Transcript Lecture 17 The Atmosphere w

The atmosphere is very thin
The Atmosphere
Troposphere
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 earth’s solid radius
99% of gasses 0.005 = 0.5% (one half of one percent) of earth’s radius
Cloud charts, radiosonde. instruments
For Wet Air add water vapor
(up to 4% around here)
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”.
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
thermosphere
Ionized Gas Auroras
in the Thermosphere
top of mesosphere
(Tropopause)
Ozone
Atmosphere protects us from incoming comets, asteroids burn up ; blocks short wavelength radiation from the Sun.
Temperature
changes in
predictable ways with
increasing altitude
Lapse Rates
The thermosphere
has very few
atoms, but they
are moving fast, so
it has high
Temperature
1. Atmosphere Layers w/ Pauses
3. Tropopause
higher at equator
2. Pressure the weight/area of air above
Ozone layer
equator
poles
4. 75% of gases
In Troposphere
6. Note change of sign of lapse rate at Tropopause (next slide)
5. lapse rate
6.5oC/km
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 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
Earth’s Spin Axis
is inclined 23½o
to its orbit around Sun
Less than 10%
of UV reaches
the surface
7% is a good
average
Proportions of Solar Radiation Reaching Earth
(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 vapor”
Water only compound in three states (liquid, gas, solid) on Earth’s surface.
Heat energy is transferred through the atmosphere as water changes from
one state to another.
The atmosphere’s heat is absorbed by water in processes
such as melting, sublimation, and evaporation.
Evaporation puts moisture (water vapor gas)
into the atmosphere
“
“
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.
Condensation releases heat to atmosphere
& forms cloud droplets
These two transfer the most
energy, are less common,
don’t cause storms
Latent Heat
• Latent heat is released to atmosphere as water
changes from a less-ordered state to a moreordered state
“Latent heating of condensation” (gas to liquid).
• Atmosphere’s 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.
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
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.
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 Earth’s surface. We will spend
most of our time considering these two.
Note the LARGE number 585 calories
Evaporation
• 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.
So do we as our water (“sweat”) evaporates
We are mostly interested in the atmosphere
losing heat.
• Tusker Beer Anecdote
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
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.
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
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 can’t hold as much water vapor as warm air.
• The Dew Point is the temperature at which air becomes
saturated with moisture, i.e. it can’t hold any more.
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.
Heat flows from hot to cold: Why?
• Warm air overlying cooler
Demo: Collisions
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
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
Moist Air vs. Dry Air 1
• Air with water vapor in it (Moist Air) is lighter
than dry air
Here’s Why:
• When water vapor H2O is added to air, other
gases are pushed aside.
• Recall that dry air is mostly Nitrogen N2 and
Oxygen O2 molecules.
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.
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)
25oC
12oC
Various Temperature Scales
72oF
53.6oF
Warm air can hold more water vapor than cold air
• For example, air with a temperature of 25oC
and an absolute humidity of 11.5 g/m3 has a
relative humidity of 50% because air 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.
Dew Point
• Condensation occurs when the air becomes
saturated with moisture (relative humidity = 100%).
• As temperature falls the
relative humidity of the air rises.
wet lapse rate
• 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.
dry lapse rate
TEMP
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).
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.
Lifting
PV=nRT
• 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. Like oil & water
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/clddev.htm
Lapse Rates
• For stable air, not rising or falling, the normal lapse
rate is 4°C to 6.5°C per 1000m (2°F 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 10°C per 1000m (5.5°F 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
6°C per 1000m (3.3°F per 1000ft) is commonly used.
http://imnh.isu.edu/digitalatlas/clima/imaging/clddev.htm
Adiabatic Processes
• 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/clddev.htm
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
2. Above cloud base,
water vapor
condenses to liquid,
releases heat, parcel
rises faster!
1. If the air reaches the dewpoint . . .
continue to rise at the wet adiabatic rate
.
.
.
before it stops, it will
Sometimes surface air is saturated with water, and a cloud forms
at the surface
FOG
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.
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
http://www.irkutsk.com/home/meteo/warmfront.jpg
Cold Front
A 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
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
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.
Ha “ More thunderstorms pass over Miami than New York in a year”
Florida Sea Breeze
Cloud names and meanings
High (>6 km) Cirrus, cirrostratus, cirrocumulus
Rule of Thumb
Middle (2-6 km) Altostratus, altocumulus
Low (< 2 km) Cumulus, stratocumulus, nimbostratus
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.
Surface winds in Northern Hemisphere
diverge CW from
high-pressure anticyclones
converge CCW on lowpressure cyclones
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 low
but Coriolis turns it nearly parallel
to lines of equal pressure (isobars)
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
Winds blowing parallel to isobars are called geostrophic winds
Winds Aloft, maybe 3 km up
Polar Jet Formation
Steep gradients
of temperature
change at the
Polar front
trigger steep
pressure
gradients, which
then forces
higher velocity
geostrophic
winds.
This is the trigger
for jet stream
flow.
Figure 11.13A
Coriolis turns these fast
winds to the right in the
northern hemisphere, along
the cell boundaries.
AT THE SURFACE
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
That’s enough for now.
Please read the book
chapter on Weather