Transcript AOS 100: Weather and Climate

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Homework Reminder
• Exam Reminder
Review of September 22nd: More
Radiation
• Kirchoff’s Law: The efficiency of absorption at a
certain wavelength of radiation is exactly equal
to the efficiency of emission in the same
wavelength
• If something is perfect at absorbing all
wavelengths of radiation (and thus perfect at
emitting all wavelengths), then we call it a
blackbody
• We can assume the Earth is a blackbody, which
means Wien’s Law and Stefan-Boltzmann’s Law
can be used
Review Continued
• Using Stefan-Boltzmann’s Law (E=σT4), and what we
know of the incoming solar radiation, we get a surface
temperature that is much too cold (about 255 K)
• The reason we are warmer is that our atmosphere
absorbs some of the Earth’s outgoing longwave radiation
and emits some of that radiation back towards the
surface
• The combination of the Sun’s shortwave radiation and
the atmosphere’s longwave radiation make the surface
as warm as we observe it
Radiation Balances
• Radiative equilibrium: the state where the rate
an object emits radiation is equal to the rate the
object absorbs radiation
• The Earth is in radiative equilibrium as a whole,
but any given location on the Earth is probably
not in radiative equilibrium
• Whether or not a particular location is receiving
more radiation than it is emitting, or vice versa,
largely determines whether that place will warm
or cool
• This process determines everything from the
diurnal cycle of temperature to the seasons
Tuesday’s Rain
• http://www.crh.noaa.gov/news/display_cm
sstory.php?wfo=mkx&storyid=31341&sour
ce=0
What can modify this cycle?
Moisture (Variables)
• Relative Humidity (RH) is defined as the ratio
of the amount of water vapor in the air to the
amount of water vapor the air can hold (given as
a percentage)
• Dewpoint is defined as the temperature the air
would have to be cooled to reach saturation
(RH=100%)
• Warmer air can hold more water vapor, so
warmer air will by definition have a higher
dewpoint
• Mixing Ratio is the ratio of the mass of water to
the mass of dry air
A Comparison
• Consider two air parcels:
- Parcel 1 has a temperature/dewpoint of
90º/60º
- Parcel 2 has a temperature/dewpoint of
30º/29º
• Parcel 1 holds much more water vapor
(i.e. has a higher mixing ratio), but parcel 2
has a much higher relative humidity (i.e. is
closer to saturation)
Types of Heat
• Sensible Heat is the sort of heat you can measure with
a thermometer
• It’s also the type of heat you feel when you step on a hot
surface with bare feet
• Latent Heat is the heat required to change a substance
from one phase to another
• This is most commonly important with water, which is the
only substance that exists on the Earth is three different
phases
• Gases are more energetic than liquids, which are more
energetic than solids, so to move up in energetic states,
energy is taken from the environment, and vice versa
Latent Heat
Expanded Energy Budget
Latent Heat Continued
• Since water going from a gas to a liquid releases
energy, this means that condensing water in the
atmosphere releases energy to the surrounding
air
• If a parcel of air is at 100% RH, and is forced to
cool, some of that water vapor will condense,
which will warm the air
• This is important for cloud formation, as
oftentimes air at 100% RH is forced to rise,
which warms the air, and in some cases can
make it keep rising (because warm air is less
dense), which causes more condensation,
creating a positive feedback
More Energy Balance
Lapse Rate(s)
• Remember, a “Lapse Rate” is merely the rate at which
temperature decreases with height
• The Environmental lapse rate is therefore simply the rate
at which the temperature of the atmosphere decreases
with height
• Sometimes, a “parcel” of air is considered:
- The dry adiabatic lapse rate (DALR) is for a parcel with
0% RH, and is about 9.8ºC/km
- The moist adiabatic lapse rate (MALR) is for a parcel
that is saturated, and is close to 6.5ºC/km in the lower
atmosphere
- The difference is due to the energy released through
phase changes
Atmospheric Soundings
• Weather balloons (radiosondes) are launched at
least twice daily from locations across the United
States and the world
• The purpose of radiosondes is to “observe” the
atmosphere above the surface of the Earth
• These radiosondes send back data about the
vertical structure of temperature, moisture,
pressure, and wind in the atmosphere (or any
other variable)
• This information is most commonly depicted in
what is known as a “Skew-T”
Adiabatic Lapse Rate Mixing Ratio
Moist Adiabatic
Lapse Rate
Temperature
Dewpoint
Temperature
What Skew-T’s Tell Us
•
-
Skew-T’s allow us to quickly determine where:
Extremely dry layers are
Saturated layers are (often clouds)
Inversions are present
Determine the likelihood of severe weather
(more on this later)
- Find “well-mixed” layers (more on this later)
- Find the lifted condensation level (LCL) and the
level of free convection (LFC) (more on these
later)
Inversions Dry Layers Clouds
Well-Mixed Layer
Inversions Clouds
Inversions Dry Layers Clouds