 23 used to
evaporate water, and about 28 heats the Earth
12
Radiation Penetration
300 meters
13
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.
14
water vapor
Water only compound in three states (liquid, gas,
solid) on Earths surface. Heat energy is
transferred through the atmosphere as water
changes from one 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 atmospheres heat is absorbed by water in
processes such as melting, sublimation, and
evaporation.
Evaporation puts moisture (water vapor gas) into
the atmosphere
Condensation releases heat to atmosphere forms
cloud droplets


These two transfer the most energy, are less
common, dont cause storms
15
Latent Heat

• Latent heat is released to atmosphere as water
  changes from a less-ordered state to a
  more-ordered state
• Latent heating of condensation (gas to
  liquid).
• Atmospheres Heat is absorbed by water as it
  changes to a less-ordered state
• Latent cooling of evaporation (liquid to
  gas)
• The amount of heat lost or gained per gram of
  water is expressed in calories of latent heat.

16
Some useful units

• One Gram is the mass of liquid water in a little
  cube, one centimeter on a side.
• A centimeter is less than half an inch. It is
  1/100 of a meter.
• A meter is 39.37 inches, so a centimeter is about
  0.394 inches
• A mole of anything is 6.023 x 1023
• 1023 means 10x10x10x10x10x x10 twenty three
  times

17
Latent heat amount the same either way

• EXAMPLE The latent heat of fusion, the heat
  released as water FREEZES, i.e. goes from liquid
  to solid, is 80 calories per gram of water.
• The reverse reaction, the conversion of ice to
  water absorbs 80 calories of heat for each gram
  of water MELTED.

18
Changes of State and Weather

• Changes in state where latent heat is released
  (freezing, condensation, precipitation vapor
  to ice )
• Changes in state where latent heat is absorbed
  (melting, evaporation, sublimation ice to vapor
  ).
• Evaporation and condensation occur over large
  areas of Earth's surface contribute significantly
  to the generation of weather phenomena and the
  redistribution of heat on Earths surface. We
  will spend most of our time considering these two.

19
Evaporation
Note the LARGE number 585 calories

• Liquid water is converted to water vapor during
  evaporation. Heat is absorbed from the atmosphere
  to convert the liquid water to a less-ordered
  form, a gas, called water vapor.
• Latent Cooling of Evaporation
• 585 calories per gram are
• absorbed by water as it
• changes to gas.
• Anything touching the water loses heat and cools.
  
• We are mostly interested in the atmosphere
• losing heat.
• So do we as our water (sweat) evaporates
• Tusker Beer Anecdote

20
Condensation

• Water vapor is converted to liquid water during
  condensation. Heat is released to the air as the
  vapor converts to the more-ordered liquid form.
  Nearby air heats up, expands, and usually rises.
• Latent Heating of Condensation
• Condensation starts at the cloud base. Cloud
  bases are made of tiny droplets of LIQUID water.
  (These may also freeze)
• 585 calories per gram are released as water vapor
  is converted to liquid water . Objects nearby
  (e.g. atoms of N2 and O2 gas in the air) gain the
  heat that is released.
• 585 calories/gram of water is really a lot of
  heat

21
Evaporation/Condensation transfers a lot of energy

• Much more latent heat is lost/gained during
  changes between liquid and gas states than during
  changes between solid and liquid states.
• Depends on the number of bonds that must be
  broken or modified between water molecules.
• During freezing/melting these bonds are altered
  but
• generally do not break as the atomic structure
  changes slightly.
• In contrast, during evaporation/condensation all
  the bonds between the molecules must be broken or
  formed, requiring much more energy.

22
Humidity

• The presence of moisture (water vapor, an
  invisible gas) in the atmosphere is measured by
  the humidity of the air.
• Humidity and condensation are closely related as
  condensation inevitably occurs when the air is
  saturated with 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
23
Relative Humidity and Dew Point

• Absolute humidity measures the amount of water
  vapor in air. Grams H2O/m3 of air
• Relative humidity measures the amount of water
  vapor in air relative to the maximum amount of
  water vapor the air could hold at that
  temperature.
• Relative humidity increases with increasing water
  vapor or decreasing temperature.
• Cold air cant hold as much water vapor as warm
  air.
• The Dew Point is the temperature at which air
  becomes saturated with moisture, i.e. it cant
  hold any more.

24
Absolute Humidity

• Absolute humidity measures the amount (mass) of
  water in a volume of air. Units are
  gramsH2O/meters3
• The absolute humidity of air
• varies with temperature
• warm air can hold more
• moisture (water vapor, a gas)
• than cold air.

25
Heat flows from hot to cold Why?

• Warm air overlying cooler water? The air will
  warm the water. Example near the equator
• Cold air over warm water? Water warms air.
  Example over the Gulf stream near Britain

Heat total kinetic (motion) energy of molecules
in a packet of air of specified volume
26
Evaporation

• When water is warmed, the bonds between the water
  molecules break as the velocity of the molecules
  increases and the liquid is converted to a gas
  phase.
• This addition of water molecules to the air
  increases the vapor density, and thus the
  absolute humidity, gramsH2O/meters3 of the air
  mass.
• EVAPORATION INCREASES HUMIDITY

27
Moist Air vs. Dry Air 1

• Air with water vapor in it (Moist Air) is lighter
  than dry air
• Heres Why
• When water vapor H2O is added to air, other
  gasses are pushed aside.
• Recall that dry air is mostly Nitrogen N2 and
  Oxygen O2 molecules.

28
Moist Air vs. Dry Air 2 OR why
moist air rises

• Water H2O weighs 18 grams per mole. Nitrogen N2
  weighs 28 grams per mole
• Oxygen O2 weighs 32 grams per mole
• The number of moles of molecules in air at
  constant T and P is constant.
• Since light water molecules displace much heavier
  molecules, air with water vapor in it is
  lighter, less dense, more bouyant.

29
Relative Humidity

• Relative humidity is expressed as a percentage.
• Relative humidity measures the amount of moisture
  in air in comparison to the maximum mass/volume
  of moisture the air would contain when saturated.
  
• Saturation is the point where
• increasing vapor density
• results in condensation (clouds)

30
25oC
72oF
12oC
53.6oF
Various Temperature Scales
31
Warm air holds more water vapor than cold
air(example from your book)

• For example, air with a temperature of 25oC and
  an absolute humidity of 11.5 g/m3 has a relative
  humidity of 50 because water at that temperature
  can hold up to 23 g/m3.
• In contrast, the relative humidity of the same
  air would be 100 if the air was cooled to
• 12oC and the moisture content remains constant
  (11.5 g/m3), because 11.5 g/m3 is all the water
  vapor the cold 12oC air can hold.

32
Dew Point

• Condensation occurs when the air becomes
  saturated with moisture (relative humidity
  100).
• As temperature falls the
• relative humidity of the air rises.
• The temperature at which
• condensation begins is
• termed the dew point.
• Condensed water forms clouds.
• A million cloud droplets may
• clump together to form a rain drop.

wet lapse rate
dry lapse rate
TEMP
33
Air Pressure and Altitude

• Air (atmospheric) pressure
• is the pressure exerted by the
• weight of the overlying column of air
• 50 of all air lies below 5.5 km
• (3 miles) of altitude, therefore air
• pressure at this altitude (500mb)
• is half of the air pressure
• at sea level (1000 mb).

34
Coalescence - Making Rain

• Condensation occurs on surfaces such as dust
  particles to form tiny cloud droplets.
• The droplets are readily kept airborne by air
  turbulence. When they become so common that they
  collide and coalesce, the larger droplets fall,
  colliding with other droplets to eventually form
  a rain-drop.
• Each rain drop contains approximately one million
  cloud droplets.
• With decreasing temperatures (less than -10oC)
  ice crystals replace water droplets.

35
Lifting
PVnRT

• When air is lifted, it expands, cools.
• Density lifting (Buoyancy Lifting) occurs when a
  warm air mass, surrounded by cooler air, rises.
  The cold, denser air pushes under the warm, low
  density air. The warm low density air is forced
  up.
• Frontal lifting occurs when warm air rises over
  cold air along a warm or cold front.
• Orographic lifting takes place
• when air is forced to rise over
• a mountain range.

http//imnh.isu.edu/digitalatlas/clima/imaging/cld
dev.htm
36
Lapse Rates

• For stable air, not rising or falling, the normal
  lapse rate is 4C to 6C per 1000m (2F per
  1000ft)
• When air rises it cools at a relatively constant
  rate.
• If the air is unsaturated, this rate, called
  the dry adiabatic rate, is 10C per 1000m (5.5F
  per 1000ft),
• For saturated cold air the wet adiabatic rate
  dry adiabatic rate. For warm air the wet
  adiabatic rate is less than the dry adiabatic
  rate. An average value of 6C per 1000m (3.3F
  per 1000ft) is commonly used.

http//imnh.isu.edu/digitalatlas/clima/imaging/cld
dev.htm
37
Adiabatic Processes
PVnRT

• An adiabatic process takes place without a
  transfer of heat between the air parcel and its
  surroundings. In an adiabatic process compression
  always results in warming, and expansion results
  in cooling
• A mass of warm dry air will rise through the
  stable air as long as the temperature of the warm
  air mass remains above that of the surrounding
  air. As it rises, it expands and cools.

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

http//imnh.isu.edu/digitalatlas/clima/imaging/cld
dev.htm
38
Example
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
39
Water vapor condenses to liquid, releases heat,
parcel rises faster!
If the air reaches the dewpoint . . .
. . . before it stops, it will
continue to rise at the wet adiabatic rate
40
Sometimes surface air is saturated with water,
and a cloud forms at the surface
FOG
41
Fronts

• 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.

42
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.
http//www.irkutsk.com/home/meteo/warmfront.jpg
43
A cold front forms when a cold air mass displaces
a warm air mass. The cold air wedges under the
warmer air while pushing it.
Cold Front
44
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 to occur
across a narrower area.
http//www.irkutsk.com/home/meteo/warmfront.jpg
45
Orographic Lifting occurs when air is forced to
rise over a mountain range
Rain Shadow Desert
46

• Orographic lifting
• Air is forced to rise over a mountain range
• Air cools with increasing altitude on the
  windward flank
• Condensation and precipitation occur at high
  elevations.
• As the air descends the lee side, it warms up
• and can absorb moisture creating a rain shadow
• Precipitation in the lee is relatively rare.

47
Sea Breeze and Land Breeze
Land heats and cools faster than water
48
Convergence lifting occurs when two air masses
collide, forcing some air upward as both air
masses cannot occupy the same space.
Thunderstorms result.
Ha More thunderstorms pass over Miami than New
York in a year
Florida Sea Breeze
49
Cloud names and meanings High (gt6 km) Cirrus,
cirrostratus, cirrocumulus Middle (2-6
km) Altostratus, altocumulus Low (lt 2
km) Cumulus, stratocumulus, nimbostratus
Rule of Thumb
50
Pressure differences cause wind

• Wind is the horizontal movement of air from areas
  of high to low pressure.
• Analogy Waves High crest Low
  trough
• Crest falls into trough. High falls into Low
• Initially winds blow from High to Low pressure
  areas.
• Winds are deflected from their course by the
  Coriolis effect. (More on this later)

51
Highs and Lows

• High-pressure regions are dominated by cool or
  cold, descending air.
• Low-pressure areas are associated with warm,
  rising air masses.
• Good weather is associated with high-pressure,
  poor weather with low-pressure.

52
Surface winds in Northern Hemisphere
converge CCW on low-pressure cyclones
diverge CW from high-pressure anticyclones
53
Lows have rising air columns

• Low-pressure systems would be rapidly dissipated
  by converging air unless the in-rushing air was
  balanced by a rising air column.
• Rising air becomes cooler and may reach
  saturation, resulting in clouds and rain.
• Lows are Wet Weather systems

54
Cyclone Evolution 1. starts with rising warm air
North
East
55
Cyclone Evolution 2 A-A warm air pushes north,
cold air east 3. B-B dense cold front
catches warm, forcing warm air mass aloft
Note the predictable sequence of clouds
56
Fair Weather Highs

• Cool air descends in high-pressure zones, warming
  as it approaches Earths surface.
• As the air becomes warmer its relative humidity
  decreases resulting in dryer air.
• Highs are dry Fair Weather systems

57
Winds Aloft in Highs and Lows

• Cyclones (Lows) require divergent airflow at
  higher altitudes to balance the convergent flow
  at the surface
• Anticyclones (Highs) must have convergent flow
  aloft to balance the divergent flow at the surface

58
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
59
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
60
Pressure Gradients
Coriolis turning
Initially wind flows from high to low but
Coriolis turns it nearly parallel to lines of
equal pressure (isobars)
61
Winds blowing parallel to isobars are called
geostrophic winds This 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
62
Winds blowing parallel to isobars are called
geostrophic winds
Winds Aloft, maybe 3 km up
63
Friction turns surface winds back toward the
pressure gradient. Near the surface, winds almost
move from High to Low pressure They spiral
counterclockwise into a Low in Northern Hemisphere
64
Thats enough for now.Please read the book
chapter on Weather
http//www.physicstoday.org/vol-59/iss-8/p74.html